{"id":2172,"date":"2026-02-12T21:28:37","date_gmt":"2026-02-12T18:28:37","guid":{"rendered":"https:\/\/planeta21vek.ru\/eng\/?p=2172"},"modified":"2026-02-24T18:46:56","modified_gmt":"2026-02-24T15:46:56","slug":"satellites-in-earth-orbit-monitor-soil-moisture-water-availability-river-flow-and-the-global-water-cycle","status":"publish","type":"post","link":"https:\/\/planeta21vek.ru\/eng\/2026\/02\/12\/satellites-in-earth-orbit-monitor-soil-moisture-water-availability-river-flow-and-the-global-water-cycle\/","title":{"rendered":"Satellites in Earth orbit monitor soil moisture, water availability, river flow, and the global water cycle"},"content":{"rendered":"

The primary objective of the SMAP mission is to assess the moisture content of the surface soil layer and its state (frozen or thawed). Another pioneering reconnaissance mission of the European Space Agency, the HydroGNSS satellites, has become an important step in expanding global understanding of water availability and the impact of climate change on the global water cycle. The SWOT (Surface Water and Ocean Topography) satellite allows for the assessment of river discharge, defined as the volume of water flowing per second at a given point along the river.<\/span><\/p>\n

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A Decade of Global Monitoring of the Water Cycle: NASA’s Soil Moisture Active and Passive Monitoring Mission. Launched in 2015, NASA’s Soil Moisture Active Passive (SMAP) mission provides more than 10 years of global L-band radiometric observations. Measurements at low frequencies (1.4 GHz or 21 cm (8 inches) wavelength) provide information on the state of the earth’s surface under all weather conditions, regardless of solar radiation. The primary goal of the SMAP mission is to estimate the moisture content of the surface soil layer and its state (frozen or thawed), according to a report in the journal Proceedings of the Institute of Electrical and Electronics Engineers (IEEE).<\/span><\/strong><\/span><\/p>\n

On the earth’s surface, soil moisture mediates the water, energy, and carbon cycles. These three cycles are the primary factors determining regional climate and regulating ecosystem functioning.<\/span><\/p>\n

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\nAn artist’s rendering of the SMAP observatory, showing the radiometer and radar. NASA\/JPL\/Caltech<\/span><\/em><\/p>\n

The SMAP satellite’s primary instrument is the L-band radiometer, which collects precise data on the surface brightness temperature. The radiometer incorporates advanced hardware and software for detecting and suppressing radio frequency interference (RFI). The radiometer measures the vertical and horizontal polarization, as well as the third and fourth Stokes parameters (T3 and T4), of microwave radiation rising from Earth. The reflector boom and assembly, which includes a deployable 6-meter (20 ft) light-mesh reflector, rotate at 14.6 revolutions per minute, creating a 1,000-kilometer (621 mi) wide swath as the SMAP satellite completes a sun-synchronous orbit around Earth. This approach allows for planetary coverage in two to three days, with precise repetition every eight days. The radiometer instrument is calibrated monthly by pointing it at deep-sky objects.<\/span><\/p>\n

The original SMAP instrument design included a companion L-band radar that operated from April to early July 2015, acquiring co- and cross-polarized radar backscatter data with a spatial resolution of approximately 1 km (0.6 mi) every three days over land. This data collection revealed the dependence of L-band signals on soil moisture, vegetation water content, and freeze-thaw conditions. The radar transmitter failed on July 7, 2015. Shortly thereafter, in August 2015, the radar receiver channels were repurposed to record global navigation satellite system (GNSS) returns, making SMAP the first fully polarimetric GNSS scatterometer in space for land surface and cryosphere studies.<\/span><\/p>\n

A decade of soil moisture observations by the SMAP satellite has led to numerous scientific advances. The data have been used to quantify the interrelationships of the three major metabolic cycles (e.g., carbon, water, and energy) on land. They have also been used to improve drought assessment and flood forecasting, as well as to enhance the accuracy of numerical weather prediction (NWP) models. Furthermore, they are used to measure the amount of liquid water and the thickness of ice sheets, as well as sea surface salinity. The following subsections describe how SMAP data are being used for a variety of societal purposes.<\/span><\/p>\n

