Measuring Open Water Evaporation

Scientists from the Desert Research Institute in Reno, the California Department of Water Resources are Measuring Open Water Evaporation with a Floating Weather Station

California's drought has made it abundantly clear how important it is to know exactly how much water is available. Since waterevaporates,it's also critical to know how much can be lost from reservoirs, especially in the process called "open-water" evaporation.

Scientists from theDesert Research Institutein Reno, the California Department of Water Resources and the US Bureau of Reclamation are placing a floating weather station in the water at Folsom Lake. It's no easy task; the high tech device sits on a buoy that has to be slowly towed out by boat to the deep part of the reservoir.

Justin Huntington is a hydrologist with DRI. He and his crew are using 600 feet of chain tied to an anchor that looks like a railroad track to keep the buoy in place.

"Let the chain out slowly by hand and then let it fall freely into the water, and then it will yank the anchor off the bow of the boat," says Huntington.

The buoy is loaded with all kinds of equipment, temperature sensors, humidity sensors, wind gauges, and net radiometers. But its primary purpose is to measure water evaporation over the reservoir. Huntington says not knowing how much water evaporates, means not knowing how much water could be available for cities and farmland.

"A ten percent error in evaporation off this reservoir is worth millions and millions of dollars in terms of water to irrigators," says Huntington.

The California Department of Water Resources has145 weather stationsthat measure water evaporation over land. It helps farmers gauge when they might need to irrigate. But less is known about evaporation over water-which could help determine how much water gets delivered. Huntington says right now, water managers measure evaporation in a large pan near water.

"It's about a four foot wide pan by about a foot deep metal pan usually on a pallet located at the dam and they measure the amount of water loss from that pan every day," says Huntington. "As you can imagine the evaporation from that pan is probably a lot different than the evaporation occurring over this big body of water."

Huntington says pan readings can be off by 30-percent. You might think that maximum evaporation would occur in the heat of the summer when temperatures are highest.

"It's more in August or September due to the heat storage effects," saysHuntington."The water body stores a bunch of heat and then it releases the heat when the air temperature is lower than the water temperature."

Source: Capital Public Radio

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