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 -~ without any adverse effects on vegetative plant growth and yield. Groundwater depth dropped from 1.2 to 2.2 m during the growing season.
Irrigated land represents approximately 18% of the cultivated land in the world and accounts for approximately one-third of the world's food supply (Higgins et al. 1987). Shallow groundwater is a problem on an estimated onehalf of the existing irrigation areas (Rhoades and Loveday 1990). Traditionally, subsurface drainage has been used to control the depth of the water table and to permit leaching of salts from irrigated soils (Rhoades 1974).
In many regions, the ability of farmers and irrigation districts to discharge drainage water into surface streams has been restricted due to the presence of salt and toxic trace elements in the water. This is the case in the Central Valley of California where, since 1986, no drainage outflows have been allowed in a 16,000 ha area due to high concentrations of selenium and other trace elements in the drain water (San Joaquin Valley Drainage Program 1990). As surface water quality standards become more stringent, it will become more difficult for drainage water from irrigated agriculture to be discharged into surface water.