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Title: Assessing satellite-based rainfall estimates in semi-arid watersheds using the USDA-ARS Walnut Gulch gauge network and TRMM-PR

Author
item AMITAI, E. - National Aeronautics And Space Administration (NASA)
item Unkrich, Carl
item Goodrich, David - Dave
item HABIB, E. - University Of Louis Pasteur
item THILL, B. - Chapman University

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2012
Publication Date: 10/24/2012
Citation: Amitai, E., Unkrich, C.L., Goodrich, D.C., Habib, E., Thill, B. 2012. Assessing satellite-based rainfall estimates in semi-arid watersheds using the USDA-ARS Walnut Gulch gauge network and TRMM-PR. Journal of Hydrometeorology. 13:1579-1588.

Interpretive Summary: Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. Detailed study of water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions. Rainfall estimates from National Weather Service (NWS) radar shown daily on popular news forecasts are now being used for water resource decisions and models. In addition, satellite based measurement from the Tropical Rainfall Measuring Mission (TRMM) are also being assessed for their ability to estimate rainfall rates. TRMM data has the potential provide estimates over large areas that are not blocked by mountains as is the ground-based NWS Radar in much of the western US. Rainfall observations from the USDA- ARS Walnut Gulch Experimental Watershed (WGEW) were used to assess the ability of TRMM satellite estimates for 25 rainy overpasses which occurred during 1999-2010. Preliminary results indicate a very good agreement between the TRMM and WGEW estimates of rainfall rates. This is an important finding as rainfall is critical in arid and semiarid regions and it not well measured over large parts of the globe.

Technical Abstract: The rain gauge network associated with the U.S. Department of Agriculture, Agricultural Research Service Walnut Gulch Experimental Watershed (WGEW) in southeastern Arizona provides a unique opportunity for direct comparisons of in-situ measurements and satellite-based instantaneous rain-rate estimates like those from the Tropical Rainfall Measuring Mission’s (TRMM) Precipitation Radar (PR). The WGEW network is the densest rain gauge network in the PR coverage area for watersheds greater than 10 km2. It consists of 88 weighing rain gauges within a 149-km2 area. On average, approximately 10 gauges can be found in each PR field-of-view (~5-km diameter). All gauges are very well synchronized. This allows generating very-high-temporal-resolution rain-rate fields, and obtaining accurate estimates of the area-average rain-rate for the entire watershed and for a single PR field-of-view. In this study, instantaneous rain-rate fields from the PR and the spatially interpolated gauge measurements (on a 100-m x 100-m grid, updated every 1-min) are compared for all TRMM overpasses in which the PR recorded rain within the WGEW boundaries (25 overpasses during 1999-2010). The results indicate very good agreement between the fields with low-bias values (<10%), and high-correlation coefficients especially for the near-nadir cases (CC >0.9). The correlation is high at overpass time, but the peak occurs several minutes after the overpass, which can be explained by the fact that it takes several minutes for the raindrops to reach the gauge from the time they are observed by the PR. The correlation improves with the new version of the TRMM algorithm (V7). The study includes assessment of the accuracy of the reference products.