How Much do Monsoon Rains Buffer Reduced Snowpack in a Colorado River Headwater Stream?

Abstract:

The Colorado River is a major water source and economic engine for the southwestern United States and relies on snow-fed headwater basins located in Colorado, Utah and Wyoming for 90% of its water supply. The North American Monsoon occurs July through September bringing significant rainfall to these headwater basins that may buffer streamflow deficiencies due to climate induced reductions in snow accumulation and melt. However, the influence of monsoon rainfall on streamflow generation remains uncertain largely due to difficulty in quantifying precipitation across mountainous watersheds and the tight coupling of climate, vegetation and topography that dictate hydrologic partitioning between evapotranspiration (ET) and runoff. Using a combination of lidar derived snow depths, precipitation raster maps, an observation network of weather and stream discharge stations and a hydrologic numerical model, we explore the importance of monsoon rain for streamflow generation over the historical record (years 1987-2019) for a Colorado River headwater basin (East River, CO 85 km²). On average, monsoons contribute 18±7% of annual water inputs to the basin, generate 10±6% the annual streamflow and increase water yield in the following year by 3±2%. For comparison, spring snow fall in April and May provides the same amount of water input as monsoon rain but produces twice the amount of streamflow. Efficiency of water inputs to generate streamflow is dictated by seasonal water and energy availability coupled with vegetation water demand, with the bulk of monsoon rains supporting ET in the conifer and aspen forests of the lower subalpine. Nonetheless, summer rains do have the potential to generate appreciable streamflow with emphasis at higher elevations in the basin where soil storage, forest cover and aridity are low; and are capable of rebounding late summer streamflow 30-70% from hypothetical reductions in snowpack. Historically, interannual variability in monsoon efficiency to generate streamflow per unit water input declines when overall snowpack is low, and annual aridity is high. This implies that the ability of monsoons to replenish streamflow in a warmer future with less snow accumulation will diminish.