HESS Hydrology and Earth System Sciences HESS Hydrol. Earth Syst. Sci. 1607-7938 Copernicus Publications Göttingen, Germany 10.5194/hess-17-1833-2013 Future humidity trends over the western United States in the CMIP5 global climate models and variable infiltration capacity hydrological modeling system Pierce D. W. 1 Westerling A. L. 2 Oyler J. 3 Division of Climate, Atmospheric Science, and Physical Oceanography, Scripps Institution of Oceanography, La Jolla, CA, USA Sierra Nevada Research Institute, University of California, Merced, USA Numerical Terradynamic Simulation Group, University of Montana, Montana, USA 14 05 2013 17 5 1833 1850 Copyright: © 2013 D. W. Pierce et al. 2013 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://hess.copernicus.org/articles/17/1833/2013/hess-17-1833-2013.html The full text article is available as a PDF file from https://hess.copernicus.org/articles/17/1833/2013/hess-17-1833-2013.pdf

Global climate models predict relative humidity (RH) in the western US will decrease at a rate of about 0.1–0.6 percentage points per decade, albeit with seasonal differences (most drying in spring and summer), geographical variability (greater declines in the interior), stronger reductions for greater anthropogenic radiative forcing, and notable spread among the models. Although atmospheric moisture content increases, this is more than compensated for by higher air temperatures, leading to declining RH. Fine-scale hydrological simulations driven by the global model results should reproduce these trends. It is shown that the MT-CLIM meteorological algorithms used by the Variable Infiltration Capacity (VIC) hydrological model, when driven by daily <i>T</i><sub>min</sub>, <i>T</i><sub>max</sub>, and precipitation (a configuration used in numerous published studies), do not preserve the original global model's humidity trends. Trends are biased positive in the interior western US, so that strong RH decreases are changed to weak decreases, and weak decreases are changed to increases. This happens because the MT-CLIM algorithms VIC incorporates infer an overly large positive trend in atmospheric moisture content in this region, likely due to an underestimate of the effect of increasing aridity on RH. The result could downplay the effects of decreasing RH on plants and wildfire. RH trends along the coast have a weak negative bias due to neglect of the ocean's moderating influence. A numerical experiment where the values of <i>T</i><sub>dew</sub> are altered to compensate for the RH error suggests that eliminating the atmospheric moisture bias could, in and of itself, decrease runoff up to 14% in high-altitude regions east of the Sierra Nevada and Cascades, and reduce estimated Colorado River runoff at Lees Ferry up to 4% by the end of the century. It could also increase the probability of large fires in the northern and central US Rocky Mountains by 13 to 60%.

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