Water Production Function
Alfalfa Water Production function or the relationship between Evapotranspiration and yield
Et = evapotranspiration or water used by crop.
y= 0.14 +0.12Et where y is metric tons/ha and Et is cm . The coefficient of determination is 0.78
The reference for this work is Sammis T. W. 1981. Yield of Alfalfa and Cotton as Influenced by Irrigation. Agronomy Journal 73:323-329
Alfalfa 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 999 C and a minimum cutoff temperature of -999 C. The Base Temperature is 5 C.
The equation for the crop coefficient (k) is: k=4.05E-1 +1.11E-3 Gdd -4.25E-7 GDD^2+3.56E-11 GDD^3 with an R^2=0.70 based on data from Artesia ,Clovis, Farmington, Las Cruces NM. The crop coefficient equation is appropriate for a flood irrigated crop with alfalfa yield around 20000 kg/ha at 0.0 moisture content.
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
Climate data should be copied from web site using Netscape. If you use Explorer, copy climate data to Notepad and then copy from Notepad to spreedsheet. Click on part of picture for instructions on how to fill out spreadsheet. A copy of the spreadsheet has been develooped that uses Samani's equation for potential evapotranspiration requiring only temperature data.