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 longterm trends, higher temperatures have exacerbated drought conditions, resulting in lower streamflow. Hydrologists are paying closer attention to this phenomenon as they compare air temperature with runoff and streamflow over time.
In the left graph, annual average surface air temperature for the Colorado River basin is plotted in red. The blue line shows the 11-year trend (running mean) from 1890 to 2010. The right graph plots the annual flow volume for the Colorado River at Lee’s Ferry, which is located in northern Arizona, downstream of Lake Powell. The dashed line is the linear trend over time. Note how air temperature trends upward in the left graph, and streamflow trends downward in the right graph. [sources: (left) https://nap.nationalacademies.org/read/11857/chapter/5; (right) https://pubs.usgs.gov/fs/2004/3062/]
What explains this inverse relationship between increasing air temperature and decreasing streamflow?
In general, higher air temperatures across our watershed are the leading cause of significantly lower snowpacks, earlier melting, and earlier peak runoff. Less snowpack means less runoff, and as streamflow timing and peak flows are advanced 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.
The illustration below shows how higher air temperatures can reduce streamflow. This is especially true in the “Interior West” (represented on the left side of the illustration). We have noticed some of the important ways that higher air temperature can reduce streamflow. Can you spot any other causes of temperature-related reductions in streamflow? [online source: https://downloads.globalchange.gov/usimpacts/allimages/3-Water/3-Hi%20res/3-Water-pg-42_top.jpg]