Changing Demands from Riparian Evapotranspiration and Free-water Evaporation in the Lower Colorado River Basin Under Different Climate Scenarios

Observed and projected trends in riparian evapotranspiration (ET) and free-water evaporation are examined in this study to help improve water demand forecasting, particularly in modeling of lower Colorado River system reservoir operations. While most previous research in the Colorado River basin have focused on the impacts of climate change and climate variability on water supply, the impacts of changing climate on water demand have not been adequately addressed. Changes in temperature, precipitation, and wind patterns are expected to increase evaporative demands in the lower Colorado River mainstream, including free-water evaporation and ET from riparian vegetation, and may also impact infiltration rates, alter cropping patterns, and change the temporal and spatial distribution of water deliveries. This study utilized historic and projected hydroclimatic variables, such as temperature, wind, and precipitation, to analyze their impacts on riparian ET and free-water evaporation in the lower Colorado River mainstream. Analysis of historic and projected hydroclimatic data shows that mean annual daily temperature in the lower Colorado River mainstream reach has increased by 0.8° Celsius (C) from the 30-year period ending in 1980 to the 30-year period ending in 2010 and is projected to increase by an additional 1.7° C by the 30-year period ending in 2060. Estimates of riparian ET and free-water evaporation indicate that the combined evaporative demands in the lower Colorado River mainstream have increased by 14,750 acre-feet, or 1.8 percent, during the 30-year period ending in 2010, and may increase by an additional 16,600 acre-feet, or 2.0 percent, during the 30-year period ending in 2060, when compared to the period from 1951 to 1980. Due the highly regulated nature of the lower Colorado River mainstream, projected increases in evaporative demands are assumed to be decoupled from changes in environmental demand. Projected changes in evaporative demands under different climate scenarios were assessed to determine their potential impacts on reservoir operations in the Colorado River basin. Increases in evaporative demands are projected to reduce the combined storage of Lake Powell and Lake Mead by a cumulative volume of 75,400 acre-feet, or 0.15 percent of total conservation capacity, based on 10-year running averages from 2020 to 2060. In addition to reductions in reservoir storage, average annual shortage volumes in the lower Colorado River basin are projected to increase by 40,000 acre-feet, or 0.30 percent, from 2012 to 2060. Trends of increasing evaporative demands in the recent past and projected increases under changing climate conditions have implications for water managers and stakeholders attempting to balance future water supply and use in the lower Colorado River basin. Incorporating dynamic evaporative demands into operational modeling will help to improve demand forecasting. Future research topics discussed in this study may help reduce uncertainties inherent in estimate riparian ET and free-water evaporation and provide further improvements to water demand forecasting