Crop coefficients to scale reference evapotranspiration

Reference evapotranspiration is scaled to the water use of chile by multiplying by the crop coefficiient which is a measure of the present of the ground that is covered with chile plants.

Figure 1 Crop coefficient for chile.

The crop coefficinet goes above 1 because the canopy of a chile crop is rougher than the canopy of grass and consequently uses more water than grass when the canopy represents a closed canopy covering more than 90 percent of the area.

The crop coefficient can also be expresssed in growing degree days since planting instead of days since planting. The chile plant growth rate is controlled by heat units and not time. Consequently , crop coefficients based on time vary between years and locations , where as crop coefficients based on growing degree days are constant under changing environment conditions.

Growing degree days GDD are calculated as:

  Gdd= (maxtemp+mintemp)/2 -basetemp

where 	maxtemp= the maximum temperature during the day
	mintemp= the minimum temperature during the day

   set maxtemp to maxcutoff if maxtemp greater than maxcutoff
   set mintemp to mincutoff if mintemp less than mincutoff

where no cutoff temperatures are used,then use 999 for the maximum cutoff and -999 for the minimum.
cutoff temperatures. For Chile Crop  (GDD) are accumulated from planting based on 
a maximum temperature cutoff of 86 F and a minimum cutoff temperature of 41 F.
The Base Temperature is 41 F

Becasue GDD can be calculated using both F or C scale the relationship is to multiple the
GDD in degree C by 9/5 to convert it to degree F