DAILY AND SEASONAL EVAPOTRANSPIRATION AND YIELD OF IRRIGATED ALFALFA IN SOUTHERN IDAHO

Daily water-use data are needed for the development of modern irrigation scheduling techniques, the optimum allocation of water and energy resources, and improved irrigation management practices. This field study was conducted to measure evapotranspiration (ET) of well-irrigated alfalfa (Medicago sativa L.) in the arid region of southern Idaho. The relationship of ET to forage yield was also investigated. The soil was Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthids) common to much of the region. Daily and seasonal ET data were calculated for seven growing seasons from measurements obtained with mechanical weighing lysimeters equipped with electronic load cells. Daily alfalfa ET was highly variable. It occasionally exceeded 10 mm d-1 and averaged 8 mm d-1 during peak ET periods. From April through October, measured ET averaged 1022 mm for three harvests per season for 5 yr when soil water was nonlimiting. Corresponding average forage yield was 17.6 Mg ha-1 (120 g kg-1 water content) giving an overall water requirement of 58.1 mm (depth equivalent) to produce 1 Mg ha-1 for forage (581 m3 Mg-1) for a water-use efficiency of 17.2 kg ha-1 mm-1. Harvest period and seasonal ET appear linearly related to pan evaporation and forage yield. The actual ET of well-irrigated, high-yielding alfalfa may be as much as 50% greater than previous estimates indicated for southern Idaho.