Deficit irrigation of cotton for increasing groundwater use in clay soils.

The interactive effects of a shallow, highly saline water table and irrigation management on crop production have not been clarified, due to difficulties in measuring crop water use. The detrimental effects of a rising water table and salinization due to irrigation can be eliminated, provided irrigation scheduling is based on a broad knowledge of crop water requirements. A deficit irrigation approach was used to (i) minimize deep percolation and (ii) study the capability of cotton (Gossypium hirsutum L.) to use water from the soil profile and from lower groundwater. Measurements of sap flow in the stem by the heat pulse method were used to determine transpiration. Transpiration of the mature cotton crop was nearly 20% lower than potential transpiration. Reduced irrigation application did not stimulate root expansion into wet, deep soil layers ; therefore, it resulted in a reduction of the transpiration rate below its maximum rate. An appreciable water uptake from groundwater did not occur, presumably because of high sensitivity of the cotton root system to high soil density. Although the leaching fraction was nearly zero, salt accumulation in the root zone during the growing season was low.