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\nObserved regulation of photosynthesis by water availability [left] and light availability [right]. Blue indicates more pronounced limitation. Photosynthetic rates for both maps were determined using solar-induced fluorescence (SIF) measurements (mW\/m\u00b2 nm sr) with the Tropospheric Ozone Monitoring Instrument (TROPOMI) on the European Union’s Copernicus Sentinel-5P mission. Water availability was determined using soil moisture (SM) measurements from the Soil Moisture Active Passive (SMAP) mission. Light availability was determined using photosynthetically active radiation (PAR) measurements with the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra and Aqua platforms. The resulting maps show the model slope (mW\/m\u00b2 \/ nm\/sr) of the estimated SIF-SM relationship under water deficit conditions [left] and the model slope (10\u207b\u00b3 \/nm\/sr) of the estimated SIF-PAR relationship under light deficit conditions [right]. Jonard et al (2022) in the journal Biogeosciences<\/span><\/em><\/p>\n

The primary scientific goal of the SMAP project is to develop observational benchmarks demonstrating the interrelationships between the water, energy, and carbon cycles on land. Soil moisture is a variable in the terrestrial branch of the water cycle. It couples the water cycle to the energy cycle by limiting the latent heat flux\u2014the change in energy during heat exchange when water undergoes a phase transition, such as evaporation at the land-atmosphere interface. Soil moisture also couples the water and carbon cycles, as evidenced by plant photosynthesis. SMAP’s global observations of soil moisture fields, combined with remote sensing of elements of the energy and carbon cycles, can reveal how these three cycles are linked in the real world, serving as a benchmark for weather and Earth system models.<\/span><\/p>\n

Photosynthesis is suppressed by both water scarcity and a lack of sufficient photosynthetically active radiation. Global maps show how soil moisture and light regulate photosynthesis. These reference observations can be used to assess how Earth system models relate to the three main metabolic cycles of the climate system.<\/span><\/p>\n

SMAP products are also widely used in applied sciences and disaster decision support systems. Soil moisture estimates based on SMAP observations provide important information for managing water-related disasters, such as monitoring agricultural drought (defined as a persistent soil moisture deficit) and flood volumes (defined as the landscape’s ability to absorb water during precipitation). The SMAP project creates a parallel, near-real-time data stream that is accessed by a number of federal and state agencies in decision support systems related to drought monitoring, food security, landscape flooding, and transportation capacity.<\/span><\/p>\n

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\nEffect of adding soil moisture data [SMAP data assimilation (DA) minus no DA] to a simulation of the Great Plains Low-Level Jet Stream (GPLLJ) using the NASA Unified Weather Forecasting Model (NU-WRF). Results show the average of 75 independent GPLLJ events. Plots correspond to the difference in wind speed with height (y-axis) and time (hours on the x-axis). Panels correspond to the jet entrance [left], the jet core [middle], and the jet exit [right]. Soil moisture data assimilation increases the intensity of the simulated GPLLJ. Dashed shading represents 99% statistical confidence. Ferguson (2020) in Monthly Weather Review.<\/span><\/em><\/p>\n

The improvements in numerical weather prediction, increased model accuracy, and reduced uncertainty in climate model forecasts achieved using SMAP are based on the assumption that solar energy contributes to weather and climate dynamics. Soil moisture has a strong influence on how available solar energy is distributed between components (e.g., sensible heat flux and latent heat flux) over land. This influence propagates through the atmospheric boundary layer and ultimately influences weather development.<\/span><\/p>\n

An example is the land surface processes that can influence the evolution of low-level jet streams (LLJs) over the US Great Plains. These jets drive mesoscale convective weather systems. Previous studies have shown that LLJs are sensitive to regional soil moisture gradients. Assimilation of SMAP soil moisture data improves forecasts of weakly synoptically driven or uncoupled LLJs compared to forecasts of coupled cyclone-driven LLJs. For example, a NASA Unified Weather Research and Forecasting model incorporating 75 NLTs at 9 km (5.6 mi) resolution with and without SMAP soil moisture data assimilation [SMAP Data Assimilation (DA) and No DA, respectively] shows how the mean absolute wind speed difference between SMAP DA and No DA increases approximately linearly over the course of the simulation, with maximum differences at 850 hPa (or mb) for the jet stream entry and core.<\/span><\/p>\n

