SMAP (Soil Moisture Active/Passive)
SMAP (Soil Moisture Active/Passive) Mission
SMAP is a NASA mission under development within the ESSP (Earth System Science Pathfinder) program. In January 2007, the Panel on Water Resources and the Global Hydrologic Cycle of NRC (National Research Council) released a report “Earth Science and Applications from Space: National Imperatives for the Next Decade and Beyond” in which the SMAP mission was ranked with the highest priority among all proposed missions, and the report recommended it for implementation in the first phase of new missions (2010-2013).
The overall objective of SMAP is to monitor global soil moisture mapping with unprecedented resolution, sensitivity, area coverage, and revisit times. The SMAP instrument concept draws heavily upon the heritage of the Hydros (Hydrosphere State) project which was cancelled by NASA due to budget constraints in late 2005. - As a consequence of the NRC report, NASA/HQ held a two-day workshop on July 9-10, 2007 to evaluate the SMAP mission as defined in the report and to identify the ancillary measurements (if any) required to accomplish mission goals. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14)
Science goals: The SMAP data will help characterize the relationship between soil moisture, its freeze/thaw state, and the associated environmental constraints to ecosystem processes including land-atmosphere carbon, water and energy exchange, and vegetation productivity. Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Since large amounts of energy are required to vaporize water, soil control on evaporation and transpiration also has a significant impact on the surface energy fluxes. Therefore, soil moisture variations affect the evolution of weather and climate over continental regions. Initialization of numerical weather prediction (NWP) models and seasonal climate models with correct soil moisture information enhances their prediction skill and extends their skillful lead-times.
Equally important are the likely societal benefits to be derived from SMAP measurements. Many of these application areas and the approach to their improvement and modernization are also present in GEOSS (Global Earth Observation System of Systems). The application areas directly addressed by SMAP measurements of soil moisture and freeze/thaw state, acquired globally and at high spatial and temporal resolutions, are as follows (Ref. 2):
1) Weather and Climate Forecasting. Soil moisture variations affect the evolution of weather and climate over continental regions. Initialization of numerical weather prediction and seasonal climate models with accurate soil moisture information enhances their prediction skills and extends their skillful lead times. Improved seasonal climate predictions will benefit climate-sensitive socioeconomic activities, including water management, agriculture, fire, flood, and drought hazards monitoring.
2) Droughts. Soil moisture strongly affects plant growth and hence agricultural productivity, especially during conditions of water shortage and drought. Currently, there is no global in situ network for soil moisture monitoring. Global estimates of soil moisture and plant water stress must be derived from models. These model predictions (and hence drought monitoring) can be greatly enhanced through assimilation of space-based soil moisture observations.
3) Floods. Soil moisture is a key variable in water-related natural hazards including floods and landslides. High-resolution observations of soil moisture and landscape freeze/thaw status will lead to improved flood forecasts, especially for intermediate to large watersheds where most flood damage occurs. The surface soil moisture state is key to the partitioning of precipitation into infiltration and runoff, and thus is one of the major pieces of information which drives flood prediction modeling. Similarly, soil moisture in mountainous areas is one of the most important determinants of landslides. In cold land regions, the timing of thawing (which can be derived from satellite radar measurements) is coincident with the onset of seasonal snowmelt, soil thaw, and ice breakup on large rivers and lakes. Hydrologic forecast systems initialized with mapped high-resolution soil moisture and freeze/thaw fields will therefore open up new capabilities in operational flood forecasting.
4) Agricultural Productivity. SMAP will provide information on water availability and environmental stress for estimating plant productivity and potential yield. The availability of direct observations of soil moisture status and the timing and extent of potential frost damage from SMAP will enable significant improvements in operational crop productivity and water stress information systems, by providing realistic soil moisture and freeze/thaw observations as inputs for agricultural prediction models.
5) Human Health. Improved seasonal soil moisture forecasts using SMAP data will directly benefit famine early warning systems particularly in sub-Saharan Africa and South Asia, where hunger remains a major human health factor and the population harvests its food from rain-fed agriculture in highly monsoonal (seasonal) conditions. In the temperate and extra-tropical latitudes, freeze/thaw measurements from SMAP will benefit environmental risk models and early warning systems related to the potential expansion of many disease vectors that are constrained by the timing and duration of seasonal frozen temperatures. SMAP will also benefit the emerging field of landscape epidemiology (aimed at identifying and mapping vector habitats for human diseases such as malaria) where direct observations of soil moisture and freeze/thaw status can provide valuable information on vector population dynamics. Indirect benefits will also be realized as SMAP data will enable better weather forecasts that lead to improved predictions of heat stress and virus spreading rates. Better flood forecasts will also lead to improved disaster preparation and response.
6) National Security. Information on surface soil moisture and freeze/thaw is critical to ground trafficability and mobility. Weather models also need maps of the soil moisture and freeze/thaw variables to initialize forecasts for low-level fog, aviation density altitude, and dust generation. SMAP soil moisture and freeze/thaw information exceed current capability in terms of resolution, sensitivity, coverage, and sensing depth. Furthermore, radar observations over oceans and water bodies yield information on ice cover at high resolution and regardless of illumination.
The SMAP mission concept includes an L-band radiometer and an L-band high-resolution radar that share a single feedhorn and parabolic mesh reflector. The radar operates with VV, HH, and HV/VH transmit-receive polarizations, and uses separate transmit frequencies for the H (1.26 GHz) and V (1.29 GHz) polarizations. The radiometer operates with V, H and U (third Stokes parameter) polarizations at 1.41 GHz. The reflector is offset from nadir and rotates about the nadir axis at 13.0 rpm, providing a conically scanning antenna beam with a surface incidence angle of approximately 40º. The reflector diameter is approximately 6 m, providing a radiometer footprint of about 40 km defined by the one-way 3 dB beamwidth.
The SMAP project is managed for NASA by the Jet Propulsion Laboratory (JPL), with participation by the Goddard Space Flight Center (GSFC). SMAP builds on the heritage and risk-reduction activities of the NASA ESSP Hydros mission.
- The SMAP Mission Concept Review was conducted on June 24, 2008. On Sept. 24, 2008, SMAP was formally approved to initiate Phase A.
- The Phase-B of SMAP started in January 2010.
- In 2011, the instrument system has completed the preliminary design review (PDR) stage, and detailed instrument design has begun.
- In May 2012, SMAP successfully passed the Key Decision Point-C (KDP-C) review and is now in Phase C of the mission.
- SMAP succesfully completed its CDR (Critical Design Review) in July, 2012.
- In May 2013, a NASA team at GSFC delivered the SMAP radiometer to JPL where it will be integrated into the SMAP spacecraft along with the L-band radar (SAR) instrument developed by JPL. 15)
Table 1: Preliminary mission requirements of SMAP
Table 2: Why Study Soil Moisture? (Ref. 26)
The radar and spacecraft have been developed in-house at JPL, leveraging previous Earth mission radar experience and adapting power and avionics elements from a recent JPL planetary mission. The radiometer is being developed at GSFC leveraging previous Earth mission radiometer experience. Mission operations and science data processing will be conducted by JPL with GSFC support.
The spacecraft design was developed concurrently and synergistically with the instrument to reduce overall observatory complexity and therefore, development effort. The spacecraft development addressed the unique challenges associated with instrument accommodation and implementation approach. Planetary avionics from a previous JPL mission were adapted to support SMAP’s high data volume and data rates, and also to support the high degree of functional integration between the instrument and spacecraft.
In addition, the spacecraft was required to maintain compatibility with several launch vehicles including the Minotaur -4+, Atlas V, Falcon 9, and most recently the Delta-2 until relatively late in the design lifecycle without slowing the development or delaying the launch ready date (the Delta-2 was selected by NASA for SMAP at CDR-time). From the spacecraft and observatory packaging and volume standpoint, a key challenge was packaging the RBA (Reflector Boom Assembly) within the most constraining vehicle: Minotaur IV+ with the 2.34 m fairing. Designing for electromagnetic compatibility was also a challenge given the sensitivity of the instruments (and especially the radiometer) to L-band emissions. The RBA and SIA (Spun Instrument Assembly) also posed a challenge to the spacecraft’s pointing control design. This also posed a challenge for the fault protection design, primarily to ensure that minor faults do not result in a spin down of the observatory with the additional loss of science observation time that would impose (Ref. 94).
