NEW MEXICO CLIMATE CENTER

New Mexico Crop Information

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

Eto = reference evapotranspiration or potential evapotranspiration referenced to grass.

Crop Coefficient uses growing degree days (GDD) accumulated from planting based on the averaging method of calculating GDD with a maximum temperature cutoff of 70 F and a minimum cutoff temperature of 40 F. The Base Temperature is 40 F

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The equation for the crop coefficient (k) is: k=-5.22E-1 -3.60E-4 Gdd +6.10E-7 GDD^2-1.50E-10 GDD^3  based on data from Farmington NM. The crop coefficient equation is appropriate for a sprinkler irrigated crop with  yield around 80000 lb./ac To convert from GDD to Date get the accumulative heat units for the current year from the weather convert page. Use the english format because the GDD days are english units. To convert GDD in degree F to GDD in degree C divide the value by 1.8.

The reference for this work is  M. S. Al-Jamal, T. W. Sammis, S. Ball and D. Smeal 1999. A new technique for obtaining a crop coefficient for onion irrigation , submitted to Ag. Water Management.