By Orestes Morfin and Nolie Templeton, PhD, CAP Colorado River Programs senior analysts
In the Colorado River Basin, the past two decades have seen reduced snowpack, low runoff and declining river flows. Why? As temperatures in the Basin have increased over the past several decades – by just over 0.5°F per decade since 1980 – scientific evidence points to warming temperatures driving aridification.
Aridification is the gradual change of a region from a wetter to a drier climate. Looking back through history there is evidence that aridification has changed living patterns across the globe, and our generation is experiencing how this phenomenon is impacting not only the Western United States, but North America.
Droughts imply a temporary condition, where it is assumed dry conditions would end with significant rain or snow. Aridification means that hotter and drier conditions are likely here to stay as our landscape and atmosphere respond to warming temperatures.
What does aridification mean for the Colorado River Basin?
We are scientists so we’d like to tell you first in an equation, and then (because our communications team insists) in words.
It starts with the water cycle we all learn in elementary school. In water balance terms:
P = Precipitation
Whether that be rainfall or snowmelt, precipitation can be thought of as the input to the system. That precipitation is essentially divvied up between infiltration (going into the ground), runoff and evaporation/evapotranspiration.
R = Runoff
Next, in water terms, we speak of the “water year.” Snow begins to accumulate in the headwater’s region (Colorado, Wyoming and Utah) in the fall, continuing into the winter. As the snow melts, it runs off into the Colorado River Basin system, ultimately making its way to our reservoirs.
But with aridification, something else is happening. Less snowpack is accumulating because warmer temperatures mean more rain and less snow. And aridification is impacting the timing of runoff. Warmer conditions translate into a shorter winter period, with runoff occurring earlier in the water year.
The result is that the accumulated snowpack is not equating to the same level of runoff into the river that we used to depend on.
Why is that?
ET = Evapotranspiration
As temperatures have increased, the atmosphere has warmed, increasing its water demand. In other words, it has become “thirstier.” A thirstier atmosphere results in increased evaporation from water bodies and increased evapotranspiration from vegetation and soils. With increased evapotranspiration, less water is available for runoff and less water flows to the Colorado River.
Put these conditions together and you see less Colorado River water overall, thus adding to the concern regarding our reservoirs – Lake Powell and Lake Mead – potentially dropping to critically low levels.