Amount of Profile Water in Early Spring Resulting from Increased Profile Water in Fall

Cost and availability of fuel, water supplies, and labor are creating interest in extending irrigation into the dormant season. In this project we assessed the amount of profile water in early spring that resulted from increased profile water in fall at three locations where soil, climate, and limited irrigation resources suggest fall irrigation has potential as a management practice. Soils were Ulysses silt loam (fine-silty, mixed, mesic Aridic Haplustoll), Amsterdam silty clay loam (fine-silty, mixed Typic Cryoboroll), and Great Bend silt loam (fine-silty, mixed Udic Haploboroll) at Tribune, Kans., Bozeman, Mont., and Redfield, S. Dak., respectively. Crops were corn (Zea mays L.) and winter wheat (Triticum aestivum L.) at Tribune; alfalfa (Medicago sativa L.) and spring wheat (Triticum aestivum L.) at Bozeman; and alfalfa, corn, spring wheat, and winter wheat at Redfield. Differences in crop growth patterns and irrigation regimes provided a broad range of fall soil water contents at each location. Profile soil water content was sampled gravimetrically at Tribune and by neutron attenuation at Bozeman and Redfield no earlier than 10 d after fall irrigation and in early spring. By multiple regression analysis, we established that profile water content change rate during winter (profile water at early-spring sampling minus profile water at fall sampling divided by the number of days) decreased as winter precipitation rate decreased and as available soil water in fall increased. The amounts of profile water in early spring resulting from 10 mm of additional fall profile water, calculated by using the long-term precipitation mean for 22 December through 31 March at each location (0.44, 0.54, and 0.52 mm/d for Tribune, Bozeman, and Redfield, respectively) were: 8.6, 6.8, and 5.0 mm at Tribune; 8.5, 7.5, and 6.6 mm at Bozeman; and 8.2, 7.0, and 5.8 mm at Redfield for available soil water values in fall of 30, 50, and 70%, respectively. Our data and calculations show fall irrigation to be an inefficient practice for many situations, even where the antecedent profile water content is well below the “field capacity” value.