The massive melting of the Greenland and Antarctic ice sheets is contributing to sea level rise, one of the most serious and immediate negative impacts of climate change. The rate of melting over the past few years has raised alarm worldwide and has affected countries with coastal populations. The cryospheric community has called for action, demanding the use of all available observing systems and models to monitor the patterns and rates of land ice melt.<\/span><\/p>\n

Surface melting influences ice cap mass loss in many ways: direct meltwater outflow from the Greenland Ice Sheet ablation zone, structural changes in the Greenland Ice Sheet infiltration zone, changes in meltwater retention and outflow boundaries, changes in the structure of Antarctic ice shelves, and destabilization of glacial outflow support mechanisms through various processes (e.g., hydraulic fracturing and calving). Long-term climate models and mass balance models rely on accurate representations of snow, firn, and ice formation processes to predict future sea levels.<\/span><\/p>\n

The SMAP L-band radiometer has a relatively long wavelength [21 cm (8 in)] compared to other Earth observation instruments. It allows measurements of the liquid water content (LWC) of ice sheets and shelves by receiving radiation from deep within the snow-firn-ice column. Relatively high LWC values \u200b\u200bonly partially absorb the radiation, making the measurement sensitive to variations in the liquid water abundance (LWA) throughout the column. Figure 4 shows the cumulative liquid water abundance for 2015\u20132023, calculated from SMAP measurements.<\/span><\/p>\n

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\nFigure 4. Total annual daily liquid water (DLW) from SMAP data for 2015\u20132023. The solid black line on each map represents the grid boundaries, and the gray color mask within the ice sheet indicates where melting has been detected, based on decreasing brightness temperature. Andreas Colliander\/Finnish Meteorological Institute.<\/span><\/em><\/p>\n

The SMAP L-band radiometer was also used to determine thin sea ice thickness [<0.5\u20131 m (<1.6\u20133.3 ft)] in both the Arctic and the Southern Ocean. Thin ice thickness data from the European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission were recalibrated for SMAP using the same fixed incidence angle. The data demonstrate good agreement and clear advantages of the combined dataset. Thin ice thickness data in the L-band are a useful complement to higher-resolution thicker ice profiles obtained from satellite altimeters (e.g., ESA’s CryoSat-2 and NASA’s Ice, Clouds and Land Elevation Satellite\u20132).<\/span><\/p>\n

The joint NASA\/SAC-D (Aquarius) project, which operated from 2011 to 2015, used the L-band Radiometer and L-band Scatterometer to create unprecedented monthly maps of global sea surface salinity at a resolution of 150 km (93 mi). The SMAP L-band radiometer not only expanded the sea surface salinity data since the end of Aquarius but also increased the spatial resolution and temporal frequency of these measurements thanks to its larger reflector and wider swath. This increased resolution and repeatability provide new and unprecedented insights into mixing processes and freshwater phenomena, coastal plume tracking, and other local oceanic features.<\/span><\/p>\n

L-band vegetation optical density (VOD), related to vegetation water content, was obtained simultaneously with soil moisture using SMAP’s dual-polarization brightness temperatures and is used to better understand global ecology. Water in aboveground plant tissue attenuates and thus depolarizes surface microwave radiation, and VOD quantifies this effect. SMAP can provide global VOD observations under all weather conditions with a temporal frequency of two to three days. Changes in VOD indicate either plant rehydration or growth. Ecologists benefit from these new ecosystem observations, which complement optical and near-infrared vegetation indices (e.g., leaf area index (LAI)) and have a higher temporal frequency, are unaffected by clouds, and do not saturate as quickly in dense vegetation.<\/span><\/p>\n

