UT CE 397 - Hydrologic Impacts of Climate Change on the Nile River Basin
Course Ce 397-
Pages 46

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Hydrologic Impacts of Climate Change on the Nile River Basin: Implications of the 2007 IPCC Climate Scenarios Tazebe Beyene1, Dennis P. Lettenmaier1 and Pavel Kabat2 Abstract The potential impacts of climate change on the hydrology and water resources of the Nile River basin are assessed using a macroscale hydrology model driven by 21st century simulations of temperature and precipitation downscaled from runs of 11 General Circulation Models (GCMs) and two global emissions scenarios (A2 and B1) archived for the 2007 IPCC report. The results show that, averaged across the multimodel ensembles, the entire Nile basin will experience increases in precipitation early in the century (period I, 2010-2039), followed by decreases later in the century (periods II, 2040-2069 and III, 2070-2099) with the exception of the eastern-most Ethiopian highlands which is expected to experience increases in summer precipitation by 2080-2100. Summarized as spatial averages over the entire Nile basin, multimodel-average Nile basin precipitation changes as percentages of the historical period 1950-99 are 115 (117), 98 (104) and 93 (96), and temperature changes (as differences in ºC from 1950-99) are 1.5 (1.3), 3.2 (2.8) and 4.4 (3.6) for the global A2 (B1) emissions scenario. These changes in precipitation and temperature resulted in streamflows at High Aswan Dam (HAD) that are 111 (114), 92 (93), and 84 (87) percent of historical simulated streamflow (1950-1999) for periods I to III, respectively, for the global A2 (B1) emissions scenario. Implications of climate change on the water resources of the Nile River basin were analyzed by quantifying the annual hydropower production and irrigation water releases at High Aswan Dam, which generally would follow changes in streamflow, increasing early in the century to 112 (118) percent, but then decreasing to 92 (97) and 87 (91) percent in Periods I and III, respectively, for the A2 (B1) emissions scenario. 1Department of Civil and Environmental Engineering Box 352700, University of Washington, Seattle WA 98195 2ALTERRA Green World Research, Wageningen University and Research Centre, P.O. Box 47, 6700 AA Wageningen, The Netherlands2 1.0 Introduction The climate of Africa is both varied and varying. Varied, because climate ranges from humid equatorial to seasonally arid and sub-tropical Mediterranean and varying because all these climates exhibit differing degrees of temporal and spatial variability. At the sub-regional scale, Africa is vulnerable to ENSO and related extreme events (drought, floods, and changes in hydrologic patterns). That portion of sub-Saharan Africa that depends entirely on the Nile River for its water supply is particularly susceptible to hydrologic changes that might be associated with a warmer climate. Flooding and droughts will be increasingly difficult to cope with in the face of increasing pressures on water supplies due to rapid population growth and dwindling resources. The Nile River basin is home to 336 million of Africa’s 850 million people. It has experienced high population growth rates and the population of the basin is expected to double between 1995 and 2025. Virtually all population projections are for continuing growth, which in turn will increase demand for natural resources among the 10 Nile River riparian countries. The potential effects of climate change on the basin have been given less attention than population growth, notwithstanding several earlier studies (e.g. Yates et al., 1998) showing that the water resources of the basin are susceptible to climate change. The IPCC Fourth Assessment Report (AR4) has resulted in a wealth of General Circulation Model (GCM) runs that have been archived in a consistent manner at the Lawrence Livermore National Laboratory Program for Climate Model Diagnosis and Intercomparison (PCMDI). These model runs provide the basis for a much more coherent analysis of possible effects of climate change using multimodel ensemble techniques (e.g., Krishnamurti et al, 2000) than has previously been possible. For instance, Maurer et al (2006) evaluated implications of projected 21st century climate for California’s water resources using PCMDI-archived output from 9 IPCC/AR4 GCMs, and Christensen and Lettenmaier (2007) evaluated implications of IPCC/AR4 climate projections for Colorado River water resources from 11 GCMs and two global emissions scenarios archived at3 PCMDI. We follow the lead of these recent studies in using multimodel ensemble methods to evaluate the implications of 21st century climate change for the Nile River basin. Future changes and uncertainties in the allocation of Nile water resources may have significant effects on local and regional economies, agricultural production, energy availability, and environmental quality (NBI, 2001, Hulme et al., 2005, Conway et al., 1993, Yates et al., 1998). Water resource planning based on the concept of a stationary climate is increasingly considered inadequate for sustainable water resources management (Mohamed et al., 2005). In addition to natural variability, which is incorporated in existing water planning methods, new water projects will have to deal with uncertainty associated with population growth and trends in climate change. Therefore, understanding the uncertainty in projected climate change over the next century, which is attributable both to uncertainty in the future emissions pathway (related to policy decisions and public response) and uncertainties in model projections (due to differing sensitivities of the GCMs to perturbations in atmospheric composition), is essential to understanding how the economy of the Nile basin will evolve, including social and environmental impacts. Water resources planning studies which typically are conducted for time horizons of several decades now require consideration of ongoing global climate change and uncertainties in the signature of future climate change. The near certainty of increased future water demand in the Nile basin (notwithstanding uncertainty as to magnitude of demand increases) contrasts with the uncertainty of climatically-induced changes in the water supply of the Nile River basin (Conway et al., 1996, Yates et al., 1998(a), Strzepek et al., 1995, Strzepek et al., 2000),


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