USGS Washington Water Science Center
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WA441 - Growing-Season Evapotranspiration from Selected Fallow Agricultural Lands of the Tule Lake National Wildlife Refuge, California - Completed FY2002
Problem - Allocation of scarce water in the Klamath Basin of southern Oregon and northern California holds many implications for agricultural and other natural-resource concerns. One source of water for critical National Wildlife Refuge (NWR) operations that has been identified by the U.S. Fish and Wildlife Service (FWS) is water conservation through reduction or curtailment of irrigation during relatively dry years for farming operations that currently are conducted on approximately 17,000 acres of the Tule Lake NWR (figure 1). Although the FWS has estimated consumptive water use by irrigated crops on the Tule Lake National Wildlife Refuge (NWR), estimates of growing-season evapotranspiration (ET) from fallow croplands are needed to compute the water savings that would be achieved by curtailing irrigation on those lands. Little is known about ET from fallow croplands in the region and the FWS has asked the USGS to investigate growing-season ET from such lands at the Tule Lake NWR.
Relevance and Benefits - Although the literature describing ET from commercially important crops is voluminous, little published information exists concerning actual ET from fallow croplands. The proposed application to this problem of the energy balance Bowen ratio (EBBR) technique will likely yield some of the first scientifically defensible ET estimates for fallow croplands in the region. Because this information will be developed principally by a neutral USGS, it will be particularly helpful to the FWS and other parties in the resolution of difficult land- and water-management issues. Given this context, the proposed investigation is consistent with "Strategic Directions in WRD Scientific Activities " (Issue 9), WRD Memorandum 95.44: "Avoiding Competition with the Private Sector" (broader goals 2, 3, 4, and 6), and the USGS Washington Water Science Center 1999 Science Plan (Issue 1).
Approach - Growing-season ET from three fallow fields that are different with respect to surface cover or drainage design, and thus are expected to have different ET rates, will be monitored continuously using the calibration-free energy balance Bowen ratio technique. Three tentatively selected fields are (1) a field that is drained by perimeter ditches only and in which a small-grain crop was grown during the 1999 growing season, (2) a field that is drained by perimeter ditches, plus a network of tile drains, and in which a small-grain crop was grown during 1999, and (3) a field with ditch and tile drains that was tilled and seeded to a cover crop during winter 2000 to simulate a fallow row-crop field. Depth of the water table below land surface will be monitored manually at selected locations using piezometers. The occurrence and percentage of surface cover by crop residues and non-crop plants, such as the cover vegetation and weedy species, will be noted or measured at intervals of from four to six weeks during May to October.