Featured in the AGU Hydrology Section March 2024 Newsletter available HERE
Since the first use of hot-air balloons for military reconnaissance, remote sensing technology has come a long way in observing and recording datasets of Earth’s environment and ecosystems. Remote sensing has thus played an important role in advancing our understanding of key hydrological variables including precipitation, soil moisture and evapotranspiration (ET), with its ability of providing consistent, regional and long-term datasets and products. The needs and applications of these datasets govern the mission stakeholders, investors and end-users. For example, commercial products may address data gaps and solve business problems that are typically driven by stakeholders from the private sector to be used for revenue-driven applications. On the other hand, freely available andimpact-driven products contribute to saving, enhancing quality of life and reducing economic damage. Lastly, scientific, and research-driven products aim to advance Earth observation or its applications in specific domains. Collectively, these products can result from a mix of public and private investments, as well as philanthropic organizations, leading to a wide range of benefits. Specifically, all these types of products have been critical in addressing economic, social and environmental challenges. As technology advances, so does the quality of data, with spatial resolution emerging as a crucial focus, particularly in managing hydrological systems at different scales where the spatial variability of hydrological processes can be decisive.
Among these advances, precipitation monitoring has been enhanced by merging datasets from various sources. Currently, the Multi-Source Weighted-Ensemble Precipitation, version 2 - MSWEP V2, with global coverage, achieves high spatial and temporal resolutions of 0.1 degrees every 3 hours. MSWEP V2 integrates gauges, satellites and reanalysis, incorporating river discharge observations for robustness. Another example is the Climate Hazards Group InfraRed Precipitation with Station Data version 2 - CHIRPS, achieving 0.05-to-0.1-degree resolutions every 6 hours. CHIRPS integrates station measurements and satellite data with a novel blending procedure. While CHIRPS excels in spatial resolution, its reliance on station data limits coverage to land analysis between 50 degrees South and 50 degrees North. Fortunately, the Global Precipitation Measurement (GPM) can extend this coverage globally with its Integrated Multi-satellite Retrieval of GPM-IMERG product at a similar resolution.
Furthermore, the development of microwave sensor technology has been crucial for global monitoring of soil moisture. However, the size constraints of these instruments have historically limited their spatial resolution. Downscaling techniques emerged to overcome this limitation, with the most recent one derived from the Soil Moisture Active and Passive (SMAP) L-band radiometer and based on the thermal inertia theory. Combining the SMAP Enhanced L2 radiometer Half-Orbit 9 km EASE-Grid Soil Moisture product with land surface temperature from the Moderate Resolution Imaging Spectroradiometer (MODIS) has resulted in a global daily 1 km resolution surface soil moisture time series. Nevertheless, in April 2024, NISAR (NASA-ISRO SAR Mission) will launch and provide active microwave measurements and thus retrievals of soil moisture at <100m resolution. Concurrently, other recent technological advancements have enabled the integration of microwave sensors onto drone-mounted systems. The Soil Moisture Company, a collaborative effort between Black Swift Technologies, Weather Stream and the University of Colorado Center for Environmental Technology, has pioneered the development of L-band Differential Correlation Radiometer technology and a soil moisture retrieval algorithm. This technique offers high-resolution soil moisture data that can be adapted to various needs and specifications.
High-resolution insights into water-plant relationships have also been achieved, leading to a revolution in water management through initiatives like OpenET. It provides daily to annual ET estimates at 30 meters resolution using public satellite and weather data by combining inputs from Landsat, Sentinel-2, GOES and more. This precision empowers sustainable decision-making for user-defined areas, enhancing resource management efficiency. Another example takes place on the International Space Station, ECOSTRESS (ECOsystem Spaceborne Thermal Radiometer Experiment on Space Station) that marks a breakthrough in how we monitor environmental and agricultural health. Through its multispectral thermal infrared radiometer, it captures the Earth's surface across five spectral bands at an impressive ~70 m resolution, identifying plant water stress and the onset of droughts with unparalleled accuracy. These data offer essential insights into how the terrestrial biosphere reacts to alterations in water availability, impacts on the global carbon cycle and enhances water conservation and agricultural resilience.
All these advancements in spatial resolution within remote sensing methods enable improved detection and monitoring of hydrological challenges, leading to better understanding and management of water resources, as well as more effective disaster response strategies.
Written by members of the AGU Remote Sensing Technical Committee
Laura Almendra-Martin (University of Florida), Debasish Mishra (Texas A&M University), Deep Shah (Texas A&M University), Leah Kocian (Texas A&M University), Vinit Sehgal (Louisiana State University), Hatim Geli (New Mexico State University), Andrew Feldman (NASA GSFC), Huilin Gao (Texas A&M University), and Jasmeet Judge (University of Florida)