Precipitation in the form of snowfall plays the dominant role in year-to-year streamflow variability in the Uncompahgre River Watershed, but air temperature also has a significant impact. In the spring, the air temperature rises, snowmelt begins, and streamflow increases. This generally begins when nighttime temperatures stay at or above freezing (see blue line in the graph below).
Observing long-term trends, higher temperatures have exacerbated drought conditions, resulting in lower streamflow. Hydrologists pay close attention to this phenomenon as they compare air temperature with runoff and streamflow over time.
Below, the graph shows the annual average surface temperature (brown line) over the Upper Colorado River Basin from 1895 through 2023; the running five-year average (green line); and the temperature trend line from 1970 to 2023 (blue line), with temperature increasing about 3 degrees F over the 53-year period. Source: https://www.ncei.noaa.gov/access/monitoring/climate-at-a-glance/
Several factors influence the flow on the Colorado River at Lee’s Ferry, mainly precipitation and snowpack over the upper basin, upstream consumptive use, and air temperature. Considering only temperature, what might explain the inverse relationship between increasing air temperature and decreasing flow?
In general, higher air temperatures across watersheds are the leading cause of significantly less snow than water, earlier melting, and earlier peak runoff. Less snowpack means less runoff.
In the graph below, note the period from 1970 to 2002 where the trend line shows declining flow while the temperature graph (above) shows markedly increasing temperatures in the Colorado River Basin over the same period. Source: https://pubs.usgs.gov/fs/2004/3062/
As streamflow timing and peak flows advance to earlier in the spring, the amount of water flowing in channels during the summer is further depleted. In more subtle ways, the increased evapotranspiration and lower soil moisture that result from higher air temperatures intensify and extend this low flow season.
Water users know all too well that higher temperatures also tend to coincide with increased human demands on the water supply. So the proverbial law of supply and demand isn’t just an economic issue; it’s a watershed concern as well!
