Lysimeter Use in Water Rights Determination

In the Western United States, a water right (the legal right to divert water from a stream) is a property right. The impact a water right has upon a stream is commonly defined through determination of the stream depletion (diversion minus return flow) that results from exercising the water right. In water rights engineering, lysimeters are utilized to provide basic data with respect to evapotranspiration (Et), return flows and plant water use from various natural sources. Studies in Colorado, using small bucket lysimeters, found that returns flows from urban lawn irrigation can be determined from the amount of water applied and the potential evapotranspiration. A cooperative study between Utah, Idaho and Wyoming, and a study in Colorado, found that build-up of soil moisture during the winter contributed to the seasonal water supply in mountain meadows. The cooperative study between Utah, Idaho, and Wyoming utilized water table lysimeters to develop procedures for the determination of the duty of water under an interstate compact and to estimate historical stream depletions. In Colorado, water table lysimeters were utilized to measure evapotranspiration in formerly irrigated meadows and to develop a methodology for determining the maximum evapotranspiration transferrable in a water rights transfer.