Revising Crop Coefficient for Washington Sate

Abstract

Abstract. Water shortages are expected to increase in the future due to the need to grow crops to feed a growing population, the increasing production of bio fuel crops, the desire to protect and restore waterways for fish, the competition for water among different sectors, and water shortages due to climate change effects. Thus, it is important to accurately predict crop water use to design efficient irrigation systems, to improve irrigation scheduling, and for hydrologic studies and equitable water‐right allocations. The Washington State evapotranspiration and consumptive irrigation water requirement guideline created in the 1980s uses now outdated ET methods applied with limited weather data. As part of this work, the water requirement guideline was updated by selecting and adapting crop coefficients (Kc) for use with ET calculations based on the ASCE standardized Penman-Monteith approach for crops that are economically important in WA. Crop coefficients were selected from AgriMet, FAO56, Wright 1982 and Allen 2006. Daily alfalfa reference ET based on the Kimberly 1982 and the ASCE standardized equations, and the daily grass reference ET based on FAO56 were determined for 211 weather stations across the state. Seasonal Crop coefficient curves for use with the ASCE standardized reference ET equation based on weather data from east central Washington were developed, since the planting dates information for crops of interest are available in this region. These curves were expressed as a function of cumulative growing degree days so as to make them applicable to all climatic regions of the state and to account for the year‐to‐year climatic variability. A database of information on water use and Kc was developed for use with GIS spatial mapping software.