Deficit Irrigation of Alfalfa for Water-Savings in the Great Plains and Intermountain West: A Review and Analysis of the Literature

Diversions of water from irrigated agriculture are occurring in the western United States to address increasing municipal and industrial demands. Deficit irrigation of alfalfa (Medicago sativa L.) could be a source of water without complete dry-up of irrigated fields. Water saving potential from alfalfa is high because it is a high water-use crop produced on 12% of the irrigated land in the United States. The objectives of this paper are to review alfalfa plant-water relations in the Great Plains and Inter-mountain West, to understand potential water savings through deficit irrigation, and to indentify management practices that maximize water-use efficiency (WUE). Alfalfa biomass yield exhibits a linear relationship to evapotranspiration (ET) with the slope of a regionally aggregated water production function of 0.16 Mg ha(-1) cm(-1). Relative ET declines 30% faster than relative biomass yield under deficit irrigation or dryland management. Because early season harvests have greater WUE, combining full irrigation in spring with no irrigation during less efficient water-use growth periods may be more effective in saving water than season-long deficit irrigation. Management practices that can influence WUE under deficit irrigation include stand age, growth stage at harvest, and alfalfa variety. A potential complication with controlled deficit irrigation of alfalfa is an uncertain contribution to ET from a water table. As alfalfa roots develop over time, a significant percentage of total ET can come from water tables shallower than 200 cm and the percentage increases as availability of water from precipitation or irrigation declines.