Examples of data use include deciphering conditions when vegetation absorbs soil water only for rehydration (i.e., an increase in VOD without a change in LAI) versus when plants grow (i.e., an increase in both VOD and LAI). The applications of VOD are expanding, and the environmental community views this product as a valuable additional source of information on the relationship between soil and plant water balance.<\/span><\/p>\n

There is currently no ground-based equivalent for this measurement method. Therefore, to fully validate this method as a new means of monitoring global ecology, field experiments using onboard instruments and ground-based sampling teams are necessary.<\/span><\/p>\n

The SMAP project collaborated with the NASA Earth Science Division’s Applied Sciences Program (now known as Earth Science in Action) and the disaster monitoring and prediction communities to integrate SMAP products into their operations before and after launch. In some operational applications requiring long-term data continuity, SMAP data continues to be used to assess current conditions and for research and development.<\/span><\/p>\n

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\nSoil moisture estimates based on SMAP data for California, Nevada, Arkansas, Mississippi, and Louisiana used by the U.S. Department of Agriculture’s (USDA) National Agricultural Statistics Service (NASS) in their weekly report for November 14\u201320, 2022. These state-specific data are available on the NASS website, linked in the text. NASS<\/span><\/em><\/p>\n

Prior to launch, the SMAP mission developed a program to explore and promote the applied and operational use of SMAP data in decision-making for the benefit of society. To achieve these goals, SMAP became the first NASA mission to establish a formal Applications Program and Early Adoption Program (EA), which ultimately became a mandatory requirement for all future NASA Earth observation satellite missions. The SMAP Early Adoption Program increases awareness of the mission’s products, expands the user community, enhances collaboration with potential users, improves understanding of SMAP data capabilities, and accelerates the dissemination and use of mission products after launch.<\/span><\/p>\n

A number of the project’s achievements were achieved primarily through active and ongoing engagement with environmental agencies and operational bodies working in the national interest. SMAP soil moisture data are used by the U.S. Department of Agriculture (USDA) to estimate crop yields both domestically and internationally. For example, the USDA’s National Agricultural Statistics Service (NASS) conducts a weekly survey of crop condition, crop status, and soil moisture on U.S. cropland. NASS conducts surveys and publishes state-level soil moisture data in the NASS Crop Progress Report.<\/span><\/p>\n

Traditional field soil moisture surveys are extensive, labor-intensive data collection processes, largely relying on responses from farmers, agricultural extension specialists, and\/or other subject matter experts who provide field observations. One of the limitations of these observations is that they rely on subjective assessments rather than quantitative metrics and can lead to spatial inconsistencies depending on the responses of individuals in the relevant counties. Furthermore, NASS crop progress reports do not include specific information on the geographic location of assessed soil moisture conditions, which is extremely useful metadata for data users. NASS has integrated the use of SMAP observations into its weekly reports during the growing season (March\u2013November).<\/span><\/p>\n

Although SMAP operates in the protected 1400\u20131427 MHz frequency range, the radiometer has been exposed to radio frequency interference throughout its operational life. Since its launch, SMAP measurements have included unauthorized transmitters operating within the passband, as well as out-of-band emissions from transmitters operating near the allocated spectrum. Previously launched SMOS and Aquarius radiometers have demonstrated the presence of global radio frequency interference in the L-band. Consequently, SMAP’s design included a new onboard digital detector to detect and filter out radio frequency interference. The radiometer generates scientific data in both time and frequency, enabling the use of several RF interference detection methods in data processing software on the ground.<\/span><\/p>\n

On-orbit data show that radio frequency interference (RFI) detection and filtering are working well and improving the quality of SMAP brightness temperature measurements. The algorithms are most effective at filtering RFI that is sparse in time and frequency, with minimal impact on the noise equivalent delta temperature (NEDT), a measure of the radiometer’s sensitivity. Some areas of the globe remain problematic, as very high and persistent RFI results in significant data loss due to the removal of contaminated data. The global RFI detection rate map for January 2025 shows a large contrast between the Eastern and Western Hemispheres, as well as between the Northern and Southern Hemispheres. Areas of isolated and strong RFI correspond to populated areas. A detection rate of 100% means that all pixels are marked and removed, resulting in data loss. Analysis of spectral information suggests that many sources are likely ground-based radar systems; However, many high-level broadband sources and low-level non-radar sources also remain. In areas of geopolitical conflict, time-frequency analysis data reveal the presence of interference covering the entire bandwidth of the radiometer receiver.<\/span><\/p>\n

