He estimated that some 360,000 acre-feet of water evaporated from Lake Tahoe each year. That's enough to lower the lake level by three feet. It's a lot more water than flows down the Truckee River in a good, water year. An acre-foot is enough water to flood an acre of land to a depth of one foot. It's 325,851 gallons, or enough water to supply three to four families for a year.

How the water impounded by the dam at Tahoe City is divided up among all the folks depending upon it is a very politically sensitive, complicated, economic issue. It's controlled, in large part, by federal court decrees. Every gallon is spoken for and fought over. Lake Tahoe is some 1465 feet deep, but it's the top few feet (6.1 feet at most) impounded by the dam that is the contentious, battle zone.

Input to Tahoe
On average, according to figures from the watermaster's office, some 500,000 acre-feet of water run into the lake from the multitude of streams in the Lake Tahoe watershed and from precipitation that falls directly on the lake's surface. If 360,000 acre-feet evaporate, that only leaves some 140,000 acre-feet to flow down to Reno and beyond. So, on average, almost three-fourths of the water impounded by the Tahoe City dam evaporates into the thin air of the high Sierra.

For decades, evaporation rates for Lake Tahoe were determined by measuring evaporation from a four-foot diameter pan installed near the lake's outlet in Tahoe City. Because the water in the pan would freeze during much of the year, measurements were only made in warmer months, and the information was extrapolated to determine an annual evaporation rate. Also, the pan has become increasingly sheltered and shaded by tree growth over the years, which could affect evaporation estimates.

Evaporation Rate
A few years ago a UC graduate student made similar measurements at the Coast Guard pier in Lake Forest. The student found that evaporation rates in Lake Forest were different from those in Tahoe City. Knowing exactly how much water is evaporated is key to developing an accurate hydrologic model for Lake Tahoe. Accurate models are vital for knowing how much water will be available for administrators to divvy up along the Truckee watershed. So, differing evaporation rates (e.g., Lake Forest and Tahoe City) are a problem, or at least raises questions.

On-going Research
Dr. Gayle Dana, who is studying Lake Tahoe's evaporation, is a hydrologist and Associate Research Professor at the University of Nevada's Desert Research Institute (DRI). She notes that actual evaporation rates are smaller than previous estimates, and that evaporation rates can potentially vary widely in different places, and at different times, around the lake. Dana explains that evaporation of water varies from hour to hour. "Wind is very influential, as is temperature, relative humidity, and solar radiation."

DRI's current research on Lake Tahoe's evaporation aims to improve management of water supplies throughout the Truckee Basin by improving accuracy of hydrologic models being developed by the U.S. Geological Survey. According to Dana, evaporation rates and ground water information are the unknowns in developing Tahoe's water budgets. She is determining heat fluxes and vapor fluxes to ultimately calculate evaporation rates.

Dr. Dana is currently measuring water vapor fluxes, temperature, radiation and wind at several locations around the shoreline. Very sophisticated instruments are being used in this study--sonic anemometers, krypton hygrometers, thermocouples, and suites of meteorological instruments. Ultimately, Dana's work will provide water modelers and managers with needed values of Lake Tahoe evaporation rates.

Why is this work so important? Tahoe water is very valuable, and just a few inches of evaporation is equivalent to several billion gallons of water that could irrigate farmland, provide habitats for endangered fishes, and provide domestic water supplies downstream.

Written and (c) 2006 by Leo Poppoff