Controls and rates of evaporation from a water supply reservoir in the Colorado Front Range

Evaporation can be a major component of a reservoir's water balance in arid, populated regions such as the western United States, yet accurate measurements are scarce. In an effort to fill this observation gap, direct measurements of evaporation from Standley Lake, a water supply reservoir in the Colorado Front Range, were collected via the eddy covariance method from March 1, 2020 through February 28, 2021. Evaporation followed a seasonal pattern, with the spring, summer, and early fall experiencing daily cycles that peaked just after midday, with measurable evaporation occurring each day for approximately 4-6 h. In the late fall and winter, the daily cycles decayed and episodic enhancements in evaporation were common, most often lasting between 18 and 36 h, some longer. Evaporation peaked in June, aligning with peak net radiation, and summertime evaporation was predominantly driven by diurnal variations in wind speed and the vapor pressure gradient. Interestingly, fall and winter saw multiple periods of negative (downward) sensible heat transfer which helped drive wintertime evaporation. The study period evaporation totaled 1377 mm, with 35 % of that occurring in summer, 27 % in fall, 21 % in spring, and 17 % in winter. This study shows that evaporation is a significant part of the energy balance of Standley Lake, and with continued observations, enables a better understanding of the impacts a changing climate and increased water demands have on water resources.

Automated summary

This study provides direct measurements of evaporation from Standley Lake, revealing its seasonal patterns and the factors influencing it. Summer evaporation is mainly driven by diurnal variations in wind speed and the vapor pressure gradient, while fall and winter are influenced by negative sensible heat transfer. Overall, evaporation plays a significant role in the energy balance of Standley Lake.