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 modifi cation were developed using weighing column lysimeters. The percentage groundwater contribution to crop water use, expressed as a function o f growing degree days for several salinities and two water table depth s, was used in the regression analysis. Use of the modified coefficien t was demonstrated by scheduling a subsurface drip irrigation system i nstalled in an area with shallow saline groundwater. Use of the modifi ed crop coefficient resulted in 25% of the cotton water requirement be ing extracted from shallow groundwater with a salinity of 5 dS m-1 wit hout any adverse effects on vegetative plant growth and yield. Groundw ater depth dropped from 1.2 to 2.2 m during the growing season.