The key challenge for the Observatory configuration is in meeting the needs of the science and the supporting subsystems (many of which have stringent demands as well) and ensuring that the spacecraft could meet all operational, pointing, environmental, and launch vehicle requirements. A major trade for the mission design is deciding “what spins”. One approach is to spin the entire Observatory, while another approach is to spin just the instrument (Ref. 12). 16) 17) 18) 19) 20)
Figure 1: Artist's view of the SMAP mission observation scheme (image credit: NASA)
The spacecraft is three axis–stabilized and employs momentum compensation via reaction wheels to accommodate the angular momentum of the spinning instrument. The spacecraft has an aluminum primary structure with a zenith deck provided for mounting the SIA and an anti-sun facing panel for mounting the radar electronics. The bus structure of the spacecraft bus employs a pentagonal box shape (size of 1.5 m x 0.9 m x 0.9 m) with internal components, including a semi-permanent frame structure with removable panels. Removable panels are organized by subsystem and serve as thermal radiators. The structure uses aluminum and aluminum honeycomb construction.
A modular design for accommodating subsystems, each on a panel, provides for ease of integration (Figure 2). A deployable, fixed solar array with three panels is utilized as the primary power source and provides about 1400 W for the observatory. Spacecraft batteries have a total 74 Ah capacity using Li-ion small cell chemistry based on NASA Aquarius/Argentina’s/CONAE Satélite de Aplicaciones Científicas-D (Satellite for Scientific Applications or SAC-D) and Kepler spacecraft heritage.
SMAP spacecraft avionics and power electronics leverage a JPL planetary heritage design based on the commercial RAD750 flight computer and PCI (Peripheral Component Interconnect) bus architecture for the C&DH (Command and Data Handling) subsystem , 1553 data bus as the observatory command and telemetry backbone, built-to-print design for telecommunication and instrument interfaces, power bus architecture (including power converters, switches, and pyro-firing circuits) for the Power and Pyrotechnic subsystem, as well as the control electronics for the Reaction Control System (thrusters and latch valves). A small number of new capabilities were added, including a 128 GB NVM (Non-Volatile Memory) capable of accommodating much larger science data storage volumes and transmission rates (130 Mbit/s downlink rate vs. 6 Mbit/s typical for planetary missions’ maximum X-band downlink rate), new engineering interface control, a high-capacity solar array interface, and a new power bus controller.
The ACS (Attitude Control Subsystem) employs high-heritage, flight-proven attitude sensors and control mechanisms. Attitude knowledge is provided by a star tracker and 12 sun sensors, which also support safing and attitude reinitialization. Redundant inertial measurement units propagate attitude knowledge between stellar attitude updates. Three large reaction wheels, with a 4th wheel used for momentum compensation, maintain momentum balance between the spacecraft and the SIA and counteract any disturbance torques. Three magnetic torque assemblies are used to manage the reaction wheel momentum and are controlled based on on-orbit magnetic field information from a single 3-axis magnetometer. Orbit position is determined via two-way Doppler tracking and propagated on-board the spacecraft.
The SMAP mission is composed of three elements – the instrument system, the spacecraft bus system, and the ground system. Together, the instrument and the spacecraft bus form the observatory, which collects information and relays it to the ground. The observatory has one portion that is 3-axis stabilized and one portion that will spin at 13 revolutions per minute. 21)
The spacecraft is responsible for avionics [including C&DH (Command and Data Handling) as well as FSW (Flight Software )], GNC (Guidance Navigation and Control), propulsion, communication to the ground and to the instruments, thermal maintenance of the de-spun portion, propulsive maneuvers, and power.
The thermal subsystem is based on a passive design. MLI coverage is optimized for survival heater power and ability to reject electronic heat. Heaters may be added as needed. The battery is thermally isolated from the supporting radiator panel, externally mounted, with dedicated radiator/MLI/heaters.
Figure 3: Illustration of the spacecraft bus configuration (image credit: NASA)
Figure 4: Basic block diagram of the SMAP observatory (image credit: NASA/JPL)
Propulsion subsystem: The monopropellant (hydrazine) propulsion subsystem employs a blow-down design. All components and processes have flight heritage, and include single titanium, diaphragm propellant tank (ROCSAT design made by ATK), eight 4.5 N monopropellant thrusters identical to MER & MSL cruise stage, redundant latch valves and pressure transducers. One of the key requirements for the thermal aspects of propulsion is to meet all range safety requirements.
Figure 5: Illustration of the propulsion subsystem (image credit: NASA)
RF communications: The subsystem is composed of redundant S-band transponders for uplink/downlink command and telemetry functions to the NASA Earth Network and Space Network (single access mode). The transponder provides for coherent Doppler tracking to support orbit determination. The high rate X-band data downlink is provided by redundant X-band transmitters (8 W output). The telecommunication antennas are mounted on a fixed (nondeployable) outrigger at the nadir end of the spacecraft. The payload downlink is in X-band at 130 Mbit/s, the TT&C data link is in S-band at 2.5 Mbit/s.
SMAP uses a single-string architecture with selective redundancy. Graceful degradation features have also been designed into the observatory where practical. The transponders, transmitters, and IRUs (Inertial Reference Units) are redundant. Propulsion latch valves are mounted in parallel for redundancy and all the thrusters are placed on a single branch. The reactions wheels are oriented and sized so that a failure in an individual wheel can be tolerated. Each magnetic torque assembly is internally redundant (via redundant windings) and magnetic field information can be provided via a ground-based model in the case of a magnetometer failure. Survival heaters and many instrument slip rings are redundantly wired. All actuators include redundant windings.
Particular attention has been paid to fault protection design to reduce the likelihood and mission impact of specific faults and also to minimize the number of fault events that cause the instrument to despin. The observatory is designed to robustly and autonomously recover attitude following the momentum change associated with a despin, but the return to science operations is a longer process resulting in undesirable science data loss. For this reason, the instrument remains spinning for all but the most severe faults (Figure 6).
Table 3: Observatory estimated mass and power breakdown
Figure 7: The unfolded solar arrays to power SMAP and the golden feedhorn for its radar and radiometer are visible in this image taken during assembly and testing (image credit: NASA) 22)
Figure 8: Photo of the SMAP leadership and test team members in cleanroom garb appear with the Spacecraft and Instrument in the background (image credit: NASA)
- NASA contracted ULA (United Launch Services LLC) in July 2012. 27)
Orbit: Sun-synchronous dawn/dusk orbit, altitude = 685 km, inclination = 98º, period = 98.5 minutes, LTAN (Local Time of Ascending Node) = 18:00 hours, exact repeat cycle = 8 days after 117 orbits, (near-global coverage of Earth can be obtained every three days, 44 orbits). — The radar data will provide the freeze/thaw measurement with 3 km spatial resolution at an interval of every two days for each location north of 45º north latitude — about the latitude of Minneapolis.
Secondary (auxiliary) payloads:
ELaNa X (Educational Launch of Nanosatellite X), which consists of three P-PODs (Poly Picosatellite Orbital Deployers) containing a total of four CubeSats (representing three CubeSat missions). The three CubeSat projects on ELaNa X include (Ref. 26): 28)
• GRIFEX (GEO-CAPE ROIC In-Flight Performance Experiment), a 3U CubeSat flight test experiment and a collaborative mission of the University of Michigan with NASA ESTO (Earth Science Technology Office) and JPL (Jet Propulsion Laboratory.
• ExoCube,a space weather nanosatellite (3U CubeSat) developed by the California Polytechnic State University (Cal Poly), San Luis Obispo. The payload is developed by NASA/GSFC.
• FIREBIRD-2 A and B (Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics), each a 1.5U CubeSat collaborative mission of the University of New Hampshire, Montana State University, LANL (Los Alamos National Laboratory), and the Aerospace Corporation.
Orbit of secondary payloads: The CubeSats will be deployed after separation of the SMAP (Soil Moisture Active Passive) observatory, into an elliptical orbit of 440 km x 670 km of 99.12º inclination.
Figure 9: A condensed version of NASA JPL's live launch coverage and animated reflector deployment (video credit: NASA JPL, provided by Astro Aerospace, Published on 1 May 2015)
Figure 10: Data availability after launch (image credit: NASA, Ref. 91)
Figure 11: The SMAP mission time line of key events (image credit: NASA) 29)
Figure 12: Line drawing of the deployed SMAP spacecraft (image credit: NASA, Ref. 26)
• February 15, 2020: Thailand is experiencing its worst drought in possibly four decades. About half of the major reservoirs in the country stand below 50 percent of capacity. River levels are so low that saltwater from the ocean is creeping upstream and affecting drinking water supplies. And in a country where 11 million people work in farming, crop production and the economy are expected to suffer. 30)
- The drought conditions in Thailand were caused by a shorter-than-normal monsoon season and below-average annual rainfall in 2019. According to the Mekong River Commission (MRC), the monsoon rains arrived almost two weeks later and departed three weeks earlier in the Lower Mekong Basin, which includes Lao PDR (Laos), Thailand, Cambodia, and Vietnam. The MRC also stated an El Niño event created abnormally high temperatures and high evapotranspiration. Thailand was predicted to be affected particularly hard; the country is only two months into the dry season and reservoirs for irrigation and drinking are already low.
