New Mexico Crop Information Sorghum coefficient(k) to calculate evapotranspiration(ET) where Et=k*Eto Eto = reference evapotranspiration or potential evapotranspiration referenced to grass.
estimated from GDD and Las Cruces N.M. climate data Crop Coefficients use growing degree days (GDD) accumulated from planting based on the averaging method of calculating GDD with no maximum and no minimum cutoff temperature . The Base Temperature is 7 C. To convert from GDD to Date get the accumulative heat units for the current year from the weather convert page. Use the metric format because the GDD days are metric units. To convert GDD in degree C to GDD in degree F multiply the value by 1.8.
The equation for the crop coefficient (k) is: k=8.93E-2 +1.59E-3 Gdd -7.462E-7 GDD^2+8.36E-13 GDD^3 with an R^2=0.74 based on data from Artesia , Clovis, Farmington, and Las Cruces NM. The crop coefficient equation is appropriate for a furrow irrigated crop non moisture stressed crop.
The reference for this work is Sammis T. W. , C. L. Mapel, D. G. Lugg, R. R. Lansford , J. T. Mc Guckin 1985. Evapotranspiration Crop Coefficients Predicted Using Growing Degree Days ASAE 28(3):773-780