Revising crop coefficients for Washington State

Tina Karimi

Water resources in Washington State are important limited. Water shortages going to increase in the future due to the need to grow additional crops to feed a growing population, growing crops to produce bio fuels, the desire to protect and restore waterways for fish, and water shortages because of climate change. It is important to accurately predict the crop water use to design efficient irrigation systems, for irrigation scheduling and management, for hydrologic studies and for equitable water‐right allocations. Crop water use can be estimated from historical weather data by calculating evapotranspiration (ET) which is the combination of water transpired from vegetation and evaporated from soil and plant surfaces. The Washington State evapotranspiration and consumptive irrigation water requirement guideline that are widely used, were created in the 1980s using out‐of‐dated methods. In this work the water requirement guideline was up-dated by revising crop coefficients (defined as the ratio of actual crop ET to reference ET). More weather data and most accurate methods were used to develop crop coefficient curves for crop 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. These values were used to convert crop coefficients for use with the ASCE standardized reference ET equation. The new crop coefficient curves were primarily developed based on the day of the year in east central Washington. These curves were converted to be based on cumulative growing degree days to make them applicable to all climatic regions of the state and to account for the year‐to‐year climatic variability. The adjustment factor during peak months and for each county was calculated to account for the weather variability across the region that may exist due to conversion of crop coefficients.

Revising crop coefficients for Washington State

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