Other Space Activities

Soil Moisture

Last updated:Jan 16, 2026

Measurement Types

Soil moisture is a key regulator of water and energy exchange within land systems, influencing plant growth, hydrology, and climate. Satellite microwave observations supplement ground-based measurements to provide the global, continuous monitoring needed to understand and manage these land–atmosphere interactions.

SMAP observations of global soil moisture from May 28 through June 4, 2019. (Image credit: NASA)

Soil moisture is the amount of water held in the pores between soil particles, typically expressed as volumetric water content (the volume of water per unit volume of soil), but also measured in some contexts on a gravimetric (mass-based) basis. Despite representing only about 0.001% of Earth’s total water, it is critical for regulating plant growth, surface water availability, and the exchange of water and energy between the land and atmosphere. Monitoring soil moisture supports agriculture by informing irrigation practices, assessing crop health, and revealing how human activities such as land use and irrigation alter the water cycle. It also contributes to understanding weather and climate processes, improving forecasts, and predicting floods and droughts. 1)

Soil moisture represents the balance between water entering, being stored within, and leaving the soil. Its distribution depends on several factors, including precipitation, soil texture, and the permeability of underlying layers. When rain infiltrates the ground, water moves through pores between soil particles until it encounters low-permeability layers or the soil becomes saturated. Extended rainfall, compacted soil layers, or the formation of a hardened surface crust after hot, dry weather can all restrict infiltration and lead to surface runoff. These processes govern how much water is available to plants and influence evaporation, groundwater recharge, and local hydrology.

Figure 1: ESA Climate Change Initiative (CCI) global soil moisture maps from January 2018 (left) and July 2018 (right) (Image credit: ESA)

Measuring soil moisture is essential for understanding how water is partitioned between the land surface and the subsurface. Soil acts as a reservoir supplying water to plants, meaning its moisture content directly affects crop productivity and irrigation demand. Beyond farming, it determines how rainfall is partitioned between infiltration and runoff and shapes the movement of water through soils and into groundwater. Human activities such as deforestation, irrigation, and land conversion modify these pathways, altering how water is stored and transported through terrestrial systems. 1)

Traditionally, soil moisture has been measured directly by collecting and weighing soil samples or using in situ sensors that detect changes in the soil’s dielectric properties. Techniques such as time domain reflectometry (TDR) and time domain transmissometry (TDT) provide precise local measurements but are labour-intensive and spatially limited, motivating the development of satellite-based methods capable of observing soil moisture globally and continuously.

Figure 2: A soil moisture map derived from SMOS and MODIS data (Image credit: NOAA NIDIS)

Measuring Soil Moisture Remotely

Because in situ observations are sparse and provide only local information, satellite remote sensing plays a central role in monitoring soil moisture at regional to global scales. Satellites measure soil moisture by detecting microwave radiation that is either emitted naturally from the Earth’s surface (passive sensors) or reflected back toward the sensor (active sensors). The sensors rely on the fact that soil’s dielectric constant increases with soil moisture, increasing the microwave reflectivity and attenuation while decreasing emissivity.

This property means microwave frequencies, particularly those in the C-, X-, and L-bands, are commonly used for soil moisture retrieval, for both active and passive sensors. Instruments measuring in the C- and X-bands measure surface or “skin” soil moisture and are more sensitive to surface roughness, whereas L-band instruments can penetrate deeper into the soil, typically sensing moisture in the top 0–5 cm. Because of their lower frequency, L-band microwaves penetrate vegetation more effectively and retain strong sensitivity to soil moisture, making them particularly well suited for global soil moisture monitoring. 4)

Active Sensors

Active microwave sensors, including scatterometers and synthetic aperture radar (SAR), retrieve soil moisture by transmitting microwave pulses toward the Earth’s surface and measuring the strength of the backscattered signal. The magnitude of the returned signal depends on the soil’s dielectric properties, surface roughness, and vegetation cover, all of which are influenced by soil moisture conditions. 20)

Active sensors, particularly SAR, offer higher spatial resolution than passive radiometers. Both types of sensor operate independently of solar illumination, enabling day-and-night, all-weather observations. However, soil moisture retrievals from active measurements can be complicated by sensitivity to surface roughness and vegetation structure, requiring careful calibration and correction to isolate the soil moisture signal.

