Hydrological modelling is a valuable tool for researchers in geography and other disciplines for studying the processes governing impacts of climate change and urban development on water resources and for projecting potential ranges of impacts from scenarios of future change. Modelling is an inherently probabilistic exercise, with uncertainty amplified at each stage of the process, from scenario generation to issues of scale, to simulation of hydrological processes, to management impacts. At the basin scale, significant factors affecting hydrological impacts of climate change include latitude, topography, geology, and land use. Under scenarios of future climate change, many basins are likely to experience changes not only in their mean hydrology, but also in the frequency and magnitude of extreme hydrological events. Impacts of climate change on water quality are largely determined by hydrological changes and by the nature of pollutants as flushingor dilution-controlled. The most significant impact of urban development on water resources is an increase in overall surface runoff and the flashiness of the storm hydrograph. The increase in impervious surface area associated with urban development also contributes to degradation of water quality as a result of non-point source pollution. Modelling studies on the combined impacts of climate change and urban development have found that either change may be more significant, depending on scenario assumptions and basin characteristics, and that each type of change may amplify or ameliorate the effects of the other. Hydrological impacts of climate change and urban development are likely to significantly affect future water resource management.

Frost processes more than doubled erosion rates in unglaciated terrain during the Last Glacial Maximum.

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Northwest of the Unite4 States include earlier peak runoff, reduced summer flo~s, and increased winter flooding. An increase in impervious surfaces, accompanied by urban development, is known to d~crease infiltration and i~crease surface runoff.