USGS Washington Water Science Center
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WA362 - Evapotranspiration in Sparse Canopies - Completed FY1997
Problem - Researchers have successfully used energy-budget methods to estimate evapotranspiration (ET) in actively-growing full canopies, such as Snively Basin on the Arid Lands Ecology (ALE) Reserve, but there are questions as to whether these methods are always appropriate in sparse canopies. In a 1991 sparse-canopy ET study on the ALE Reserve, the Bowen-ratio method estimated 41 percent of the ET measured by weighing lysimeters. In a similar investigation in 1992, the results varied -- some days the Bowen-ratio estimate of ET was higher than that measured by the lysimeters, while on others it was close to or lower than the ET measured by the lysimeters. Because weighing lysimeters are the most direct method of measuring ET, reasons for the different results obtained with the weighing lysimeters and the Bowen-ratio method need to be investigated and resolved.
In sparse canopies, major assumptions in the methods may be invalidated due to advection from adjacent or distant wet or dry areas, inadequate fetch, uneven terrain, dry conditions, or other environmental influences. The presence of one or more of these conditions can lead to air and vapor pressure measurements that are unrepresentative of the instrumented site. Hence, collected data and subsequent ET calculations would be in error.
Objectives - The objectives of this study are to 1) investigate the use of several methods to calculate ET in sparse canopies on the ALE Reserve and determine under what environmental conditions each provides accurate or inaccurate estimates of ET using weighing-lysimeter ET as the standard; 2) continue data collection at the sparse-canopy site in the Black Rock Valley, established in 1992; 3) develop ET databases for use in present and future studies; 4) refine deep-percolation models for the ALE Reserve, and 5) develop a deep-percolation model for the Black Rock Valley.
Benefits - Knowledge gained from completion of this project will be of interest to hydrologists, climatologists, water-resources managers, and others interested in the measurement of ET in arid and semiarid areas in the U.S. and other countries. The findings may require researchers to reconsider present recommendations on fetch extent, type, and density for making micrometeorological measurements to calculate ET with energy-budget methods in sparse-canopy wildland vegetation. Very few studies have compared Bowen-ratio calculated ET with that from a more direct method, such as weighing lysimeters, under wildland conditions. This type of investigation is urgently needed to help researchers evaluate what conditions might result in the collection of data that would be unrepresentative of the area being measured and thereby lead to erroneous Bowen-ratios and ET estimates.
Approach - Four sites will be instrumented with Bowen-ratio and Penman-Monteith data-collection equipment. Two sites will be located adjacent to two pairs of weighing lysimeters in areas of sparse-canopy grassland and sagebush grassland on the ALE Reserve. A third site will be established in a sparse canopy on the ALE Reserve that has level terrain and a large, uniform fetch, which are conditions that are ideal for accurate Bowen-ratio data collection. The fourth site will be located in sparse-canopy sagebrush grassland in the Black Rock Valley. Collected data will be used to calculate ET for each site, refine deep-percolation models used to estimate ET for the ALE Reserve, and develop a deep-percolation model for the Black Rock Valley. ET estimates obtained through energy-budget methods, the deep-percolation models, and weighing lysimeters will be compared and analyzed to check for advection or other influences. Conditions that are present when ET estimates differ will be documented. Reasons for the differences will be investigated.
WA343 - Long-Term Evapotranspiration Network, Phase I - Completed FY1991
Problem - 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 of these components is evapotranspiration (ET). Not only is long-term ET information not available, but short-term ET data is extremely sparse and does not include most of the statewide climate and land-cover regimes. Long term estimates of ET would greatly enhance estimation of other water-budget components and reduce the overall error in such estimates.
Objectives - To develop a long-term ET monitoring network for the State of Washington. To evaluate equipment, intrabasin variability in climate parameters, and the relation of Bowen ratio to Penman site information.
Approach - We will establish two sites in a basin. This basin will also be monitored for other hydrologic variables. The sites will be a Bowen ratio site (provides data for calculating actual ET), and a Penman site (provides data for calculating potential ET). The first part of the study will be to assess equipment, intrabasin variability in climate parameters, and relation of Bowen ratio to Penman site information. Thus, the feasibility of locally calibrating the Penman site potential ET data to the Bowen actual ET data would be evaluated. This would allow for future ET sites to consist of the more trouble-free Penman equipment. Those future sites would then only need to be calibrated periodically.
WA362 - Estimating Actual ET using Bowen Radio and Penman Combination Methods - Phase II - Completed FY1992
Problem - Evapotranspiration (ET) is a major component of the hydrologic cycle and one of the most difficult to accurately quantify. Good estimates of ET are needed for increasing the accuracy of other water-budget components, notably ground water. Since long-term ET data are generally not available, stations to collect such data need to be established in the major climate and plant regimes throughout Washington State.
Objectives - The objectives are to accurately define ET in several major climate and plant regimes in Washington, begin to develop long-term ET data bases for these regimes, and investigate a method simpler than the Bowen ratio method for calculating ET from routinely collected data.
Approach - The approach to this study would be to continue monitoring ET at the initial site established in Phase I (project WA343), and to establish two new sites in eastern Washington. Data collected would be used to calculate ET through the Bowen ratio equation and the Penman-Monteith equation. The data and ET results will be analyzed to determine whether a simplified Penman approach might be used.
WA362 - Estimating Actual ET using Bowen Radio and Penman Combination Methods, Phase III - Completed FY1994
Problem - Evapotranspiration (ET) is a major component of the hydrologic cycle and one of the most difficult to accurately quantify. Good estimates of ET are needed for increasing the accuracy of other water-budget components, notably ground water. Additionally, long-term values of ET will provide verification for often unproven methods of estimating ET used in water-balance studies.
Objectives - The objectives are (1) to accurately define ET in several major climate and plant regimes in Washington by continuing to develop long-term ET data bases for these regimes, and (2) to compare ground-water recharge values obtained using estimated ET data with recharge values obtained using field-collected ET data.
Approach - This study will continue to monitor ET at the initial site established in phase I and will establish one new site in western Washington. Data collected will be used to calculate ET through the Bowen-ratio equation and the Penman-Monteith equation. For the part of the study comparing field-based ET values to model-estimated ET values, simulations for ground-water recharge will be run using the Jensen-Haise method of ET estimation. Another simulation will utilize field-based ET data.