- The drought has caused saltwater intrusion in some areas in Thailand’s water supplies, where water from the sea is encroaching up the river. There’s not enough fresh river water flowing down to the push saltwater away from cities, said Senaka Basnayake, the Director of Climate Resilience at the Asian Disaster Preparedness Center in Thailand.
- “This is one of the signs showing the drought situation in low-lying areas in Thailand this year is worse than before,” said Basnayake, who is also a member of the SERVIR-Mekong team. SERVIR-Mekong is a joint project between NASA and United States Agency for International Development (USAID) that uses remote sensing to provide support for several areas in the Mekong River Basin, such as protection of food and water resources, disaster risk reduction, and sustainable development.
- The SERVIR-Mekong team develops several drought-related products to generate drought and crop yield forecasts in the Mekong area. The information is used by the MRC to help provide accurate information for farmers trying to plan their water usage. According to the team’s Regional Drought and Crop Yield Information System, the drought severity in Thailand is above 90 percent across the country and could persist at that level until the end of March.
- Thailand, one of the world’s leading sugar exporters, is expected to produce up to 30 percent less sugar compared to previous years, making this possibly the worst season in five years. On February 17, the Department of Royal Rainmaking and Agricultural Aviation (DRRAA) is expected to begin its annual relief plan, which includes filling local reservoirs.
- Other countries in the Lower Mekong Basin are also expecting a severe dry season in the upcoming months. According to Vietnam’s Ministry of Agriculture and Rural Development, the Mekong Delta located in southwestern Vietnam received about 8 percent less precipitation during last year’s rainy season. Vietnam has already started to experience saltwater intrusion, which are damaging many rice fields in the Trá Vinh province. More than 10,000 hectares of winter-spring rice are experiencing a water shortage in the province. If water supplies continue to run low and saltwater intrusion continues, about 94,000 hectares of rice fields could be affected across the Mekong River Basin area.
Figure 13: A severe drought in Thailand is affecting agriculture and drinking water in the region. The map shows soil moisture anomalies, or how much the water content near the land surface was above or below the norm for southeast Asia from January 1 to February 7, 2020. The measurements were derived from data collected by the Soil Moisture Active Passive (SMAP) mission, the first NASA satellite dedicated to measuring the water content of soils. SMAP’s radiometer can detect water in the top 5 cm of the ground. Scientists use that surface layer data in a hydrologic model to estimate how much water is present even deeper in the root zone, which is important for agriculture (image credit: NASA Earth Observatory, image by Lauren Dauphin using soil moisture data from NASA-USDA and the SMAP Science Team. Story by Kasha Patel)
• November 19, 2019: Getting stuck on a muddy road is a hassle for anyone, but for the U.S. Army it could be far more serious - a matter of life and death in some parts of the world. That's one of the reasons the U.S. Air Force HQ 557th Weather Wing is now using data about soil moisture from a NASA satellite in the weather forecasts, warnings and advisories that it issues for the Army and the Air Force. 31)
- NASA's Soil Moisture Active Passive (SMAP) spacecraft, launched in 2015 and managed by NASA's Jet Propulsion Laboratory in Pasadena, California, measures the amount of water in the top two inches (5 cm) of soil. Near-real-time SMAP data began flowing into Air Force computers on Nov. 19 to be used within the modeling environment powered by NASA's Land Information System (LIS). This implementation will be the first instance of assimilating SMAP data in an operational, near-real-time environment in the world.
- Besides dictating how muddy or dry the land surface is, soil moisture is also a key weather-maker: It evaporates into water vapor, rises and condenses into clouds. "The forecasting model output is going to be different depending on whether soil is dry or moist," said Frank Ruggiero, the lead engineer for USAF's Numerical Weather Modeling program, run by Hanscom Air Force Base in Lexington, Massachusetts. "Sometimes those differences can significantly affect the overall forecast."
- In 2005, the Air Force's 557th Weather Wing at Offutt Air Force Base near Omaha, Nebraska, was looking to replace its existing Land Information System. That year, they partnered with NASA's Goddard Space Flight Center in Greenbelt, Maryland, to collaborate on the development of the LIS, a software framework that integrates satellite and ground-based data using advanced mathematical techniques to improve performance of weather and climate computer models.
- "The initial idea was to use NASA's state-of-the-art modeling system to move new research findings and data sources into Air Force operations more rapidly," said Goddard's Sujay Kumar, who, with Christa Peters-Lidard, is the technical lead for transitioning the LIS to the Air Force. "That kick-started a 15-year relationship with the Air Force."
- The NASA-Air Force LIS team began researching the feasibility of adding SMAP data to the Land Information System even before the satellite was launched. The Air Force forecasting tools were already using soil moisture data from older satellites, Kumar said. "But once we had SMAP data and conducted several studies to look at the quality of it, we were pleased to find that its high information content was likely to improve the accuracy of the soil moisture characterization in the Air Force systems," Kumar added.
Military Weather Forecasts Are Different
- The Army and Air Force have specific information needs that civilian weather forecasts don't address. "We tend to be very focused on weather impacts, and some are unique compared to impacts for civilians," Ruggiero said. "You, as a regular person, might want to know if it's mostly sunny or mostly cloudy so you'll know if it's a good beach day. But clouds affect the signal of a lot of the instrumentation we use, so we need detailed, 3D cloud depiction to know how much our signal is going to be affected."
- That doesn't mean, however, that the military has to build a weather forecasting model from scratch. Instead, it enhances a civilian weather model - the Unified Model, developed by the United Kingdom's Met Office - to create a system that can produce the tailored forecasts its users need.
- The Air Force is a partner of the Met Office and about 10 other weather agencies that utilize and adapt the Unified Model. It has been using the model - considered one of the best in the world - as the basis for its own primary forecasting model since 2015. Because other U.S. coalition partners, including the U.K., Australia and South Korea, also use it, the Air Force is assured that in joint actions, the partners operate with the same basic weather assumptions.
A Wide, and Widening, Value
- Scientists in the Met Office also look at new capabilities created by the partners and can choose to adopt them into the Unified Model. Jerry Wegiel, a NASA Goddard support scientist stationed at Offutt Air Force Base is NASA Goddard's lead liaison for the Unified Model partnership. "The developers of the Unified Model are in the process of adopting [NASA's Land Information System] as a result of this multi-agency collaboration. It's currently under validation and verification, but they're going down the road of transferring this capability into their own operational suites in the future," Wegiel said. "The Air Force is providing value back to the Unified Model partnership."
- Wegiel pointed out that the LIS has other users besides weather forecasters, among them, the USDA (U.S. Department of Agriculture). "The USDA was one of the very first adopters. Once a month they produce agricultural forecasts that are used in places like the Chicago Mercantile Exchange, and that affects U.S. policies on global agriculture sectors as we try to position U.S. commodities to remain world leaders on the competitive stage." With users in a wide array of fields, including military and civilian weather forecasters worldwide, SMAP's soil moisture data in the LIS are benefiting people around the globe.
• May 28, 2019: The continental United States recently finished its soggiest 12 months in 124 years of modern recordkeeping. The results are visible in satellite measurements of fresh water. 32)
- From May 1, 2018, to April 30, 2019, the lower 48 states collectively averaged 36.20 inches (919.48 mm) of precipitation, a full 6.25 inches (158.75 mm) above the mean. The previous record (April 2015 to March 2016) was 35.95 inches. According to the National Centers for Environmental Information, ten U.S. states had their wettest 12 months, and three others were in the top three. Many of them were clustered in the Mid-Atlantic and Midwest regions.
- According to the May 21 report from the U.S. Drought Monitor, just 2.72 percent of the contiguous U.S. was in drought, among the lowest levels in two decades of records. California is completely out of drought for the first time since 2011. As recently as February 2018, one-third of the United States was in drought.
Figure 14: This map shows how groundwater has responded to the unusually wet year. The colors depict the wetness percentile; that is, how the amount of groundwater on May 13, 2019, compares to all Mays from 1948 to 2012. Blue areas have more abundant groundwater than usual for the time of year, and orange and red areas have less. The map is based on multiple types of meteorological data (precipitation, temperature, etc.) integrated within an advanced computer model developed by scientists at NASA’s Goddard Space Flight Center (image credit: NASA Earth Observatory image by Lauren Dauphin and Joshua Stevens using soil moisture data from the NASA-USDA SMAP team and using GRACE data from The National Drought Mitigation Center at the University of Nebraska-Lincoln, and rainfall data from The Iowa Environmental Mesonet The Iowa Environmental Mesonet (IEM). Story by Mike Carlowicz)
- The map of Figure 15 shows soil moisture anomalies, or how much the water content near the land surface was above or below the norm on May 11–13, 2019. The measurements are derived from data collected by the SMAP (Soil Moisture Active Passive) mission, the first NASA satellite dedicated to measuring the water content of soils. SMAP’s radiometer can detect water in the top 5 cm of the ground. Scientists use that surface layer data in a hydrologic model to estimate how much water is present even deeper in the root zone, which is important for agriculture.