Figure 3: Active Sensor (Image credit: NASA)

Passive Sensors

Passive microwave sensors retrieve soil moisture by measuring the natural microwave emission from the land surface, expressed as brightness temperature. As soil moisture increases, microwave emissivity decreases, leading to a reduction in observed brightness temperature, particularly at lower frequencies such as L-band. 20)

Passive sensors provide a more direct and physically robust link to soil moisture and are well suited for generating long, consistent time series for climate and hydrological applications. Their main limitations are relatively coarse spatial resolution and sensitivity to radio-frequency interference and dense vegetation cover.

Figure 4: Passive Sensor (Image credit: NASA)

GNSS Reflectometry

Global Navigation Satellite System Reflectometry (GNSS-R) is an emerging technique that exploits signals transmitted by navigation satellites and reflected from the Earth’s surface. Variations in the reflected signal strength and coherence are related to surface reflectivity, which in turn is influenced by soil moisture, surface roughness, and vegetation.

GNSS-R offers high temporal sampling and cost-effective coverage using small satellite constellations, complementing traditional active and passive microwave observations. However, retrieval algorithms are still under development, and separating soil moisture effects from other surface characteristics remains an active area of research.

Figure 5: Hypothetical GNSS Reflectometry Setup (Image credit: Norwegian University of Science and Technology)

Example Products

SMAP Level-3 Soil Moisture

The SMAP Level-3 Soil Moisture product is derived from L-band radiometer observations acquired by NASA’s Soil Moisture Active Passive (SMAP) mission, using geophysical retrievals and spatial interpolation to estimate near-surface soil moisture on a global grid. The dataset provides daily global coverage from 31 March 2015 onwards and supports applications including agricultural monitoring, drought and flood assessment, land-surface initialization for weather and climate models, hydrology, and ecosystem studies. 6) 7) 8)

Table 1: Key features of SMAP Level-3 product
Parameter	Brightness temperature and soil moisture
Platform	SMAP
Sensor	SMAP L-band radiometer
Temporal coverage	31 March 2015 to present
Temporal resolution	1 day
Spatial resolution	9 km x 9 km
Spatial coverage	N: 90, S: -85.044, E: 180, W: -180

Figure 6: The variations of the soil moisture in summer (June - August) across the Tibetan Plateau, as measured by SMAP Level-3 product. The bottom chart shows mean soil moisture across longitudes. (Image credit: Li et al 2018)

SMOS Level-2 Soil Moisture Products

The SMOS Level-2 Soil Moisture products are derived from L-band brightness temperature observations acquired by the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS) instrument onboard the Soil Moisture and Ocean Salinity (SMOS) mission, using inversion algorithms to retrieve near-surface soil moisture at swath level. The datasets provide global coverage every one to three days at a spatial resolution of around 35–50 km and support applications in hydrology, agriculture, drought monitoring, and land–atmosphere process studies. 9) 10)

In addition to the standard Level-2 product, a Near-Real-Time (NRT) version is generated using a neural-network retrieval approach to deliver soil moisture estimates with latencies of less than three hours, enabling time-critical applications such as operational flood and drought monitoring and numerical weather prediction. 9) 10)

Figure 7: Comparison of SMOS’s near-real time soil moisture (NRT SM) (left) and Level 2 soil moisture (right) products for one orbit on 27 May 2012 (Image credit: ESA)

ESA CCI Soil Moisture

The ESA Climate Change Initiative (CCI) Soil Moisture project generates harmonised, long-term Climate Data Records of global soil moisture by merging observations from multiple active and passive microwave satellite sensors using consistent processing and cross-calibration approaches. Updated annually and spanning more than four decades, the initiative provides ACTIVE, PASSIVE, and COMBINED soil moisture products that support climate research, hydrological and drought analysis, and long-term studies of land–atmosphere interactions by offering a stable and consistent view of global soil moisture variability. 11) 12) 13)

Figure 8: Soil moisture anomalies during 2022 in ESA CCI SM. Data is not recorded from the grey areas due to dense vegetation. (Image credit: ESA)

Related Missions

SMAP

NASA’s Soil Moisture Active Passive (SMAP) mission was launched on 31 January 2015, with support from the Canadian Space Agency (CSA). SMAP measures the moisture content of the top soil layer and also determines whether the surface is frozen or thawed. The satellite carried two L-band instruments, an active radar and a passive radiometer. Following the radar failure in 2015, the mission has continued using the radiometer alone. SMAP provides global maps of soil moisture every two to three days using its radiometer. SMAP data are widely used for agricultural monitoring, drought assessment, and improving weather and hydrological forecasts.