The issue of radio frequency interference is further addressed through official spectrum management channels and official reports, which include geolocation coordinates of sources, interference levels, their frequency of occurrence during the observed period, and spectral information\u2014all of which assist field agents in identifying potential violators. The reports are sent to NASA’s Spectrum Management Office and then forwarded to the country of interest through the Satellite Interference Reporting and Resolution System.<\/span><\/p>\n

The current SMAP science dataset is presented in the table. The main data are grouped into four levels, designated L1\u20134. L1 data represent the instrument’s L-band brightness temperature in Kelvin and include all four Stokes parameters (i.e., horizontal and vertical polarization, as well as the third and fourth Stokes parameters). The data include both the 6:00 AM equatorial crossing (descending range) and the 6:00 PM equatorial crossing (ascending range). The user has access to quality flags indicating the conditions under which measurements are available for each project. L1B data are organized by time and include measurements in the longitudinal and transverse directions. L1C data are presented on the Equal-Area Scalable Earth V2 (EASE2) grid with polar and global projections. L2 data are geophysical data (i.e., soil moisture, VOD, and binary freeze\/thaw classification on a fixed Earth grid). L2 data, acquired over half an orbit, are publicly available within 24 hours of acquisition. L3 products are daily summaries and include all half-orbits for that day.<\/span><\/p>\n\n

Table: The Active Passive Soil Moisture Science Product Suite is available through the National Snow and Ice Data Center, one of NASA’s Distributed Active Archiving Centers.<\/span><\/p>\n

\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n
Product type<\/span><\/td>\nProduct Description<\/span><\/td>\nResolution (grid display)<\/span><\/td>\nGranule size<\/span><\/td>\n<\/tr>\n
SPL1BTB<\/span><\/td>\nGeolocation data, calibrated brightness temperature in chronological order.<\/span><\/td>\n36 km<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL1CTB_E<\/span><\/td>\nBackus-Gilbert interpolated, calibrated brightness temperature in chronological order<\/span><\/td>\n(9 km)<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL1CTB<\/span><\/td>\nGeolocated, calibrated brightness temperature on the Equal-Area Scalable Earth V2 (EASE2) grid.<\/span><\/td>\n36 km<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL1CTB_E<\/span><\/td>\nInterpolated and calibrated Backus-Gilbert brightness temperature on the EASE2 grid<\/span><\/td>\n(9 km)<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL2SMP<\/span><\/td>\nSoil moisture and vegetation optical depth radiometer<\/span><\/td>\n36 km<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL2SMP_E<\/span><\/td>\nSoil moisture and vegetation optical density measured by a radiometer based on the SPL1CTB.<\/span><\/td>\n(9 km)<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL2SMAP_S<\/span><\/td>\nCopernicus Sentinel-1 SMAP\/Soil Moisture Radiometer<\/span><\/td>\n3 km<\/span><\/td>\nSentinel-1<\/span><\/td>\n<\/tr>\n
SPL3SMP<\/span><\/td>\nDaily data from the global composite radiometer of soil moisture and vegetation optical depth based on SPL1CTB<\/span><\/td>\n36 km<\/span><\/td>\nDaily\u2013Global<\/span><\/td>\n<\/tr>\n
SPL3SMP_E<\/span><\/td>\nDaily data from the global composite radiometer of soil moisture and vegetation optical depth based on SPL1CTB_E<\/span><\/td>\n(9 km)<\/span><\/td>\nDaily\u2013Global<\/span><\/td>\n<\/tr>\n
SPL3FTP<\/span><\/td>\nDaily freeze\/thaw summary based on SPL1CTB<\/span><\/td>\n36 km<\/span><\/td>\nDaily\u2013Global<\/span><\/td>\n<\/tr>\n
SPL3FTP_E<\/span><\/td>\nDaily freeze\/thaw summary based on SPL1CTB_E<\/span><\/td>\n(9 km)<\/span><\/td>\nDaily\u2013Global<\/span><\/td>\n<\/tr>\n
SPL4SMAU<\/span><\/td>\nSoil moisture in the surface layer and root zone<\/span><\/td>\n9 km<\/span><\/td>\n3 hours – Global<\/span><\/td>\n<\/tr>\n
SPL4CMDL<\/span><\/td>\nNet ecosystem carbon exchange<\/span><\/td>\n9 km<\/span><\/td>\nDaily\u2013Global<\/span><\/td>\n<\/tr>\n
SPL1BTB_NRT<\/span><\/td>\nAccurate real-time location, chronological brightness temperature calibration.<\/span><\/td>\n36 km<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
SPL2SMP_NRT<\/span><\/td>\nSoil moisture as measured by a near-real-time radiometer<\/span><\/td>\n36 km<\/span><\/td>\nHalf-orbit<\/span><\/td>\n<\/tr>\n
L2\/L3 SMOS SM<\/span><\/td>\nSoil moisture and VOD in SMOS based on SMAP algorithms<\/span><\/td>\n(9 km)<\/span><\/td>\nSemi-Orbital\/Daily Global<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