Figure 15: NASA Earth Observatory image by Lauren Dauphin and Joshua Stevens using soil moisture data from the NASA-USDA SMAP team and using GRACE data from The National Drought Mitigation Center at the University of Nebraska-Lincoln, and rainfall data from The Iowa Environmental Mesonet The Iowa Environmental Mesonet (IEM). Story by Mike Carlowicz)
- Much of the East and Midwest had an extremely damp autumn in 2018; land-falling category 5 hurricanes Michael and Florence dropped copious amounts of rainfall in the late summer; and California has been soaked by sporadic atmospheric river events and the effects of a mild El Niño. But there is no one explanation for the extreme precipitation of the past year. It does, however, fit with long-term increases in overall precipitation and with heavy rainfall events in our changing climate.
- “I do not have an explanation for the weather systems that caused the heavy precipitation, but sea surface temperatures in the western Atlantic and Gulf of Mexico have been generally well above normal over the past year. This has surely added to the atmospheric water vapor content available to the precipitating weather systems,” said Ken Kunkel, a climatologist with the National Oceanic and Atmospheric Administration. “The pattern of precipitation over the past 12 months indicates general wetness over most parts of the U.S. but does not match projections of the future, which show increases mostly in the northern U.S. Thus, the recent wetness probably has explanations in addition to, or instead of, just anthropogenic forcing.”
- In the Fourth National Climate Assessment, released by the U.S. Global Change Research Program in 2018, scientists reported: “a national average increase of 4 percent in annual precipitation since 1901 is mostly a result of large increases in the fall season. Heavy precipitation events in most parts of the United States have increased in both intensity and frequency since 1901...The frequency and intensity of heavy precipitation events are projected to continue to increase over the 21st century. Mesoscale convective systems (organized clusters of thunderstorms) in the central United States are expected to continue to increase in number and intensity in the future.”
- Writing for The Washington Post, meteorologist Jason Samenow reflected on a record-setting year of rain in the nation’s capital: “The historic rainfall over the past year is somewhat of a random occurrence. It is mostly a result of weather patterns that have frequently arranged themselves, by chance, in an optimal way to squeeze water from the sky. Yet, at the same time, this record-wet year has occurred against a longer-term backdrop of climate warming and increasing precipitation extremes. In other words, climate change probably intensified the rain and increased the chance it would become a record breaker.”
• May 17, 2019: Our oceans and the complex "conveyer belt" system of currents that connects them play an important role in regulating global climate. The oceans store heat from the Sun, and ocean currents transport that heat from the tropics to the poles. They release the heat and moisture into the air, which moderates climate nearby. But what happens if part of that conveyer belt jams? 33)
Figure 16: This animation shows a time lapse of sea surface salinity and soil moisture from NASA's Soil Moisture Active Passive (SMAP) satellite from April 2015 through February 2019 (image credit: NASA/JPL-Caltech/GSFC)
- It's not a theoretical question. Scientists have observed that a major ocean current called the Indonesia Throughflow, which provides the only tropical connection between the Pacific and Indian oceans, slows dramatically near the surface during the Northwest Asia monsoon season - usually December through March. And a team of scientists, led by Tong Lee of NASA's Jet Propulsion Laboratory in Pasadena, California, has figured out why.
- "We have found that this current, which is a very important element of the global ocean current system, is significantly affected by local precipitation," Lee said. "It is fairly common knowledge that winds drive ocean currents. In this case, however, the precipitation is actually a dominant factor during the monsoon season."
- It's a discovery that will improve our understanding of complex Earth processes. During this season, about 10 feet (3 meters) of rain fall over the maritime continent, a region of Southeast Asia between the Indian and Pacific oceans through which the Indonesia Throughflow current travels. This influx of local rain reduces the pressure force that drives the current through the region.
- How does that work?
- Gravity causes water to travel "downhill" from areas of relatively higher sea level toward areas of lower sea level unless opposed by another force. In the tropical Pacific, trade winds also influence the flow of water. They blow from east to west, causing ocean currents to transport large amounts of water from the U.S. westward toward Asia. This raises the sea level on the Asian side of the Pacific Ocean and provides enough force to keep the Indonesia Throughflow moving, connecting the two oceans.
- However, the influx of rain during monsoon season temporarily but significantly raises the local sea level in the Indonesian seas that sit between the Pacific and Indian oceans enough to essentially eliminate the downhill flow. Think of it like a ball rolling freely downhill versus a ball on a flat surface, which has little momentum to move forward.
- Although the slowing of this current is primarily seasonal, it still affects the amount of heat transported from the Pacific Ocean to the Indian Ocean, which can change regional climate in Southeast Asia.
- ”The increase in local sea level due to the seasonal freshening of seawater pushes against the normally higher sea level from the Pacific Ocean," said Lee. "It restricts the surface flow of this current during the monsoon season, which prevents a lot of the heat normally carried by the current from making its way to the Indian Ocean."
- Furthermore, since all of these currents are connected globally, less warm water is transported into the Indian Ocean, and in turn, less warm water is transported from the Indian Ocean to the Atlantic Ocean over the long term. So the Indonesia Throughflow - one element of a much larger system - can have a significant effect thousands of miles away from where it flows.
- The results of this study will help to improve climate models by enabling scientists to factor in these effects and changes. Titled "Maritime Continent water cycle regulates low-latitude chokepoint of global ocean circulation," the study was recently published in Nature. 34)
- NASA satellite data, particularly ocean salinity measurements from the Soil Moisture Active Passive (SMAP) satellite, were instrumental in these findings. Although SMAP was designed primarily to measure soil moisture, its radiometer is also able to measure sea surface salinity. The results of this paper demonstrate the utility of SMAP salinity data in exploring changes in the water cycle, sea level, ocean circulation and climate.
• August 10, 2018: July 2018 was the driest July in Australia since 2002. The dry month exacerbated an ongoing drought that had already ruined large swaths of grazing land and cropland. New South Wales has been hit the hardest, according to a news reports. About 99 percent of the state was in drought heading into August. 35)
- Drought becomes relevant to farmers and ranchers when there are deficits in the amount of water available to plants in the “root-zone,” which generally includes the top 200 cm (80 inches) of soil (Figure 17).
- “Agricultural drought can irreversibly damage crops and hamper yield formation, leading to economic losses and food insecurity,” said Iliana Mladenova, a scientist at NASA’s Goddard Space Flight Center.
- The map is derived from data collected by the SMAP (Soil Moisture Active Passive) mission, the first NASA satellite dedicated to measuring the water content of soils. SMAP’s radiometer can detect the amount of water in the surface layer of the soil; that is, the top 5 cm (2 inches) of the ground. Scientists use that surface layer data in a hydrologic model to estimate how much water is present even deeper in the root zone. Analyzing this moisture can tell you whether there is enough water for plants to function properly and to achieve an optimal yield.
- “Timely and accurate knowledge of the root-zone soil moisture is very important for farmers, stakeholders, and agricultural agencies,” Mladenova said. “Soil moisture provides the necessary information to track and monitor drought development and occurrence, and predict its impact on end-of-season yields.”
Figure 17: The map shows root-zone soil moisture anomalies—how much the moisture content was above (green) or below (brown) the norm—on July 28, 2018. Soils were extremely dry in eastern Australia, which is a major agricultural area (image credit: NASA Earth Observatory, image by Joshua Stevens using soil moisture data from NASA-USDA and the SMAP Science Team, story by Kathryn Hansen)
• June 1, 2018: Data from the first NASA satellite mission dedicated to measuring the water content of soils is now being used operationally by the U.S. Department of Agriculture (USDA) to monitor global croplands and make commodity forecasts. 36)
- The SMAP (Soil Moisture Active Passive) mission, launched in 2015 , has helped map the amount of water in soils worldwide. Now, with tools developed by a team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, SMAP soil moisture data are being incorporated into the Crop Explorer website of the USDA's Foreign Agricultural Service, which reports on regional droughts, floods and crop forecasts. Crop Explorer is a clearinghouse for global agricultural growing conditions, such as soil moisture, temperature, precipitation, vegetation health and more.
- "There's a lot of need for understanding, monitoring and forecasting crops globally," said John Bolten, research scientist at Goddard. "SMAP is NASA's first satellite mission devoted to soil moisture, and this is a very straightforward approach to applying that data."
- Variations in global agricultural productivity have tremendous economic, social and humanitarian consequences. Among the users of these new SMAP data are USDA regional crop analysts who need accurate soil moisture information to better monitor and predict these variations.
- "The USDA does crop forecasting activities from a global scale, and one of the main pieces of information for them is the amount of water in the soil," said Iliana Mladenova, a research scientist at Goddard.
