Spring Precipitation Deficits Exacerbate Streamflow Declines in the Upper Colorado River Basin During the Millennium Drought

Abstract

The Colorado River heavily relies on the annual pulse of snowmelt from the Rocky Mountains within the Upper Colorado River Basin (UCRB), where over 90% of streamflow originates. Since 2000, the Colorado River has been delivering less water per unit precipitation than in preceding decades. The over 40 million people reliant on this water are concerned, where did that water go? Prior research has suggested the water loss is linked to warming temperatures, but prior work has not focused on the importance of seasonality in snow-dominated basins, which provide over 80% of Colorado River water. Spring weather must be considered because it controls when snow melts and how much energy is available for evaporation when the soil is wettest. These impacts make spring weather a critical contributor to annual streamflow. We found the most significant drop in seasonal precipitation since 2000 was during spring, with a decrease of 15-26% across Colorado headwater basins. These drier spring conditions bring about fewer clouds and result in warmer temperatures. Spring temperatures are better correlated with spring precipitation than with the temporal temperature trend associated with global warming. The greater radiation at the surface from cloud-free skies increases evaporative water losses, particularly in areas where snow has recently disappeared, leading to lower surface albedo and more net radiation available for evapotranspiration. Lower elevation headwater basins, where snowmelt occurs earlier, experienced the most substantial annual streamflow decreases of up to 41%. Refining spring precipitation estimates will be imperative for future water availability forecasts in snow-dominated water resource regions throughout the world.