This paper presents the feasibility of estimating discharges from Roseires Reservoir (Sudan) for the period from 2002 to 2010 and Aswan High Dam/Lake Nasser (Egypt) for the periods 1999-2002 and 2005-2009 using satellite altimetry and imagery with limited in situ data. Discharges were computed using the water balance of the reservoirs. Rainfall and evaporation data were obtained from public domain data sources. In situ measurements of inflow and outflow (for validation) were obtained, as well. The other water balance components, such as the water level and surface area, for derivation of the change of storage volume were derived from satellite measurements. Water levels were obtained from Hydroweb for Roseires Reservoir and Hydroweb and Global Reservoir and Lake Monitor (GRLM) for Lake Nasser. Water surface areas were derived from Landsat TM/ETM+ images using the Normalized Difference Water Index (NDWI). The water volume variations were estimated by integrating the area-level relationship of each reservoir. For Roseires Reservoir, the water levels from Hydroweb agreed well with in situ water levels (RMSE = 0.92 m; R 2 = 0.96). Good agreement with in situ measurements were also obtained for estimated water volume (RMSE = 23%; R 2 = 0.94) and computed discharge (RMSE = 18%; R 2 = 0.98). The accuracy of the computed discharge was considered acceptable for typical reservoir operation applications. For Lake Nasser, OPEN ACCESSRemote Sens. 2014, 6 7523 the altimetry water levels also agreed well with in situ levels, both for Hydroweb (RMSE = 0.72 m; R 2 = 0.81) and GRLM (RMSE = 0.62 m; R 2 = 0.96) data. Similar agreements were also observed for the estimated water volumes (RMSE = 10%-15%). However, the estimated discharge from satellite data agreed poorly with observed discharge, Hydroweb (RMSE = 70%; R 2 = 0.09) and GRLM (RMSE = 139%; R 2 = 0.36). The error could be attributed to the high sensitivity of discharge to errors in storage volume because of the immense reservoir compared to inflow/outflow series. It may also be related to unaccounted spills into the Toshka Depression, overestimation of water inflow and errors in open water evaporation. Therefore, altimetry water levels and satellite imagery data can be used as a source of information for monitoring the operation of Roseires Reservoir with a fairly low uncertainty, while the errors of Lake Nasser are too large to allow for the monitoring of its operation.

The information of water level variations in lakes and reservoirs is essential for many applications such as water resources management. The conventional in-situ measurements of water levels based on gauge station are often rarely available or difficult to be shared to the public especially for lakes in the developing countries and transboundary basins. Satellite altimetry provides an alternative solution to obtaining water level variations in lakes. In this study, the laser altimetry product GLAS/ICESat L2 Global Land Surface Altimetry (ICESat-GLA14) data was used to derive water levels in a narrow long-shaped reservoir in the Nile Basin, i.e. Roseires Reservoir in Sudan. A total of 19 water levels were derived from ICESat-GLA14 during the period 2003-2009. Compared with the in-situ measurements, all 19 ICESat-derived water levels had a R 2 of 0.88 and RMSE of 138.80 cm. After excluding two water levels (occurred on June 21, 2004 and June 8, 2005) with very large standard deviation (> 1 m), other 17 ICESat-derived water levels were in excellent agreement with in-situ measurements with R 2 of 0.99 and RMSE of 16.96 cm. In practical situation, the high standard deviation can be considered as an indicator for unreliable water level which is better excluded for further application. Two problematic water levels showed high overestimation of the in-situ measurements, and they occurred at the lowest water level situations and during rainy season. The exact reason for such high overestimation needs to be investigated in the future study.Index Terms-ICESat, altimetry, lake and reservoir, water level, Nile Basin.