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Title:   Author  item
                                            Yilmaz, Mustafaitem
                                            Anderson, Martha
  Submitted to: Meeting Abstract
                                    Publication Type: Abstract Only
                                    Publication Acceptance Date: 5/20/2011
                                    Publication  Date: 7/7/2011
                                    Citation: Yilmaz, M.T., Anderson, M.C. 2011. ALEXI evapotranspiration estimate analysis over the Nile basin[abstract]. Managing Water under Climate Change in the Blue Nile Headwaters Workshop. 2011 CDROM.Interpretive Summary:
                                Technical Abstract: The ability to obtain skillful water budget cycling is essential to many hydrological and agricultural applications. Observations obtained from local meteorological and hydrological stations are very accurate; however, they suffer from the sparsely located nature of the stations. Furthermore, the availability of consistent observations is further limited over basins especially when the region of interest lay over multiple countries. This is particularly true for Nile basin, as exchange of information is further limited by the cross-border political issues. Hence, skillful remote sensing based water balance estimates are critical for these regions. Among the water budget closure elements, water storage change over long periods of time vanishes where the total amount of water left in the system as runoff is small relative to the other elements of the water budget. Hence, magnitude wise, precipitation and evapotranspiration are the two most important components of long term water budget calculations. There are many remotely sensed satellite based precipitation products that are obtained through the microwave based sensors. However, there are only limited remote sensing based evapotranspiration products available. Among them Atmosphere-Land Exchange Inverse (ALEXI) is a thermal-based remotely sensed daily evapotranspiration product with high spatial resolution (5km). Using geostationary satellite that provides frequent estimates of land surface temperature, ALEXI relates the clear sky morning temperature gradient to the sensible heat flux estimates where evapotranspiration is obtained as a residual from the energy balance closure. ALEXI has been extensively evaluated and tested over US and Europe through available dense ground networks. In this talk, we are going to discuss the ALEXI evapotranspiration estimates and its accuracy over the Nile region where the available ground observations are relatively limited. We will show that ALEXI evapotranspiration estimates over Nile Basin matches the model based water balance estimates and more skillful estimates than model simulations especially over regions where the amount of incoming water to the system is unknown. We will demonstrate that ALEXI is very critical to obtain closed consistent water balance estimates for the entire Nile region.


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