Ensemble Evaporation Predictions from Remote Sensing in the Nile Basin

Friday, 19 December 2014: 5:30 PM
Wim GM Bastiaanssen1,2, Rutger Hofste2, Gabriel B Senay3, Martha C. Anderson4, Albert van Dijk5, Henk Pelgrum6, Abdulkarim Hussin Seid7, Diego Miralles8, Bart van den Hurk9, Yoshihide Wada10, lisa-Maria Rebelo11 and Vladimir Smakhtin11, (1)UNESCO-IHE� Institute for Water Education, Integrated Water Management, Delft, Netherlands, (2)Delft University of Technology, Water management, Delft, Netherlands, (3)USGS EROS, Sioux Falls, SD, United States, (4)USDA ARS, Pendleton, OR, United States, (5)Australian National University, Canberra, Australia, (6)eLEAF, Spatial modelling, Wageningen, Netherlands, (7)Nile Basin Initiative, Decision Support Systems, Entebbe, Uganda, (8)Gent University, Hydrology, Gent, Belgium, (9)Royal Netherlands Meteorological Institute, Atmospheric sciences, De Bilt, Netherlands, (10)Utrecht University, Department of Physcial Geography, Utrecht, 3584, Netherlands, (11)International Water Management Institute Sri Lanka, EcoHydrology, Vientianne, Laos
Water scarcity is increasing globally and is most evident in arid zones. Most rainfall is evaporated and runoff coefficients of 5 to 10% are common in arid zone river basins. Evaporation is the most important hydrological process, not only because of its magnitude, but also because it can be managed and regulated by withdrawals, irrigation equipment, agricultural practices, land use changes and soil treatments. Hence, evaporation can be modified and the looming water crisis prompt us to think more careful on how water is consumed and the services and benefits we render on return in terms of agricultural production, ecosystem services, hydropower, leisure etc. Several lead research groups have developed global evaporation products, at least for the African continent. Most of these products have a pixel size varying between 1 to 3 km, and this is a reasonable tradeoff between what is technically preferred (evaporation by land use class) and what can be operationally inferred from the newest earth observation satellites (100 to 1000 m pixels with revisit time of 1 to 5 days). The evaporation variability from monthly SSEBop, ALEXI, CMRSET, NBI version of MOD16, GLEAM and LandSAF model outputs for the main land use classes of the Nile will be demonstrated for the period 2005 to 2012. For 2007, there is also an evaporation data set from ETLook available. The largest variabilities occur on irrigated land, open water bodies and flood plains. The evaporation predictions are compared against flux tower data, and the water balance of paired catchments in Ethiopia and Southern Sudan. It is proposed to use ensemble averages and spreads of actual evaporation values for applications in water management, rather than using one single value and one single model. Some first thoughts on ensemble averaging will be provided. Ensemble evaporation values will be applied in the Water Accounting Plus (WA+) system, being a new analytical framework for water resources assessment reporting that is under development by IWMI, FAO and UNESCO-IHE (www.wateraccounting.org).