 <\/p>\n<\/div>\n

The SMAP project also produces L4 data, which is the result of data assimilation. L4 products use other environmental observations, such as precipitation, air temperature and humidity, land-surface radiative fluxes, and additional land-use and soil texture information, to derive soil moisture estimates at the surface (nominally 0\u20135 cm (0\u20132 in)) and in the subsurface (e.g., from the root zone to one meter). The data assimilation system is a combination of the model and measurements and, therefore, allows for the determination of the diurnal cycle of land surface conditions. The data assimilation system also provides estimates of surface carbon, energy, and water fluxes, such as evaporation, runoff, gross primary productivity (GPP), and respiration. The difference between GPP and respiration represents the net ecosystem exchange, which is the net source\/sink of the carbon cycle on land.<\/span><\/p>\n

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\nPercentage of pixels on a 0.25\u00b0 grid for January 2025 flagged for removal by active and passive soil moisture mode RFI detection algorithms. Priscilla N. Mohammed\/GSFC<\/span><\/em><\/p>\n

The SMAP product suite also includes near-real-time (NRT) brightness temperature and soil moisture data for use in operational weather forecasting applications. NRT is designed to deliver data to users within three hours of measurement acquisition. NRT utilizes SMAP’s predicted antenna orientation (instead of telemetry) and model-predicted auxiliary data (soil temperature) to support operational centers that require more than three hours of data to update weather forecast models. SMAP currently meets the required and targeted latency requirements for NRT.<\/span><\/p>\n

Two other data processing projects combine synergistic data with other (co-located) satellite measurements. The SPL2SMAP_S project combines SMAP L-band radio radiance measurements with C-band synthetic aperture radar (SAR) measurements from ESA’s Copernicus Sentinel-1 mission. The SAR data have high resolution and enable the production of combined estimates of surface soil moisture at resolutions of 1 and 3 km (0.62 and 1.8 mi). However, high-resolution soil moisture information is only available with the combined SMAP and Sentinel-1 measurements. The update rate for this product is limited and can be up to 12 days.<\/span><\/p>\n

The SMOS\u2013SMAP combined passive L-band radiometry data provides global, near-daily estimates of surface soil moisture, essential for modeling rapid hydrological processes such as gravity drainage and recharge flow. These parameters are only partially determined by SMAP, with data updates occurring every two to three days. This product interpolates SMOS multi-angle data to an SMAP dip angle of 40\u00ba and utilizes all SMAP algorithms, including correction for the influence of water bodies on SMAP brightness temperature, as well as ancillary data for geophysical inversions of soil moisture and VOD, ensuring consistency. The SMAP\u2013SMOS combined data product may not be available daily in some regions, such as Japan, parts of China, and the Middle East, where radio frequency interference affects data collection.<\/span><\/p>\n