SMOS

Launched on 2 November 2009, the Soil Moisture and Ocean Salinity (SMOS) mission is part of the European Space Agency’s (ESA) Earth Explorer programme. It was the first satellite to use L-band microwave measurements to observe global variations in soil moisture and sea surface salinity. SMOS carries the Microwave Imaging Radiometer using Aperture Synthesis (MIRAS), a passive microwave 2-D interferometric radiometer that detects changes in natural microwave emissions from Earth’s surface. These data improve understanding of the global water cycle and support advances in weather and climate modelling. 14) 15)

MetOp

The Meteorological Operational (MetOp) satellite series is a collaboration between ESA and EUMETSAT. MetOp-A, launched in 2006 and retired in 2021, was followed by MetOp-B launched in 2012, and MetOp-C in 2018. Each satellite carries 11 instruments, including the Advanced Scatterometer (ASCAT), which measures radar backscatter from the land and ocean surface. While primarily designed for ocean wind observations, ASCAT data are now accepted as a source for the derivation of global land products such as soil moisture. The EUMETSAT reprocessed soil moisture dataset covers the period from 2007 to 2018 and provides consistent long-term records for climate and hydrological studies. 16) 17) 18)

GCOM-W

Japan’s Global Change Observation Mission (GCOM), managed by the Japan Aerospace Exploration Agency (JAXA), aims to track long-term variations in Earth’s environment. The first satellite, GCOM-Water (GCOM-W), also known as “Shizuku,” was launched in May 2012, followed by the climate-focused GCOM-Climate (GCOM-C) in December 2017. The key instrument for soil moisture monitoring is the Advanced Microwave Scanning Radiometer-2 (AMSR2) carried on GCOM-W, which measures microwave emissions from the surface. GCOM-W produces near-real-time soil moisture data through its AMSR2 Unified Level-2B product, offering daily measurements of global surface soil moisture at a spatial resolution of 25 km.

HydroGNSS

HydroGNSS is an ESA Scout mission designed to demonstrate the use of Global Navigation Satellite System Reflectometry (GNSS-R) for measuring key hydrological climate variables. Launched in November 2025, the mission consists of two small satellites that collect data on soil moisture, wetland inundation, freeze–thaw states, and above-ground forest biomass. By capturing signals reflected from Earth’s surface, HydroGNSS provides cost-effective, frequent observations that complement existing microwave missions. 23)

FY-3

The FengYun-3 (FY-3) series is a constellation of polar-orbiting meteorological satellites operated jointly by the China Meteorological Administration (CMA) and the China National Space Administration (CNSA). Each satellite carries 12 instruments, among them the Microwave Radiation Imager (MWRI), which measures brightness temperatures across multiple frequencies. MWRI data are used to estimate global surface soil moisture by relating microwave emissivity to soil and vegetation properties. FY-3 provides daily and monthly soil moisture products that are routinely validated against SMAP and in situ observations, supporting weather forecasting and agricultural applications. 19)

Sentinel-1

Sentinel-1 is a constellation of C-band radar imaging satellites developed and operated by the European Space Agency (ESA) under the Copernicus Programme. The first satellite, Sentinel-1A, was launched in 2014, while Sentinel-1B ceased operations in August 2022. Sentinel-1C, launched in December 2024, continues the mission. Using synthetic aperture radar (SAR), Sentinel-1 provides high-resolution, all-weather, day-and-night observations of Earth’s surface. Its data are widely used to estimate soil moisture, particularly for agricultural monitoring, flood detection, and land-surface process studies, often in combination with optical and passive microwave data.

