Evaluation of Coupled Model Intercomparison Project Phase 5 historical simulations in the Colorado River basin

Abstract

The Colorado River basin (CRB) is the primary source of water in the southwestern United States. A key step to reduce the uncertainty of future streamflow projections in the CRB is to evaluate the performance of historical simulations of general circulation models (GCMs). In this study, this challenge is addressed by evaluating the ability of 19 GCMs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) and four nested regional climate models (RCMs) in reproducing the statistical properties of the hydrologic cycle and temperature in the CRB. To capture the transition from snow‐dominated to semi‐arid regions, analyses are conducted by spatially averaging the climate variables in four nested sub‐basins. Most models overestimate the mean annual precipitation (P) and underestimate the mean annual temperature (T) at all locations (up to +140% and −4.9 °C, respectively). A group of models capture the mean annual run‐off at all sub‐basins with different strengths of the hydrological cycle, depending on the level of P overestimation. Another set of models overestimate the mean annual run‐off, due to a weak cycle in the evaporation channel. An abrupt increase in the mean annual T of ~0.8 °C is detected at all locations around 1980 from the observed and most of the simulated time series. However, no statistically significant monotonic trends emerge for both P and T. All models simulate the seasonality of T quite well. The phasing of the seasonal cycle of P is reproduced fairly well in one of the upper, snow‐dominated sub‐basins. Model performances degrade in the larger sub‐basins that include semi‐arid areas, because several GCMs are not able to capture the effect of the North American monsoon. Finally, the relative performances of the climate models in reproducing the climatologies of P and T are quantified to support future impact studies in the basin.