USING REMOTE SENSING & MODELING FOR EVALUATING HYDROLOGIC FLUXES, STATES, & CONSTITUENT TRANSPORT PROCESSES WITHIN AGRICULTURAL LANDSCAPES
Title: Evaluation of a Drought Index Based on Thermal Remote Sensing of Evapotranspiration Over the Continental U.S.
Submitted to: Journal of Climate
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 15, 2010
Publication Date: April 5, 2011
Citation: Anderson, M.C., Hain, C., Wardlow, B., Pimstein, A., Mecikalski, J., Kustas, W.P. 2011. Evaluation of a drought index based on thermal remote sensing of evapotranspiration over the continental U.S. Journal of Climate. 24:2025-2044.
Interpretive Summary: Accurate and timely spatial information about drought onset, duration and impacts around the globe is critical to addressing issues of rapid response and food security. Drought indices based on measurements of precipitation are of limited utility in many parts of the world lacking dense ground-based rain gauge/radar networks, and therefore there is utility in developing indices based on satellite data that can be widely and robustly implemented using minimal ground-based information. This paper describes a new satellite drought index, using surface temperature obtained from thermal-band satellite imagery as a proxy for soil moisture. The Evaporative Stress Index (ESI) reflects temporal anomalies in the ratio of actual-to-potential evapotranspiration, and shows good spatial and temporal correlation with standard precipitation based drought indices over the continental U.S. Evapotranspiration estimates are generated using the ALEXI surface energy balance algorithm applied to thermal band imagery supplied by the Geostationary Operational Environmental Satellites (GOES). The ESI is generated in near-real time over the U.S. at 10-km spatial resolution. Work is underway to expand product generation to global coverage using international geostationary satellite datasets.
The reliability of standard meteorological drought indices based on measurements of precipitation is limited by the spatial distribution and quality of currently available rainfall data. Furthermore, they reflect only one component of the surface hydrologic cycle, and cannot readily capture non-precipitation based moisture inputs to the land-surface system (e.g., irrigation) that may temper drought impacts, or variable rates of water consumption across a landscape. This study assesses the value of a new drought index based on remote sensing of evapotranspiration (ET). The Evaporative Stress Index (ESI) quantifies anomalies in the ratio of actual to potential ET (PET) mapped using thermal-band imagery from geostationary satellites. The study investigates the behavior and response timescales of the ESI through a retrospective comparison with the Standardized Precipitation Indices and Palmer Drought Index suite, and with drought classifications recorded in the United States Drought Monitor (USDM) for the 2000-2009 growing seasons. Spatial and temporal correlation analyses suggest that the ESI performs similarly to short-term (up to 6 month) precipitation-based indices, but at higher spatial resolution and without requiring any precipitation data. Unique behavior is observed in the ESI where non-precipitation moisture inputs are likely to be important, for example in areas of intense irrigation or shallow water table. Normalization by PET serves to isolate the ET signal component due to soil moisture variability from that of the radiation load. This study suggests that the ESI is a useful complement to the current suite of drought indicators, with particular added value in parts of the world where rainfall data are sparse or unreliable.