Although the SMAP radar failed shortly after launch, the collected data were used to develop an active-passive (AP) algorithm for SMAP, which will be applied to combined SMAP radiometer data and radar data from the NASA\u2013Indian Space Research Organization (ISRO) Synthetic Aperture Radar (NISAR) mission\u2014a recently launched L-band synthetic aperture radar mission designed to obtain global soil moisture data at a spatial resolution of 1 kilometer (0.62 mi) or better. The high-resolution product could facilitate the development of SMAP data applications (e.g., monitoring agricultural productivity, forest fires, and landslides).<\/span><\/p>\n

An upcoming mission meets some, but not all, of SMAP’s measurement requirements and desired improvements. The European Union’s Copernicus Microwave Radiometer (CIMR) mission is a proposed multi-channel microwave radiometry observatory, including an L-band and four other microwave channels, using a large grid reflector. The grid reflector is similar to that used on SMAP, but larger. SMAP’s successful demonstration of rotating, large, deployable grid antennas for Earth observations proved useful for the CIMR design.<\/span><\/p>\n

In terms of RF interference detection capabilities, CIMR will also use an approach similar to SMAP. With respect to instrument thermal noise (NEDT) and data latency, CIMR meets or approaches the desired performance of the next mission and is equal to or better than SMAP in most parameters. CIMR’s native L-band resolution is ~60 km (37 mi); however, the measurements are coincident, and the higher-resolution measurements in this configuration allow for the reconstruction of L-band radiometry with higher resolution than CIMR’s L-band. It may be possible to combine the L- and C-bands to obtain an L-band reconstruction product with a resolution of ~15 km (9 mi) based on coincident and overlapping measurements. Due to CIMR’s wide swath, a data update rate of one day is possible.<\/span><\/p>\n\n

The SMAP mission was launched in 2015 and has collected scientific data for over 10 years. Thanks to the unique instrument characteristics and operating conditions, the SMAP observatory’s global low-frequency microwave radiometry has yielded estimates of surface soil moisture, vegetation optical density, and freeze-thaw conditions that surpass the results of previous and ongoing studies. These data are widely used in Earth system science and are also applied in natural disaster research.<\/span><\/p>\n

The Earth system science and applications communities are actively using high-quality global L-band radiometry data collected over a decade. The intensity and diversity of scientific use of SMAP data is evident in the number of publications in peer-reviewed journals that contain SMAP or Soil Moisture Active Passive in the title or abstract and that utilize SMAP data (e.g., search: www.webofscience.com database). The authors acknowledge that many publications avoid this search approach. The bibliography currently includes over 1,700 records and over 20,000 citations, covering several elements of Earth system science, including hydrology and the regional and global water cycle, oceanography and atmospheric sciences, the cryosphere, global ecology, and microwave remote sensing technologies.<\/span><\/p>\n

The OpenET program, a public-private collaboration led by consortium partners NASA, the U.S. Geological Survey, California State University, Monterey Bay, the Environmental Defense Fund, the Desert Research Institute, Google Earth Engine, and HabitatSeven, provides information on evapotranspiration.<\/span><\/strong><\/span><\/p>\n

Evapotranspiration is the movement of water from the soil to the atmosphere through plants and direct evaporation from the earth’s surface. OpenET serves as an essential tool for water resource management, irrigation planning, drought monitoring, and fire risk assessment.<\/span><\/p>\n

The OpenET Data Explorer tool was initially launched in 2021, with data available for 17 western US states. The Farm and Ranch Management Support (FARMS) tool, launched in early 2025, provides an easy-to-use interface designed for individual farmers and livestock producers. At the time of FARMS’s release, data was available for 27 states. On December 15, 2025, data became available for 48 states across all tools.<\/span><\/p>\n

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\nThe continental US map features free OpenET water data, now covering the 48 states instead of the westernmost 27. The color scale displays total annual evapotranspiration\u2014the amount of water lost to the atmosphere through vegetation and evaporation\u2014from orange (low) to dark blue (high). OpenET<\/span><\/em><\/p>\n

