Assessment of climate impacts on hydrology and geomorphology of semiarid headwater basins using a physically-based model.

Abstract

The response of watershed erosion rates to changes in climate is expected to be highly non-linear and thus demands for mechanistic approaches to improve our understanding of the underlying causes. In this study, the integrated geomorphic component tRIBS-Erosion of the physically-based, spatially distributed hydrological model, tRIBS, the TIN-based Real-time Integrated Basin Simulator, is used to analyze the sensitivity of small semi-arid headwater basins to projected climate conditions. Observed historic climate and downscaled realizations of general circulation models from CMIP3 inform the stochastic weather generator AWE-GEN (Advanced WEather GENerator), which is used to produce two climate ensembles representative of the past historic and future climate conditions for the Walnut Gulch Experimental Watershed (WGEW) area in the Southwest U.S. The former ensemble incorporates the stochastic variability of the observed climate, while the latter includes the stochastic variability and the uncertainty of CMIP3 multi-model climate change projections. The climate ensembles are used as forcing input to the hydrogeomorphic model that is applied to seven headwater basins of WGEW. The basin response in terms of runoff and sediment yield for climate ensembles representative of the historic past and future is simulated and probabilistic inferences on future changes in catchment runoff and sediment transport are drawn. The application of the model to multiple catchments also identifies the scaling relationship between specific sediment yield/runoff and drainage basin area. The study reveals that geomorphic differences among catchments influence the variability of sediment yield, as affected by possible future climates, much more as compared to runoff, which is instead strongly dominated by the climate forcing. Despite a large uncertainty inherent to climate change projections and imposed by the stochastic climate variability, the basin sediment yield is predicted to decrease in almost all of the considered cases despite a slight possibility of runoff increase. The study further highlights the importance of addressing watershed sediment dynamics within a stochastic framework that explores the wide range of responses corresponding to natural variability.