CHARACTERIZING DROUGHT, SNOW, AND LANDSCAPE HYDROLOGY IN THE COLORADO RIVER BASIN USING A HIGH RESOLUTION, GRID-BASED WATER BALANCE MODEL

Drought in the Colorado River Basin (CRB) is impacting the water supply for over 40 million people in 7 states. Compared to past droughts, the current drought is unique because of the combined trends of increasing air temperature and decreasing precipitation starting at about the turn of the century. To analyze the effects of the current drought relative to the historical climate, the Basin Characterization Model (BCM) was used to simulate the monthly water balance for the CRB using a high-resolution 270-meter grid with about 9.5 million cells. Climate inputs were developed using gridded monthly (~4-km) PRISM precipitation and maximum and minimum monthly air temperature data, downscaled to the CRB, from October 1895 through October 2021. Parameters defining physical basin properties were calculated using available geospatial data. The BCM uses an energy-balance approach to simulate potential evapotranspiration (PET). Actual evapotranspiration (ET), soil moisture, snow accumulation, snow melt, recharge, runoff, and climatic water deficit (CWD, defined as PET minus ET) are simulated using climate inputs, simulated PET, calculated soil storage capacities, estimated bedrock and alluvium permeability, and monthly vegetation parameters. Parameters affecting snow accumulation and melt, ET, and sub-basin discharge were calibrated using snow water equivalent (SWE) data, published monthly ET maps for CONUS, and streamflow data. The CWD was used to identify drying soils and landscapes relative to conditions prior to about WY 2000 and was used to identify differences in the spatial distribution of drought severity. To quantify the severity of drought between locations in the CRB, monthly drought indices and cumulative departures from long-term means were calculated for CWD, runoff, and recharge. Preliminary results showed the 20-year mean total available water (recharge + runoff) declined by 18% for the upper CRB and by 37% for the lower CRB compared to conditions at the end of the century. Consistent trends of increasing CWD and decreasing SWE starting from about WY 1991 for the Lower CRB and WY 1999 for the upper CRB were identified. Multi-year drought indices showed severe drought in terms of recharge for the lower CRB, whereas results for the upper CRB showed severe drought in terms of both recharge and runoff.