Acceleration of the Water Cycle over the Colorado Headwaters Region

A number of studies have indicated that one of the expected results of climate change will be an acceleration of the water cycle due to the enhanced moisture available due to a warmer climate. The representation of the water cycle in most climate models, however, is relatively poor due in part to the relatively coarse resolution as well as the difficulty of representing convection using existing convective parameterizations and the poor representation of complex topography. As a result, there is some uncertainty regarding the magnitude of the acceleration and its seasonal dependence. To overcome these deficiencies, a high resolution (4 km) cloud resolving simulation of the current climate (continuous eight years starting 2001) and a perturbed moister climate using the Pseudo Global Warming approach was performed using the WRF Climate model over the Colorado Headwaters region. The current simulation was driven by the North American Regional Reanalysis with 32 km resolution and 3 hourly updates. The future simulation was driven by the same NARR reanalysis with enhanced moisture and temperature from a CCSM A1B AR4 simulation of the global future climate out to 50 years. The headwaters region of Colorado that includes, among others, the Colorado, Platte, Rio Grande and Arkansas Rivers, is one of the key source regions for water in the Southwest as ~85% of the streamflow for the Colorado River comes from snowmelt in this region. The performance of the WRF model for precipitation, temperature and snowpack was verified to be reasonable for both winter and summer during the 2001-2008 period using SNOTEL observations at ~100 sites. Based on this good performance, an analysis of the residence time of various the various components of the water cycle was performed. This paper will present the change in residence time of water in its various phases as a function of season, region both above and below ground due to the warmer and moister climate. A comparison will also be made to current climate models for the same region and time period.