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Title: Using USDA ALEXI evapotranspiration product to obtain skillful water budget estimations over Nile river basin

item Yilmaz, Mustafa
item Crow, Wade
item Anderson, Martha

Submitted to: BARC Poster Day
Publication Type: Abstract Only
Publication Acceptance Date: 3/23/2011
Publication Date: 4/27/2011
Citation: Yilmaz, M.T., Crow, W.T., Anderson, M.C. 2011. Using USDA ALEXI evapotranspiration product to obtain skillful water budget estimations over Nile river basin. BARC Poster Day, April 27, 2011. Beltsville, MD.

Interpretive Summary:

Technical Abstract: Accurate estimation of the water balance estimation is essential to many hydrological, agricultural, and meteorological applications, which use observation- or model-based water balance elements. Observations obtained from local meteorological and hydrological stations are very accurate; however, they usually do not conserve the water (input water is not equal to output water) and suffer from the sparsely located nature of these stations, which is pertinent especially for the developing countries. On the other hand, models provide consistent continuous spatial and temporal coverage; though, they are heavily dependent on the parameterization of the specific model that is used. Hence, to obtain consistent water budget information over areas with minimum available ground observations, we still heavily rely on the models. Majority of the hydrological models estimate evapotranspiration (ET; observationally least available but one of the most important elements of the water balance) using the information about the input water to the system (i.e. precipitation, ground water, irrigation, etc.). As a result, the accuracy of the obtained water balance elements heavily depend on the accuracy of these information; if poor, then the resulting water balance would be unskillful. However, majority of the hydrological models neglect to take into account the groundwater table or the irrigation effects. As a result, they produce inaccurate water balance estimates over areas with significant groundwater or irrigation contribution. Hence, ET estimates obtained independently from input water information is crucial and complementary to obtain accurate water balance information. Atmosphere-Land Exchange Inverse (ALEXI) is a remote sensing based modeled ET product of US Department of Agriculture (USDA). Using satellite estimates of land surface temperature, ALEXI relates the clear sky morning temperature gradient to the sensible heat flux estimates where high spatial resolution (5km) daily ET information is obtained as a residual from the energy balance closure. Hence, ALEXI estimates ET without using any input water information. In this study, we are evaluating the ALEXI model based ET over the Nile Delta. Given the nature of the sparsely located ground monitoring networks in this region, and the existence of dense irrigated agricultural fields and shallow groundwater table (especially in the upstream of the Nile Delta), ALEXI offers unique ET information over these areas, where a skillful water balance estimate through the current hydrological models may require ancillary data that are almost non-existent over these regions. This study would be useful in obtaining improved ET information to complement the water balance estimates.