NEW MEXICO CLIMATE CENTER

New Mexico Crop Information

Cotton coefficient(k) to calculate evapotranspiration(ET) where Et=k*Eto

Eto = reference evapotranspiration or potential evapotranspiration referenced to grass.

Crop Coefficients use growing degree days (GDD) accumulated from planting based on the averaging method of calculating GDD with a maximum temperature cutoff of 30 C and a minimum cutoff temperature of 12.5 C. The Base Temperature is 12.5 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=4.24E-4 +1.20E-3 Gdd +4.62E-7 GDD^2-5.77E-10 GDD^3 with an R^2=0.76 based on data from Artesia and Las Cruces NM.The crop coefficient equation is appropriate for a furrow irrigated crop with cotton yield around 1800 kg/ha or 1600 lb/ac

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