The crop coefficient used to determine the daily water use by cotton was found to be sensitive to air temperature, as expressed in average daily heat units of degree days above 60 degrees F. When the evapotranspiration (ET) of the crop was determined using a Penman-type equation for the reference ET, then the crop coefficient was also found to be sensitive to the average daily wind speed. When the reference ET came from evaporation pan data, then the daily crop-pan coefficient was a function of heat units but independent of wind speed. The crop-pan coefficient becomes independent of temperature after cut-out, when the plant growth switches from the vegetative stage to the boll-development stage. A new procedure was developed to determine the crop coefficient, using a field test of 6 different water application rates through a subsurface drip system on a 2-acre plot.