USGS Washington Water Science Center
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Assessment of water resources for sound management practices requires reliable continuous, long-term information on the components of the water budget. One of the most important components is evapotranspiration (ET)- the loss of water to the atmosphere from the earth's surface by evaporation and by transpiration through plants, but ET is one of the most difficult components to quantify accurately. In general, long-term ET information for Washington is not available, and short-term information is extremely sparse and does not include most of the climate and land-cover regimes. Establishing a long-term database of ET data for the major climate and plant regimes in Washington would greatly enhance the ability to estimate other water-budget components and reduce the overall error in such estimates.
To help develop a long-term ET monitoring network and accurately define ET in several major climate and plant regimes in Washington, the USGS is establishing stations in selected basins in the arid eastern part of the State. Data needed to calculate ET using two different approaches will be collected at the sites. If results from the Penman approach compare favorably with those from the Bowen ratio approach, then the Penman approach, which uses data-collection equipment that requires less-frequent calibration and maintenance, could be used at future sites.
USGS scientists also are estimating ET during a part of the growing season for selected habitat types at Klamath Marsh and Lower Klamath National Wildlife Refuge in northern California. The estimates are needed to evaluate ecosystem water balances. The study includes developing numerical models for predicting ET from those types of habitat.