- The USDA has used computer models that incorporate precipitation and temperature observations to indirectly calculate soil moisture. This approach, however, is prone to error in areas lacking high-quality, ground-based instrumentation. Now, Mladenova said, the agency is incorporating direct SMAP measurements of soil moisture into Crop Explorer. This allows the agriculture analysts to better predict where there could be too little, or too much, water in the soil to support crops.
- These soil moisture conditions, along with tools to analyze the data, are also available on Google Earth Engine. There, researchers, nonprofit organizations, resource managers and others can access the latest data as well as archived information.
- "If you have better soil moisture data and information on anomalies, you'll be able to predict, for example, the occurrence and development of drought," Mladenova said.
- The timing of the information matters as well, she added — if there's a short dry period early in the season, it might not have an impact on the total crop yield, but if there's a prolonged dry spell when the grain should be forming, the crop is less likely to recover.
- With global coverage every three days, SMAP can provide the Crop Explorer tool with timely updates of the soil moisture conditions that are essential for assessments and forecasts of global crop productivity.
- For more than a decade, USDA Crop Explorer products have incorporated soil moisture data from satellites. It started with the Advanced Microwave Scanning Radiometer-E instrument aboard NASA's Aqua satellite, but that instrument stopped gathering data in late 2011. Soil moisture information from ESA's (European Space Agency) SMOS (Soil Moisture and Ocean Salinity) mission is also being incorporated into some of the USDA products. This new, high-quality input from SMAP will help fill critical gaps in soil moisture information.
- SMAP is managed for NASA's Science Mission Directorate in Washington by the agency's Jet Propulsion Laboratory in Pasadena, California, with instrument hardware and science contributions made by Goddard.
Figure 18: With data from NASA's Soil Moisture Active Passive satellite, researchers can monitor the amount of water in soils to identify areas prone to droughts or floods. In this map created with SMAP data from May 18-May 18, 2018, soils that are wetter than normal are seen in greens, while those that are drier than normal are seen in browns (image credit: Joshua Stevens/NASA Earth Observatory)
• Spring 2018: The Amazon is the largest river in the world in terms of the amount of freshwater it carries and discharges into the Atlantic Ocean. This animation clearly shows the freshwater discharges into the Atlantic, seen as dark blue. 37)
Figure 19: The animation, which runs from March 27, 2015 to April 16, 2018, is produced using data from NASA’s Soil Moisture Active Passive Mission (SMAP). A key feature is that the freshwater does not stay along the coast but is moved by ocean currents. The North Brazil current carries the freshwater north, where it is then captured and moved eastward by the Atlantic Equatorial Countercurrent. Freshwater entering the Gulf of Mexico from the Mississippi River can be seen at the top (image credit: NASA/JPL/PODAAC)
• August 30, 2017: Images of soil moisture conditions in Texas near Houston, generated by NASA's SMAP (Soil Moisture Active Passive) satellite before and after the landfall of Hurricane Harvey can be used to monitor changing ground conditions due to Harvey's rainfall. As seen in the left panel, SMAP observations show that soil surface conditions were already very wet a few days before the hurricane made landfall (August 21/22), with moisture levels in the 20 to 40 percent range. Such saturated soil surfaces contributed to the inability of water to infiltrate more deeply into soils, thereby increasing the likelihood of flooding. After Harvey made landfall, the southwest portion of Houston became exceptionally wet, as seen in the right panel image from August 25/26, signaling the arrival of heavy rains and widespread flooding. 38)
Figure 20: Soil moisture conditions in Texas near Houston, generated by NASA's SMAP satellite before and after the landfall of Hurricane Harvey can be used to monitor changing ground conditions due to Harvey's rainfall (image credit: NASA)
• January 16, 2017: The top 5 cm of topsoil on all of Earth's landmasses contains an infinitesimal fraction of the planet's water—less than 0.001%. Yet because of its position at the interface between the land and the atmosphere, that tiny amount plays a crucial role in everything from agriculture to weather and climate, and even the spread of disease. The behavior and dynamics of this reservoir of moisture have been very hard to quantify and analyze, however, because measurements have been slow and laborious to make. 39)
- That situation changed with the launch in 2015 of the NASA SMAP (Soil Moisture Active Passive) satellite, designed to provide globally comprehensive and frequent measurements of the moisture in that top layer of soil. SMAP's first year of observational data has now been analyzed and is providing some significant surprises that will help in the modeling of climate, forecasting of weather, and monitoring of agriculture around the world. These new results are reported in a study of MIT (Massachusetts Institute of Technology). 40) 41)
- The SMAP observations are providing an unprecedented level of detailed, worldwide information on the amount of water in those top 5 cm of soil, collected globally every two to three days. Dara Entekhabi says this is important because this thin layer is a key part of the global water cycle over the continents, and also a key factor in the global energy and carbon cycles.
- Precipitation on land, and the evaporation of that moisture from the land, "transfers large amounts of energy" between the continents and the atmosphere, Entekhabi says, and the Earth's climate would be drastically different without this element. The oceans, containing 97% of Earth's water, provide a major role in storing and releasing heat, but over land that role is provided by the moisture in the topmost layer of the soil, albeit through different mechanisms. That moisture "is a tiny, tiny fraction of the water budget, but it's sitting at a very critical zone at the surface of the land, and plays a disproportionately critical role in the cycling of water," he says. "It plays a significant role in moderating climate, on seasonal and annual timescales."
- Understanding these cycles better, thanks to the new data, could help make weather predictions more accurate over longer timescales, which could be an important boon for agriculture. Several federal agencies have already begun using the SMAP data, Entekhabi says, for example, to help make forecasting of drought and flood conditions more accurate. "The satellite is providing an extraordinary quality of surface soil moisture information that makes this analysis possible," he says. The satellite's primary mission of three years is about halfway over, he says, but the team is working on applying for an extended mission that could last as much as a decade.
- One of the big surprises from the new data is that this top level of soil preserves a "memory" for weather anomalies, more so than had been predicted from theory and earlier, sparser measurements. Memory refers to the persistence of effects from unusually high or low amounts of rainfall. Contrary to most researchers' expectations, it turns out that these effects persist for a matter of days, rather than just a few hours. On average, about one-seventh of the amount of rain that falls is still present in that topmost layer of soil three days after it falls—and this persistence is greatest in the driest regions.
- The data also show a significant feedback effect that can amplify the effects of both droughts and floods, Entekhabi says. When moisture evaporates from wet soil, it cools the soil in the process, but when the soil gets too dry that cooling diminishes, which can lead to hotter weather and heat waves that extend and deepen drought conditions. Such effects "had been speculated," he says, "but hadn't been observed directly."
- The ongoing SMAP mission also provides educational opportunities that help to verify and calibrate the satellite data. With minimal equipment, students can participate in hands-on lessons in data collection, using measurement methods that are considered the gold standard. For example, they can gather a sample of soil in a fixed volume such as a tuna can, and weigh it before and after drying it out. The difference between the two masses gives a precise measure of the soil's moisture content in that volume, which can be compared with the satellite's moisture measurement.
- Even young students "can carry out 'gold standard' measurements, and all it takes is a kitchen scale and an oven," Entekhabi says. "But it's very labor-intensive. So we have engaged with schools around the world to do these measurements."
Figure 21: Global map and associated averages, by zone, of a new measure of how long it takes for soil moisture from rainfall to dissipate (estimated soil moisture water cycle fraction), produced from one year of data from NASA’s Soil Moisture Active Passive mission (image credit: MIT/NASA/JPL-Caltech)
• December 2016: The SMAP was launched in January 2015 and has been providing science data since April 2015. Though designed to measure soil moisture and despite the demise of its radar in July 2015, the SMAP sensor is very capable to measure ocean winds in storms at a resolution of 40 km. The L-band radiometer V-pol and H-pol channels keep excellent sensitivity to ocean surface wind speed even at very high wind speeds and they are only very little impacted by rain. The presence of polarimetric channels, which measure all 4 Stokes parameters on the SMAP radiometer, also allows retrieving wind direction. 42)
- L-band satellite radiometers allow measuring the intensity of tropical and extratropical storms. The reason is that the wind-induced surface emission at L-band increases approximately linearly with wind speed between 18 and 65 m/s and shows no signs of saturation at any wind speeds nor is it significantly affected by rain. This provides a distinct advantage over many other active and passive spaceborne sensors, whose signal saturates in high winds or which degrade in rain. The spatial resolutions of spaceborne passive L-band sensors that are currently operating (SMOS and SMAP) are limited to about 40 km.