Application areas of OpenET include:<\/span><\/p>\n

Water Use Reporting: California’s Central and South Delta Water Agencies use OpenET to support water use reporting to the state, resulting in increased reporting by landowners and significant cost savings for farmers.<\/span><\/p>\n

Irrigation Management: From individual landowners to large companies including Gallo and Sun Pacific Farming, OpenET helps farmers of all sizes track plant water use to optimize irrigation, maintain or increase yields, and reduce costs.<\/span><\/p>\n

Groundwater Accounting and Management: Water agencies in California and Nebraska are using OpenET, an open-source groundwater accounting platform developed by the Environmental Defense Fund and its partners, to help farmers track and manage their water resources. Agencies in Oregon and Kansas are developing pilot projects using this accounting platform.<\/span><\/p>\n

On 28 November 2025, the European Space Agency’s first reconnaissance mission, the HydroGNSS satellite, launched, marking a major step in expanding global understanding of water availability and the impact of climate change on the global water cycle.<\/span><\/strong><\/span><\/p>\n

Two HydroGNSS satellites were launched into orbit at 19:44 CET aboard a SpaceX Falcon 9 rocket as part of the Transporter-15 mission from the Vandenberg Space Center in California. Both satellites will use innovative technology called Global Navigation Satellite System (GNSS) reflectometry to literally “find water,” according to the ESA press service.<\/span><\/p>\n

This process involves the reception of L-band signals from navigation systems such as GPS and Galileo by HydroGNSS satellites. These navigation satellites transmit L-band microwave signals, which are modified when reflected off the Earth’s surface. HydroGNSS satellites will compare these reflected signals with the signals they receive directly from GNSS satellites to obtain valuable information about properties related to the water cycle and much more.<\/span><\/p>\n

To achieve this, each HydroGNSS satellite is equipped with a receiver for constructing delay and Doppler shift maps. It consists of two antennas: a zenith antenna, which tracks direct GNSS signals, and a nadir antenna, which collects reflected signals and processes them to construct delay and Doppler shift maps. Using this technology, two small satellites orbiting the Earth at 180 degrees apart will measure soil moisture, freeze-thaw conditions, flooding, and aboveground biomass.<\/span><\/p>\n

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\nHydroGNSS satellite: searching for water. ESA<\/span><\/em><\/p>\n

These data will be crucial not only for deepening our understanding of the Earth’s water cycle, but also for supporting applications such as flood forecasting and agricultural planning.<\/span><\/p>\n

Additionally, by monitoring flooding and wetland areas, HydroGNSS will help identify wetlands\u2014important ecosystems that can be significant sources of methane\u2014often hidden under tree canopies.<\/span><\/p>\n

Information on freeze-thaw states will provide insight into the balance of surface radiation, energy and carbon exchange with the atmosphere, and the behavior of subsurface permafrost at high latitudes.<\/span><\/p>\n

Meanwhile, data on aboveground biomass will help assess forest carbon stocks and their role in the global carbon cycle.<\/span><\/p>\n

Inspired by the spirit of “New Space,” the scouts champion flexibility and innovation, using small, intelligent satellites to scale down proven technologies or test bold new ways to observe our planet. Each mission goes from concept to launch in just three years, with a modest budget of \u20ac35 million, covering all stages\u2014from design and construction to orbital operations.<\/span><\/p>\n

ESA’s Director of Earth Observation Programmes, Simonetta Celi, said: “The launch of HydroGNSS, ESA’s first Scout mission, marks a major milestone for this new family of fast, low-cost Earth observation missions, and we extend our thanks to the mission’s prime contractor, SSTL\u2026 The launch also marks an important step in the development of our FutureEO programme, where Scouts embody a fast, flexible, innovative and cost-effective approach, complementing our larger Earth Explorer missions\u2026 We now look forward to how HydroGNSS will use GNSS reflectometry to provide valuable information on key hydrological variables that shape Earth’s water cycle.”<\/span><\/p>\n