Abstract
Abstract. The changing availability of freshwater resources is likely to be one of the most important consequences of projected 21st century climate change for both human and natural systems. However, substantial uncertainty remains regarding the precise impacts of climate change on water resources, due in part due to uncertainty in GCM projections of climate change. Here we explore the potential impacts of climate change on freshwater resources in a humid, tropical catchment (the River Mitano) in the Upper Nile Basin of Uganda. Uncertainty associated with GCM structure and climate sensitivity is explored, as well as parameter specification within hydrological models. These aims are achieved by running pattern-scaled output from seven GCMs through a semi-distributed hydrological model of the catchment (developed using SWAT). Importantly, use of pattern-scaled GCM output allows investigation of specific thresholds of global climate change including the purported 2 °C threshold of "dangerous" climate change. In-depth analysis of results based on the HadCM3 GCM climate scenarios shows that annual river discharge first increases, then declines with rising global mean air temperature. A coincidental shift from a bimodal to unimodal discharge regime also results from a projected reduction in baseflow (groundwater discharge). Both of these changes occur after a 4 °C rise in global mean air temperature. These results are, however, highly GCM dependent, in both the magnitude and direction of change. This dependence stems primarily from projected differences in GCM scenario precipitation rather than temperature. GCM-related uncertainty is far greater than that associated with climate sensitivity or hydrological model parameterisation.
Highlights
Historical (20th century) periods of wetter and drier conditions in the Upper Nile region have demonstrated the vulnerability of human and natural systems to changes in the availability of freshwater resources (Tate et al, 2004; Conway, 2005)
This paper addresses an urgent need for improved understanding of projected climate change impacts on water resources in the Upper Nile Basin, and the uncertainty associated with such projections of hydrological change
This analysis is achieved through hydrological modelling of the River Mitano, a catchment in southwestern Uganda that drains into Lake Edward
Summary
Historical (20th century) periods of wetter and drier conditions in the Upper Nile region have demonstrated the vulnerability of human and natural systems to changes in the availability of freshwater resources (Tate et al, 2004; Conway, 2005). This paper addresses an urgent need for improved understanding of projected climate change impacts on water resources in the Upper Nile Basin, and the uncertainty associated with such projections of hydrological change. This analysis is achieved through hydrological modelling of the River Mitano, a catchment in southwestern Uganda that drains into Lake Edward. Following calibration the hydrological model is forced with a range of climate change scenarios, designed to allow investigation of uncertainty between different GCMs and climate sensitivities This represents an advancement of previous research into climate change impacts on surface and sub-surface hydrology in the Mitano River Basin (Mileham et al, 2009), which did not directly consider uncertainty between different GCMs or climate sensitivities. The scenarios explored permit direct investigation of the impacts of specific (and policy relevant) thresholds of climate change on water resources including the hypothesised 2 ◦C threshold for so-called “dangerous” climate change
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