ABSTRACT
Near-surface wetness conditions change rapidly with the weather, which limits their usefulness as drought indicators. Deeper stores of water, including root-zone soil wetness and groundwater, portend longer-term weather trends and climate variations; thus, they are well suited for quantifying droughts. However, the existing in situ networks for monitoring these variables suffer from signi–cant discontinuities (short records and spatial undersampling), as well as the inherent human and mechanical errors associated with the soil moisture and groundwater observation. Remote sensing is a promising alternative, but standard remote sensors, which measure various wavelengths of light emitted or režected from Earth’s surface and atmosphere, can only directly detect wetness conditions within the –rst few centimeters of the land’s surface. Such sensors include the Advanced Microwave Scanning Radiometer-Earth Observing System (AMSR-E), C-band passive microwave measurement system on the National Aeronautic and Space Administration’s (NASA) Aqua satellite, and the combined active and passive L-band microwave system currently under development for NASA’s planned Soil Moisture Active Passive (SMAP) satellite mission. These instruments are sensitive to water as deep as the top 2 and 5 cm of the soil column, respectively, with the speci–c depth depending on vegetation cover. Thermal infrared (TIR) imaging has been used to infer water stored in the full root zone,
11.1 Introduction .................................................................................................. 261 11.2 Satellite Gravimetry...................................................................................... 262 11.3 Gravity Recovery and Climate Experiment ................................................. 263 11.4 Hydrological Science Enabled by GRACE ...................................................264 11.5 Unique and Challenging Aspects of GRACE Data ......................................265 11.6 Disaggregating and Downscaling GRACE Data ..........................................266 11.7 Drought Monitoring with GRACE ...............................................................268 11.8 Future Prospects ........................................................................................... 272 Acknowledgments .................................................................................................. 274 References .............................................................................................................. 274
with limitations: auxiliary information including soil texture is required, the TIR temperature versus soil water content curve becomes žat as wetness increases, and dense vegetation and cloud cover impede measurement. Numerical models of land surface hydrology are another potential solution, but the quality of output from such models is limited by errors in the input data and trade-offs between model realism and computational ef–ciency.