• October 20, 2016: A NASA analysis of a 2015 Texas flood is the first to document the full life cycle and impacts of a flood on both land and ocean. Using data from NASA's SMAP satellite and other satellite instruments, the study traced the event's chronology — starting with rains that fell weeks before the flood and ending with an unusually shaped plume of freshwater that lingered in the Gulf of Mexico months later, with a potential for significant impacts on the gulf's marine life. 43) 44)
- The flood occurred in southeastern Texas on May 23-24, 2015, when record rainstorms topped off weeks of sustained rainfall. According to the National Weather Service, 37.3 trillion gallons of water (14 x 109 m3) fell on Texas in May 2015 — enough to cover the entire state 20 cm deep in water (Texas has a size of 694100 km2). "The sheer amount of water was shocking," said coauthor J. T. Reager of NASA's Jet Propulsion Laboratory, Pasadena, California. "Of course it had an impact on land — 11 people died, and property was lost. We thought, this has to have an impact on the ocean as well."
- The research team used measurements from SMAP with observations from five other NASA satellites to create a comprehensive timeline and map of the flood and its regional effects. SMAP measures both soil moisture (water retained in the top several cm of the ground) and sea surface salinity, which allows scientists to calculate how much freshwater is mingled with the saltwater of the ocean.
- When so much rain fell on waterlogged soils, there was nowhere for it to go but downstream. South Texas is one of the parts of the country most in need of stream gauges, according to the U.S. Geological Survey, so ground-level streamflow data are limited. The new analysis found that eight Texas rivers drained huge volumes of water into the Gulf of Mexico, with peak discharge rates as high as 1,700 m3/s of water.
- Ocean currents carried this large plume of freshwater eastward along the Louisiana coastline during June 2015. In July, it merged with the Mississippi River's outflow, the downstream product of spring precipitation and snowmelt high water from 40% of the contiguous United States.
- The combined Texas and Mississippi plumes formed what Reager calls "a huge, very weird, horseshoe shape" with legs extending southward into the central gulf. "We had never seen a plume shaped like this in six years of observations of sea surface salinity," Fournier said, referring to the European SMOS (Surface Moisture and Ocean Salinity) satellite data. "Looking at the circulation in the gulf, you can say the unusual features of this plume are not due to the Mississippi River. They're due to the Texas flood."
- A strong ocean current in the east of the gulf and a warm eddy in the west played roles in forming the horseshoe and pulling its legs southward toward the central gulf (Figure 22).
- Although river water is called freshwater, that doesn't mean it's pure H2O and nothing else. Rivers carry pollutants to the sea, and that's what makes the location and size of the freshwater plume important. "Riverine waters bring lots of nutrients to the ocean, and that can have impacts," Fournier explained. Ocean algae use nitrogen and phosphorus in the runoff — mainly the product of fertilizers — to grow and multiply, and the algae concentrations can become much larger than normal. When these overabundant algae die, they sink and decompose in a process that uses up the oxygen in seawater, sometimes creating dead zones that are so oxygen-starved no fish or plants can live there.
- The Gulf of Mexico has a variable but huge dead zone, usually ranked largest or second largest in the world. Its average area is about the same as the state of Connecticut. The gulf is also one of the world's largest seafood providers. When its dead zone expands, there are impacts to both fish and the regional economy; for example, fishing boats have to travel farther off shore to find fish, leading to higher fuel costs and a less profitable industry.
- After the Texas flood, the gulf's dead zone was about 28% larger than average: the size of Connecticut and Rhode Island combined. Fournier will delve deeper into the biogeochemical impacts of the flood on the gulf waters in a follow-up study. This type of research could lead to predictive models for the influences of the high levels of pollution on gulf fisheries.
- Reager noted that the complementary land and ocean measurements of SMAP greatly facilitated the team's analysis. "With the existing suite of NASA satellites, we have the opportunity to see the full extent of regional water cycle extremes and the impacts of heavy rains, saturation flooding and intense runoff on both land and ocean," he said. "SMAP really provides a new and important complement to the tools in the NASA Earth science tool belt."
- Besides SMAP's measurements, the study used a wide array of other NASA observations: precipitation data from the TRMM (Tropical Rainfall Measuring Mission) and GPM (Global Precipitation Measurement) mission, water storage observations from the GRACE (Gravity Recovery And Climate Experiment) mission, ocean color observations from the MODIS (Moderate Resolution Imaging Spectrometer), and altimetric satellite measurements of ocean currents from the Jason series of satellites.
Legend to Figure 22: SSS = Sea Surface Salinity. PSS =Practical Salinity Scale (to derive salinity from precise instrument measurements of seawater electrical conductivity, temperature and pressure).
• July 2016: Since the loss of the radar, the SMAP project has been conducting two parallel activities to enhance the resolution of its soil moisture products. The first explores the Backus Gilbert optimum interpolation and de-convolution techniques based on the oversampling characteristics of the SMAP radiometer. The second investigates the disaggregation of the SMAP radiometer data using the European Space Agency’s Sentinel-1 C-band SAR (Synthetic Aperture Radar) data to obtain soil moisture products at about 1 to 3 km resolution. In addition, SMAP’s L-band data have been found useful for many applications, including vegetation opacity, ocean surface salinity and hurricane ocean surface wind mapping. Highlights of these new applications will be provided. 45)
- The SMAP project has been conducting a Cal/Val (Calibration/Validation) program to assess the quality of soil moisture and freeze/thaw products. The Cal/Val program includes the comparison of data with many core Cal/Val sites, where spatially distributed in situ sensors are deployed within the reference grids, data comparison with sparse in-situ networks, intersatellite comparison, and field campaigns.
- The preliminary Cal/Val assessment has indicated excellent quality of the soil moisture and freeze/thaw products. The key findings include 1) the ubRMSE (unbiased Root Mean Square Error) of the passive soil moisture product has met the 4 percent volumetric accuracy requirement, 2) soil moisture retrieval bias for some Cal/Val sites or land cover types remains to be reduced, and 3) the detection of freeze/thaw states has well exceeded the requirement of 80 percent classification accuracy.
- Figure 23 illustrates a comparison of the passive soil moisture product with the in situ data acquired from the TxSON (Texas Soil Moisture Observation Network) Cal/Val site. The time series comparison clearly indicates the changes of soil moisture due to precipitation events and subsequent dry downs. All three algorithms produce very similar ubRMSE and correlation. The bias is smaller for SCA-V (Single Channel V-pol), leading to a smaller RMSE than the other two algorithms for the TxSON site. However, the SCA-H (Single Channel Algorithm H-pol) has a better performance for some other sites. After we average the statistics using the data from all core Cal/Val sites, SCA-V has a slightly better performance overall for the beta-level products, and is consequently selected as the current baseline product for L-2 radiometer-only soil moisture.
Figure 23: Comparison with the in situ data from the TxSON Cal/Val site. The upper panel illustrates the time series comparison. The bottom right indicates the locations of in situ sensors in the 36 km grid. The bottom middle indicates the scatter plot of satellite retrieved VSM (Volumetric Soil Moisture) and reference (in situ) VSM. The statistics of differences is provided in the bottom left panel for Single Channel Algorithm H-pol (SCA-H), Single Channel V-pol (SCA-V) and DCA (Dual Channel Algorithm). Soil moisture results from SMOS are also shown (image credit: NASA/JPL)
- Science recovery: One of SMAP’s key science requirements is to produce soil moisture products at a spatial resolution of better than 10 km for hydrometeorology applications. This was to be achieved by using high-resolution radar data to disaggregate brightness temperatures on 9 km resolution grids. Unfortunately the SMAP radar transmitter ceased operation on July 7, 2015. Since then the SMAP project has been conducting two tasks to enhance the resolution of radiometer data products. One of the tasks will take advantage of the oversampling characteristics of the SMAP radiometer data along scan and along track. Two resolution enhancement techniques are being considered, including the Backus Gilbert (BG) optimum interpolation and deconvolution. The other task will match up the SMAP radiometer data and Sentinel-1’s C-band SAR data, and will then apply the SMAP AP (Active-Passive) algorithm to produce the higher resolution soil moisture products. These two tasks will produce complementary products; it is expected that the BG or deconvolution techniques will lead to a global product with a spatial resolution of about 20 to 30 km resolution, while the SMAP/Sentinel-1 AP algorithm will produce soil moisture with resolution as high as 1 km, but only for regions where the Sentinel-1 data are available.
- Figure 24 provides an illustration of the BG interpolation algorithm applied to a half orbit of simulated SMAP radiometer brightness temperatures. The interpolation was carried out on 3 km grids aligned with the along-track and across-track coordinate system. The interpolated data will then be averaged to produce data on the EASE-2 (Equal-Area Scalable-2 Earth Grid) at 9 or 18 km resolution.
