Characterizing ground water use by safflower using weighing lysimeters

Decades of irrigation on the west side of the San Joaquin Valley without sufficient drainage have created large areas where shallow ground water (<1.5 in) has become a problem for agriculture. Because drainage outflow is restricted as a result of environmental concerns, reducing the amount of irrigation applied is a farm management solution for this situation. One option to reduce the amount of irrigation water is to include shallow ground water use as a source of water for crop production when scheduling irrigation. The objective for this study is to describe soil water fluxes in the presence of saline, shallow ground water under a safflower crop. Two weighing lysimeters, one with and one without shallow saline ground water were used to measure crop evapotranspiration of surface drip irrigated safflower. A saline water table (14 dS/m) was maintained in one of the lysimeters. Ground water use as part of crop evapotranspiration was characterized using hourly measurements of the water level in a ground water supply tank (Mariotte bottle). Ground water contribution of up to 40% of daily crop water use was measured. On a seasonal basis, 25% of the total crop water use originated from the ground water. The largest ground water contribution was shown to occur at the end of the growing season, when roots are fully developed and stored soil water in the root zone was depleted. The applied irrigation on the crop grown in the presence of a water table was 46% less than irrigation applied to the crop without a water table. The reduction of irrigation was obtained by using the same irrigation schedule as on the lysimeter without ground water, but through smaller applied depths per irrigation event. (C) 2002 Elsevier Science B.V. All rights reserved.