CROP COEFFICIENTS FOR IRRIGATING COTTON IN THE PRESENCE OF GROUNDWATER

A cotton crop coefficient was modified to account for the contribution of shallow groundwater to crop water use. The data used in the modification were developed using weighing column lysimeters. The percentage groundwater contribution to crop water use, expressed as a function of growing degree days for several salinities and two water table depths, was used in the regression analysis. Use of the modified coefficient was demonstrated by scheduling a subsurface drip irrigation system installed in an area with shallow saline groundwater. Use of the modified crop coefficient resulted in 25% of the cotton water requirement being extracted from shallow groundwater with a salinity of 5 dS m-1 without any adverse effects on vegetative plant growth and yield. Groundwater depth dropped from 1.2 to 2.2 m during the growing season.