References

1) Waterscapes, “What is water irrigation and what purpose does it serve?,” URL: https://waterscapes.co.uk/blog/what-is-water-irrigation-and-what-purpose-does-it-serve

2) K Centre for Ecology & Hydrology, “COSMOS-UK Soil Moisture,” URL: https://cosmos.ceh.ac.uk/soilmoisture

3) European Space Agency (ESA), “Nearly four decades of soil moisture data now available,” URL: https://www.esa.int/Applications/Observing_the_Earth/Space_for_our_climate/Nearly_four_decades_of_soil_moisture_data_now_available

4) B. P. Mohanty et al., “Soil Moisture Remote Sensing: State-of-the-Science,” 2017, Vadose Zone Journal, URL: https://acsess.onlinelibrary.wiley.com/doi/10.2136/vzj2016.10.0105

5) University of Reading, “Soil moisture monitoring with satellite radar,” 26 July 2021, URL: https://blogs.reading.ac.uk/weather-and-climate-at-reading/2021/soil-moisture-monitoring-with-satellite-radar/

6) National Snow and Ice Data Center (NSIDC), “SMAP Enhanced L3 Radiometer Global and Polar Grid Daily 9 km EASE-Grid Soil Moisture, Version 5,” URL: https://nsidc.org/data/spl3smp_e/versions/5

7) NASA Earthdata, “Earth Observation Data Basics – Standard Data Products,” URL: https://www.earthdata.nasa.gov/learn/earth-observation-data-basics/standard-data-products

8) Li C., Lu H., Yang K., Han M., Wright J.S., Chen Y., Yu L., Xu S., Huang X., Gong W., “The Evaluation of SMAP Enhanced Soil Moisture Products Using High-Resolution Model Simulations and In-Situ Observations on the Tibetan Plateau,” Remote Sensing, 31 March 2018, URL: https://www.mdpi.com/2072-4292/10/4/535#

9) Copernicus Documentation, “Soil Moisture and Ocean Salinity (SMOS) – Data,” URL: https://documentation.dataspace.copernicus.eu/Data/ComplementaryData/SMOS.html

10) European Space Agency (ESA), “SMOS Data – ESA Earth Online,” URL: https://earth.esa.int/eogateway/missions/smos/data

11) European Space Agency (ESA), “Soil Moisture project – CCI,” URL: https://climate.esa.int/en/projects/soil-moisture/

12) TU Wien, “Soil Moisture CCI,” URL: https://www.tuwien.at/en/mg/geo/climers/research/soil-moisture/cci

13) A. Gruber et al., “Evolution of the ESA CCI Soil Moisture climate data records and their underlying merging methodology,” Earth Syst. Sci. Data, 11, 717-739, 2019, URL: https://essd.copernicus.org/articles/11/717/2019/

14) European Space Agency (ESA), “SMOS Data – ESA Earth Online,” URL: https://earth.esa.int/eogateway/missions/smos/data

15) European Space Agency (ESA), “MIRAS – Microwave Imaging Radiometer using Aperture Synthesis,” URL: https://earth.esa.int/eogateway/instruments/miras

16) EUMETSAT/OSI SAF, “ASCAT Soil Moisture at 25 km Swath Grid – MetOp,” URL: https://navigator.eumetsat.int/product/EO:EUM:DAT:METOP:SOMO25

17) European Space Agency (ESA), “About ASCAT – MetOp,” URL: https://www.esa.int/Applications/Observing_the_Earth/Meteorological_missions/MetOp/About_ASCAT

18) NASA Earthdata, “Advanced Microwave Scanning Radiometer 2 (AMSR2) Instrument Overview,” URL: https://www.earthdata.nasa.gov/data/instruments/amsr2

19) Japan Meteorological Agency (JMA), “Introduction of FengYun‑3/MWRI soil moisture product and its applications,” URL: https://www.data.jma.go.jp/mscweb/en/aomsuc12/presentation/S5_05.pdf

20) IT Sensing, “Types of Remote Sensing: Passive vs Active Sensors,” URL: https://www.itsensing.com/types-of-remote-sensing-passive-vs-active-sensors/