ECONOMICS AND GROUNDWATER.

Groundwater withdrawals for cotton production in the arid Southwest account for about 15 percent of all groundwater withdrawals. In Arizona, cotton's proportion of groundwater withdrawals is much higher - 34 percent. Drip irrigation of cotton can greatly reduce groundwater applications - in Arizona applications have usually been reduced by 30-50 percent of typical furrow irrigation applications. This economic analysis predicts that drip irrigation on cotton might be profitable in Arizona, given a yield increase of about a bale per acre. A review of the literature and interviews with researchers and cotton producers suggest that yield is often, but not always, improved. Key factors that affect drip yields are the soil, climate and management. Drip irrigation tended to increase yields on medium to coarse soils, where the weather was hot and dry, and where good management was applied. Conversely, there were often no yield increases where level, fine soils prevented deep percolation or runoff from conventional furrow systems, and which slowly released moisture to the plant. Yield increases may also be restricted where the growing season is shorter, cooler, and wetter.