Floating Weather Station Will Measure Water Evaporation

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

Scientists from the Desert Research Institute in 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.

 (Scientists with Desert Research Institute and the California Department of Water Resources prepare to drop anchor.)

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.

 (Solar panels power weather station buoy)

“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 has 145 weather stations that 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," says Huntington. "The water body stores a bunch of heat and then it releases the heat when the air temperature is lower than the water temperature.”

 (Justin Huntington with the Desert Research Institute stands in front of the floating weather station.)

Weather is simply different over water. This solar-powered floating weather station will gather near real-time measurements over water. Scientists at DRI, DWR and the Bureau of Reclamation will then pair that information with data from satellites and land based evaporation data. If the technology proves successful, more floating weather stations could be launched.

“It does no good just to know what evaporation is off Folsom Reservoir," says Huntington. "We need to know what evaporation is off all the reservoirs off the west slope of the Sierra and country wide.”

This floating weather station cost the three agencies about $80,000. Future stations wouldn’t be as costly. But Huntington says an improved picture of water supply is worth every penny, particularly during the drought. 

You can soon find near real time weather data from the weather station on Folsom Lake here.