Future humidity trends over the western United States in the CMIP5 global climate models and variable infiltration capacity hydrological modeling system

摘要

Global climate models predict relative humidity (RH) in the western US willdecrease at a rate of about 0.1–0.6 percentage points per decade,albeit with seasonal differences (most drying in spring andsummer), geographical variability (greater declines in the interior),stronger reductions for greater anthropogenic radiative forcing, and notablespread among the models. Although atmospheric moisture content increases,this is more than compensated for by higher air temperatures, leading todeclining RH. Fine-scale hydrological simulations driven by the global modelresults should reproduce these trends. It is shown that the MT-CLIMmeteorological algorithms used by the Variable Infiltration Capacity (VIC)hydrological model, when driven by daily T_min, T_max,and precipitation (a configuration used in numerous published studies), donot preserve the original global model's humidity trends. Trends are biasedpositive in the interior western US, so that strong RH decreases are changedto weak decreases, and weak decreases are changed to increases. This happensbecause the MT-CLIM algorithms VIC incorporates infer an overly largepositive trend in atmospheric moisture content in this region, likely due toan underestimate of the effect of increasing aridity on RH. The result coulddownplay the effects of decreasing RH on plants and wildfire. RH trends alongthe coast have a weak negative bias due to neglect of the ocean's moderatinginfluence. A numerical experiment where the values of T_dew arealtered to compensate for the RH error suggests that eliminating theatmospheric moisture bias could, in and of itself, decrease runoff up to14% in high-altitude regions east of the Sierra Nevada and Cascades, andreduce estimated Colorado River runoff at Lees Ferry up to 4% by the endof the century. It could also increase the probability of large fires in thenorthern and central US Rocky Mountains by 13 to 60%.