Irrigation Scheduling with Planned Soil Water Depletion

Abstract

A two-year study was initiated in the spring of 1990 on a Keith silt loam soil (Aridic Argiustoll) in northwest Kansas to determine if irrigation scheduling with planned soil water depletion could be used successfully for irrigated corn (Zea mays L.) as a method of conserving and protecting groundwater resources without reducing yields. The study was conducted using surface irrigation in small dead-level basins. Planned soil water depletion was attempted by allowing a small additional daily deficit (0, 1, or 2 mm/day) to accumulate in irrigation amounts as scheduled by an evapotranspiration (ET)-based water budget. The daily deficit amounts were imposed on three irrigation levels, heavy (1.25 ET), normal (1.00 ET), and deficit (0.75 ET) which represented a range of management by irrigators. The plant-available soil water at physiological maturity was related linearly to irrigation amounts. However, the plant-available soil water at physiological maturity was reduced by only 25 mm for each 100 mm reduction in irrigation. Imposition of a small daily deficit of 1 mm/day after tasseling resulted in yield reductions of 7, 1, and 3% for the heavy, normal, and deficit irrigation management levels, respectively. The 1 mm/day deficit resulted in irrigation savings of approximately 12, 9, and 0% for the three respective irrigation management levels and generally resulted in slight reductions in available soil water at physiological maturity. In some cases, the imposition of the 1 mm/day deficit had little effect on the total seasonal irrigation amount, but simply shifted the irrigation event to a later date. The larger 2 mm/day daily deficit after tasseling reduced yields by 7, 9, and 15% for the three respective irrigation levels and reduced irrigation amounts by 19, 26, and 25%. Yields were related linearly to irrigation and water use with a reduction in irrigation or water use reflected by yield reductions. Water use efficiencies were similar whether planned soil water depletion was used or not. Therefore, from a water conservation standpoint, irrigation scheduling with planned soil water depletion was not justified.