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The water cycle is something we’ve all heard about. Water falls from the sky, soaks into soils and forms streams and rivers that fill lakes or flow into oceans. Evaporation returns water to the atmosphere, and the cycle continues. When we’re concerned with having too much or not enough water, we must measure and model the water cycle to better manage water resources, yet accurate measurements can be challenging.

Along the ����ֱ�� Front Range and in many arid and semi-arid regions world-wide, surface reservoirs store meltwater from the alpine snowpack. The water is then allocated and distributed downstream to water rights holders during the dry, summer months. Accurate accounting of this water is required to assure that the agreed-upon distributions are made. Water inputs and outputs from these reservoirs must therefore be known, including how much water evaporates. The water ‘lost’ through evaporation is especially important since it can be a large term in the water cycle. Reservoir managers in ����ֱ�� must release water in an amount equal to the volume evaporated from the reservoir. This release compensates downstream users for the water that they would have had access to without the added evaporation loss from the reservoir.

Historically, evaporation pans, a large dish filled with water located near a reservoir, have been used to measure evaporation. Although the concept is simple and evaporation pans are practical, they rarely provide an accurate estimate of actual reservoir evaporation. Improper placement, and the small volume of water stored compared to a reservoir, typically results in large overestimates of evaporation in the summertime, large underestimates in the fall, and no estimates in the winter when the pans are removed.

In an effort to help manage water resources, Professor Blanken and his PhD candidate, Holly Roth, are directly measuring evaporation from Standley Lake reservoir in Westminster ����ֱ�� using a state-of-the-art method seldom used over reservoirs in the American West. As recently featured on , they installed a suite of meteorological sensors used to measure evaporation. These measurements have been collected since 2020, and help water managers to allocate precisely the correct volume of water, thereby saving water for times when it’s really needed. Their goal is to use a reservoir water balance model to extend their measurements to other reservoirs across the Front Range. As the Front Range’s population and air temperature continues to increase, and the snowpack decreases, their hope is that this research will help conserve this important resource. According to Professor Blanken, “Just as people and plants need water, so does the atmosphere. For each degree Celsius increase in air temperature, the atmosphere’s demand for water vapor increases by 7%, so it’s very important that that we measure and account for all sources of water loss including evaporation.”