Figure 24: Illustration of the Backus-Gilbert interpolation algorithm applied to simulated SMAP radiometer data. Left panel simulated data; middle panel: BG output; right panel: difference between simulation and BG interpolation (image credit: NASA/JPL)
- The potential of the AP soil moisture algorithm for combined SMAP radiometer/Sentinel-1 radar data is indicated in Figure 25. The Sentinel-1 SAR data was used to disaggregate the SMAP radiometer data at a spatial resolution of 1 km for a site in Southern Canada. The high-resolution features are clearly visible in the soil moisture image although the algorithm parameters require optimization. We will be applying the SMAP Cal/Val program to the SMAP/Sentinel-1 AP soil moisture product, and will be conducting the SMAPVEX campaign in 2016 for direct validation of the disaggregated brightness temperature data.
- In spite of the loss of SMAP radar in July 2015, the SMAP radiometer has been performing very well. The radiometer soil moisture products have been used to monitor the severe flooding caused by heavy rainfall in Texas in May-June and in South Carolina in early October 2015. More recent flooding events in Missouri and South America in December 2015 were also indicated in the SMAP soil moisture products (Ref. 45).
• June 2016: NASA submitted a paper at the EUSAR conference 2016, presenting an analysis of the short coverage period of the SMAP L-band SAR which experienced a malfunction in July 2015. 46)
- In summary, the surface soil moisture was retrieved globally by systematically correcting for the effects of vegetation and soil surface roughness. The retrieval is enabled by employing physical-models of radar forward scattering for individual vegetation types to account for vegetation scattering and absorption, and by constraining the surface roughness effect using time-series observations. The L-band SMAP multi-polarized (HH/VV/HV) σ°data acquired globally every three days were used from mid-April to early July, 2015. Assessment was conducted over 13 rigorously-chosen core validation sites covering a wide range of biomass types, biomass amount, and soil conditions (Figure 26). The soil moisture retrieval reached an accuracy of 0.063 m3/m3 RMSE (Root Mean Square Error), a bias of -0.002 m3/m3, and a correlation of 0.54. The successful retrieval demonstrates that the physically-based retrieval method is capable of characterizing soil moisture over diverse conditions of soil moisture, surface roughness, and vegetation on a global scale.
- Figure 27. shows a global image developed using the SMAP algorithms. The regions that are expected to be very dry (i.e., the Sahara desert) and wet (i.e., the Amazon Basin) reflect the expected levels of retrieved soil moisture. The global patterns show the expected soil moisture variability. Compared with the radiometer-based retrievals in the inset of Figure 27, the global spatial patterns are in agreement but there are some regional bias differences between the two. The radar-based retrievals are less dry over the Sahara desert than the radiometer-based estimates. The bare surface forward model assumes no topography and no penetration, while the topography in the area is expected to increase σº (more than the topography does on the radiometer data) and penetration depth may reach 1 m. Subsurface feature such as rocky terrain may cause retrieval errors. Over shrublands, the two products match well in Namibia, the southwest of the United States, southern Argentina, and Australia. The rainforest appears wetter in the radar product than in the radiometer output, but this is an area where the performance of both products is expected to be poor.
Figure 27: SMAP radar-based soil moisture for one 8-day cycle of June 19 to 26, 2015. The inset shows the SMAP radiometer-baseline retrievals over the same period. Except for a few orbit gaps, missing data (grey) are mostly due to the frozen condition (permanent and dynamic) and failed retrievals (Australia), image credit: NASA/JPL
• April 19, 2016: In 2015, Ethiopia endured its worst drought in decades. While there is an indication that this April has been drier than normal in some areas, the strength of the drought this year will not be known until farther along in 2016. 47)
- For now, satellite data can help show the state of conditions on the ground important for agriculture (Figure 28). Ethiopia is now in its secondary crop season—the “belg”—a short rainy season from March to May. Successful harvest of belg crops depends on adequate rainfall.
- Planting in the highlands was already underway in early March 2016, as the belg rains had an early start in February. But rainfall since February has been unevenly distributed across Ethiopia, according to the NOAA Climate Prediction Center. Moisture deficits have grown in the central, southwestern, and eastern parts of the country. The situation could change in coming weeks, as heavy rainfall was forecasted for eastern and southern Ethiopia, which would help to reduce the deficit.
- Another way to gauge growing conditions is by analyzing the NDVI (Normalized Difference Vegetation Index), a measure of how plants absorb visible light and reflect infrared light. Drought-stressed vegetation reflects more visible light and less infrared than healthy vegetation.
- The map of Figure 29 is based on data from MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s Aqua satellite. The map shows the NDVI anomaly: it contrasts vegetation health from March 29 to April 5, 2016, against the long-term average from 2000–2015. Brown areas show where plant growth, or “greenness,” was below normal. Greens indicate vegetation that is more widespread or abundant than normal for the time of year. Grays depict areas where reliable data were not available, usually due to cloud cover.
- Most of the country’s food is produced during the main growing season, the “meher,” which is a longer rainy season that begins in June and runs through at least August. While it remains to be seen what the 2016 meher will bring, the effects of the 2015 drought are still being felt. According to news reports, more than 10 million Ethiopians are relying on food aid, and others need farming supplies to revive the area’s agriculture.
Figure 28: The map shows how soil moisture in Ethiopia, averaged from April 1 to April 14, 2016, differed from conditions one year earlier. The map is based on data from NASA’s SMAP (Soil Moisture Active Passive) satellite, which can measure soil moisture in the top 5 cm of the ground. Yellow and green indicate areas where the top layer of soil became wetter; red areas became drier (the resolution is about 9 km/ pixel), blue areas are lakes (image credit: NASA Earth Observatory, images by Joshua Stevens and Jesse Allen)
Figure 29: MODIS data of the Aqua mission. The map shows the NDVI anomaly: it contrasts vegetation health from March 29 to April 5, 2016, against the long-term average from 2000–2015. Brown areas show where plant growth, or “greenness,” was below normal. Greens indicate vegetation that is more widespread or abundant than normal for the time of year. Grays depict areas where reliable data were not available, usually due to cloud cover (image credit: NASA Earth Observatory, images by Joshua Stevens and Jesse Allen)
• Nov. 27, 2015: NASA scientists are auditioning the radar aboard Sentinel-1A European satellite to see how well it stands in for the radar that broke down aboard the U.S. space agency’s newly launched Soil Moisture Active Passive (SMAP) satellite in July. Sentinel’s C-band radar is not the only radar flying in space — or even the closest substitute for the lost SMAP L-band radar — but it is the only one that will trail SMAP closely enough to gather timely radar images of the swath of Earth that SMAP covers with its radiometer. 48)
• Nov. 25, 2015: Compared to the amount of water stored elsewhere on the planet, the amount in the soil is minuscule. But that small volume has great significance around the world. Measurements of soil moisture are relevant for a variety of applications, particularly farming. There’s more to it, however, than the simple fact that plants need water to grow. 49)
- Knowing something about the moisture in the soil is important before, during, and after the growing season. For example, will mud prevent a tractor from safely plowing through the fields? How much water will fruits, nuts, and vegetables have available at each stage of growth, from germination through harvest? What is the forecast for crop yields around the world? How will the amount of moisture and agricultural output affect trade policy and food aid? Satellite and ground-based sensors are helping scientists find out.
- The colors of the map (Figure 30),
whose data was acquired in the period May 27-31, 2015 with the radar
and radiometer instruments on SMAP, show the volume of water contained
in a volume of soil. Dark green and blue areas are progressively
wetter, up to a ratio of about 0.5; at that point, the ground is
- At the same time, ground-based sensors monitor soil moisture over small areas — typically less than one square meter. The image of Figure 31 shows the locations of more than 1,200 ground-based stations across the United States.
- Networks of ground-based stations have become expansive in the United States, particularly in Oklahoma. In that state, stations are part of an environmental monitoring network called Mesonet, which was conceived after a disastrous flood struck Tulsa, Oklahoma, in 1984. Today, more than 100 stations across Oklahoma measure soil moisture at depths down to 60 cm. Mesonet is just one of 31 networks, which together account for nearly 1,500 stations in North America.
- Data from ground sensors, updated every 30 minutes, can help farmers quickly figure out where there is inadequate moisture in their fields. The plot below the map shows the level of detail provided by data from the Coastal Sage UCI (University of California, Irvine) station in California.
- These in situ networks do not cover all areas of the United States, and certainly not all of the planet. To fill in the gaps, some scientists estimate global soil moisture by running computer models loaded with precipitation, temperature, and humidity data. Gathering all of the data to run such models can take as long as two to three months, which makes realtime applications impossible.
- “What we really want is soil moisture information that can be used to understand how plants are growing and what’s going on in the atmosphere right now,” said Susan Moran, a hydrologist with USDA’s Agricultural Research Service and chair of the SMAP Applications Working Group. “We have to get soil moisture information to the agriculture community, and the only way to do that is from satellites.”
