Evapotranspiration and Net Irrigation Water Requirements for Nevada

Evapotranspiration (ET) of water is a primary component of the hydrologic cycle and is becoming more significant as increasing demands are placed on finite water supplies in Nevada and across the western U.S. Local, state, and federal water resource agencies have a great need for accurate estimates of net irrigation water requirements for evaluating irrigation development, transfers of irrigation water for municipal use, and litigation of water rights applications and protests. The major objective of this study is to update estimates of crop ET and net irrigation water requirements for Nevada. Estimates of ET and net irrigation water requirements are made for all major crops grown in Nevada at over 200 locations using National Weather Service weather stations located throughout the state. The methods for estimating ET follow the new ASCE-EWRI Standardized Penman-Monteith approach, while the net irrigation water requirement for crops is estimated using a dual crop coefficient and daily soil water balance approach. Results show that in central and northern parts of Nevada, the net irrigation water requirement for alfalfa, the primary crop grow in Nevada, is less than the typical permitted amount of irrigation water rights of 4 ac-ft/ac, indicating that if irrigators wish to change or sell their water rights to more consumptive uses, they would not be able to transfer the entire water right. These revised estimates of ET and net irrigation water requirements for various crops will be used for updating basins water budgets and establish the amount of irrigation water that is available for water transfers in the future.