Simulation of present and future discharges at the Nile River upstream Lake Nasser

The objective of this study is to simulate present and future discharges at the Nile River upstream Lake Nasser. For this purpose a rainfall-runoff model is integrated in an existing model which describes the water distribution in the upper Nile; RIBASIM-NILE. The latter is a result of the Lake Nasser Flood and Drought Control / Integration of Climate Change uncertainty and flood risk (LNFDC/ICC) project and described in (MWRI/Deltares, 2009a). In the LNFDC/ICC project, RIBASIMNILE is used to describe effects of developments upstream the High Aswan Dam on Nile discharges, with the focus on Lake Nasser inflows. For the purpose of this study the HBV rainfall-runoff model (Bergstrom & Forsman, 1973) is used Where RIBASIM-NILE is forced by sub-catchment runoff. The combination of the two models is referred to as the Nile Hydrological Simulation Model (NHSM). To assess the performance of the NHSM in simulating discharges under the current climate it is forced by observed rainfall and potential evapotranspiration for the years 1961 till 2000. The first 20 years are used for model calibration and the second 20 years for model validation. To simulate future discharges, NHSM meteorological forcing is derived from simulated series by three Global Circulation Models (GCMs) under two SRES climate emission scenarios (see Nakicenovic & Swart (2000)). Bias from GCM simulations is removed by a correction of derived rainfall and evapotranspiration to the observed 1961-1990 mean monthly climatology. Thereafter, the performance of NHSM-GCM combinations in simulating the current climate is assessed by comparison of simulated and observed mean monthly discharges and interannual variability. Finally, NHSM-GCM simulations under the two SRES scenarios are used to describe the 2065 and 2100 hydro-climates. Results of NHSM calibration and validation are satisfying on the scale of main tributaries; The White Nile, draining the Great Lakes district and the Blue Nile and Atbara River draining the Ethiopian Highlands. The hydrographs of observed and simulated discharges show a good overall agreement and long term volume errors fall between acceptable limits. However, especially on sub-catchments belonging to the White Nile performance is considerably lower ranging from a poor representation of discharge peaks to a structural underestimation of discharges. Although some poor results are related to errors in model forcing others are related to the NHSM. It is presumed that performance will increase when improvements are made in the description of rainfall-runoff processes in HBV and the representation of lakes and swamps in RIBASIM-NILE. The performance of NHSM-GCM in simulating 1961-1990 discharges is low. Uncorrected meteorological forcing derived from GCM simulations shows a high bias compared to the observed climatology. After the bias correction the spatio-temporal representation of observed meteorological forcing is insufficient, especially for rainfall. This is revealed when actual evapotranspiration simulations by NHSM with observed and simulated meteorological forcing are compared. Actual evapotranspiration is a function of bias corrected potential evapotranspiration and the state of the NHSM (soil moisture storage, lake levels, etc.). Differences in states of the NHSM forced by observed and simulated forcing are caused by remaining differences in the spatio-temporal domain of bias corrected rainfall. Where actual evapotranspiration differs, so will the amount of rainfall being available as runoff and river discharge. Simulation of present and future discharges at the Nile River upstream Lake Nasser Summary IV NHSM-GCM simulations for the 2065 and 2100 hydro-climates result in a high degree of randomness. Therefore, uncertainty in trends regarding the future climate and discharges is high. Though, literature agrees on the high uncertainty in predicting the future Nile climate, some peculiar results cannot be explained. More detailed studies to the performance of the used GCMs in representing the local climate are required to give conclusive answers. For now, uncertainty in the simulated future hydro-climates is too high for being useful in water resources management. In conclusion on the objective to this research it is found that NHSM performance with observed meteorological forcing at the scale of main tributaries is satisfactory. For sufficient performance on sub-catchment scale, improvements on NHSM simulations of White Nile basin have to be made. Regarding the simulation of future discharges, an attempt is made to provide representative future meteorological forcing series for the NHSM. Discharges simulated by NHSM-GCM combinations show to much randomness and uncertainty in trends to have sufficient predictive value. The methods by which representative future meteorological forcing is derived are to be improved to increase predictive value within satisfactory limits. Further research is recommended to improve the NHSM performance on sub-catchment scale. NHSM performance on sub-catchment scale can be improved. This can be achieved by (1) improving the quality of meteorological forcing, (2) changing the distribution of HBV, (3) improving the representation of river-lake dynamics in RIBASIM-NILE and (4) an integrated calibration of HBV and RIBASIM-NILE. Future meteorological forcing series with a lower level of uncertainty can be derived by improving the method by which GCM simulations are directly used as meteorological. When results are still unsatisfying, statistics of observed forcing can be manipulated with trends derived from GCM simulations to achieve forcing representing a future period. Beyond the scope of this research a recommendation is made to use the NHSM in assessing climate change in combination with other socio-economic and river developments in the Nile basin. Furthermore RIBASIM-NILE can be used to assess the impacts of climate change in relation to mitigating river basin management strategies.