Figure 30: The map above shows the amount of moisture in the top 5 cm of the ground across the United States (resolution: 9 km/pixel). It was produced with data collected from May 27–31, 2015, with the radar and radiometer instruments on NASA’s SMAP satellite (image credit: NASA Earth Observatory, Joshua Stevens and the SMAP science team)
• October 23, 2015. SMAP data so far: 50)
- Exceptional quality global L-band radiometry – science acquisition continuing
- SMAP data provides overlapping global L-band radiometry together with SMOS
- Potential for long-term and high-revisit water cycle observations (land and ocean)
- SMAP radar loss on July 7 is set-back for community
- Science recovery efforts now underway taking advantage of substantial SMAP radiometer oversampling and Sentinel-1A (and -1B) joint observations
- Science uses of SMAP in characterizing both land and ocean branches of the water cycle underway at the same time.
• October 8, 2015: It was rain that wouldn't quit. A weather system fueled by warm moisture streaming in from the Atlantic Ocean on Oct. 3 and 4 relentlessly dumped between 0.3 and 0.6 meters of rain across most of South Carolina. The result was rivers topping their banks and dams bursting. Catastrophic flooding followed across most of the state, which has left residents in some areas without power or clean drinking water. 51)
- Tracking and predicting the deluge, both as rain and then as floodwater, are the first steps to help protect people in harm's way. State and federal emergency managers have been on the front lines of this natural disaster since it began, armed with weather and flood forecasts from the National Weather Service. NASA has supported these efforts with information based on data from Earth-observing satellites in space.
- The GPM (Global Precipitation Measurement) mission of NASA and JAXA provided regular data on the amount of rain falling across the region. These data, managed by NASA's Goddard Space Flight Center in Greenbelt, Maryland, confirmed the record-breaking rainfall totals and fed into other systems that helped forecast the extent of the flooding in the region.
- As essential as following the rain is knowing what happens when it reaches the ground. The NASA SPoRT (Short-term Prediction Research and Transition Center) at MSFC (Marshall Space Flight Center) in Huntsville, Alabama, runs a computer simulation model called the NASA Land Information System for most of the central and eastern United States, including the Carolinas. The SPoRT Project used rainfall data from the GPM mission and measurements from NASA's SMAP (Soil Moisture Active Passive) satellite in this near realtime simulation, and provided the NWS (National Weather Service) forecast offices with experimental soil moisture data that can aid short-term flood forecasting. Soil moisture describes the ability of the ground to absorb water, like a sponge. During the record-setting rainfall event, soil moisture across South Carolina reached 75 to 100 % saturation. At these high levels of saturation, the soil no longer has the sponge capacity to hold the water, so it flows over the land — leading to flash floods.
- GPM rainfall data are used in another NASA-funded flood prediction tool that follows water on the ground, the Global Flood Monitoring System, developed and run by researchers at the University of Maryland, College Park. The Global Flood Monitoring System takes current satellite rain data and combines them with a model of the land surface — showing slopes, streams and rivers — and soil moisture to project the likelihood of flooding based on when rivers broke their banks in the past. The FEMA (Federal Emergency Management Agency) and the Department of Homeland Security use these maps to help tell where the flooding is now, after the rainfall, and where flooding may continue in the days ahead as floodwaters move downstream.
- Even before the storm clouds cleared, efforts to assess the damage from space had begun. NASA's ARIA (Advanced Rapid Imaging and Analysis) project at NASA/JPL in Pasadena, California, developed plans to analyze before-and-after data from synthetic aperture radars (SAR) flown by ASI (Italian Space Agency) and JAXA. These data can see through clouds to detect changes in the urban environment, such as storm damage to buildings and roads.
Figure 32: Relative soil moisture over the Carolinas on Oct. 5, 2015, shown in percent. Green indicates the soils are 50 to 65% saturated, while blues and purples indicate 65 to 100% saturated. Saturated soils can hold no more water so those areas will flood (image credit: NASA/MSFC)
• October 5, 2015: The beta version of Level-2 and Level-3 radiometer data from NASA’s SMAP (Soil Moisture Active Passive) mission is now available at the NSIDC/DAAC (NASA National Snow and Ice Data Center/ Distributed Active Archive Center). These data use input L-band brightness temperature measurements retrieved from the SMAP radiometer to produce global soil moisture estimates. 52)
- As of Sept. 9, 2015, the SMAP Science Data System (SDS) has begun forward processing of the beta (Version 1) Level-2 and -3 radiometer data, which will be made available at the NSIDC/ DAAC within 24 hours of satellite observation for the Level-2 data and within 50 hours for the Level-3 data. Note that reprocessing of the data from March 31, 2015 to Sept. 9, 2015 to Version 1 will begin at the end of October 2015.
Figure 33: SMAP radiometer-only soil moisture between Oct. 3 to 5, 2015 (image credit: NASA/JPL)
• September 2, 2015: Managers of the SMAP observatory have determined that its radar, one of the satellite's two science instruments, can no longer return data. However, the mission, which was launched in January to map global soil moisture and detect whether soils are frozen or thawed, continues to produce high-quality science measurements supporting SMAP's objectives with its radiometer instrument. 53)
- The SMAP spacecraft continues normal operations, and the first data release of soil moisture products is expected in late September. "Although some of the planned applications of SMAP data will be impacted by the loss of the radar, the SMAP mission will continue to produce valuable science for important Earth system studies," said Dara Entekhabi, SMAP Science Team lead at the Massachusetts Institute of Technology in Cambridge.
- On July 7, 2015, SMAP's radar stopped transmitting due to an anomaly involving the radar's high-power amplifier (HPA). The HPA is designed to boost the power level of the radar's pulse to more than 500 W, ensuring the energy scattered from Earth's surface can be accurately measured. The SMAP project at NASA/JPL formed an anomaly team to investigate the HPA issue and determine whether normal operation could be recovered. A series of diagnostic tests and procedures was performed on both the spacecraft and on the ground using flight spare parts.
- Following an unsuccessful attempt on Aug. 24 to power up the radar unit, the project had exhausted all identified possible options for recovering nominal operation of the HPA and concluded the radar is likely not recoverable.
- NASA has appointed a mishap investigation board to conduct a comprehensive review of the circumstances that led to the HPA anomaly in order to determine how the anomaly occurred and how such events can be prevented on future missions. JPL also will convene a separate failure review board that will work with the NASA investigation.
- The nearly three months of coincident measurements by the two instruments are a first of their kind. The combined data set allows scientists to assess the benefit of this type of combined measurement approach for future missions. Scientists now are developing algorithms to produce a freeze-thaw data product at 40 km resolution from the radiometer data. They also are evaluating whether the 40 km radiometer soil moisture resolution can be improved.
- Based on the available SMAP mission data, scientists have identified other useful science measurements that can be derived from the radiometer data, such as sea surface salinity and high winds over the ocean surface. Over the next several months, the SMAP project and NASA will work to determine how to implement these new measurements into the project's data products.
Figure 34: A three-day composite global map of surface soil moisture as retrieved from SMAP's radiometer instrument between Aug. 25-27, 2015 (image credit: NASA) 54)
• August 6, 2015: In a major milestone, scientists have completed their initial calibration of the two instruments on NASA's new Soil Moisture Active Passive (SMAP) satellite observatory, launched Jan. 31, 2015. The work paves the way for upcoming activities to validate SMAP science data against ground measurements, leading to the planned release of SMAP soil moisture data products to the international science community this fall and the release of fully validated data next spring. 55)
- The calibration activities reveal SMAP's radar calibration stability to be within 1 decibel, and SMAP's radiometer calibration stability within 1 Kelvin. The calibration work allows SMAP data users to familiarize themselves with partially calibrated, or "beta" data, while the full calibration activities are being completed. The "beta-level" radiometer and radar data products were made available for public release July 31 through the NSIDC (National Snow and Ice Data Center) in Boulder, CO and the ASF (Alaska Satellite Facility) in Fairbanks, AK. The fully calibrated data will be released in early November of 2015.
- SMAP's minimum three-year mission will map global soil moisture and detect whether soils are frozen or thawed. The mission will help scientists understand the links between Earth's water, energy and carbon cycles; help reduce uncertainties in predicting weather and climate; and enhance our ability to monitor and predict natural hazards such as floods and droughts.
- SMAP radiometer data have been processed to map microwave emissions from Earth's surface, expressed as brightness temperatures in Kelvin and at a horizontal spatial resolution of about 40 km. SMAP's radar began regular operations on April 13, but stopped transmitting July 7 due to an anomaly that is still being investigated by the SMAP team at JPL. The available radar data have been processed to produce coarse-resolution 5 km x 30 km global data products and high-resolution (1 km) data products over land surfaces and coastal oceans.
Figure 35: Artist's rendering of the SMAP satellite. The width of the region scanned on Earth's surface during each orbit is about 1000 km (image credit: NASA/JPL,Caltech)