Precipitation Storage Efficiency During Fallow in Wheat-Fallow Systems.

Wheat-fallow production systems arose in order to stabilize widely ranging wheat yields that resulted from highly variable precipitation in the Great Plains. Historically, precipitation storage efficiency (PSE) over the fallow period increased over time as inversion tillage systems used for weed control were replaced first with less drastic tillage and later with no-till systems that employed herbicides for weed control. The objectives of this study were to compare PSE in conventionally tilled (CT) and no-till (NT) wheat-fallow systems over 10 seasons at Akron, CO against previously published values of fallow PSE and to identify meteorological conditions that may be influencing PSE. Soil water measurements were made at four times during each fallow period: following wheat harvest, and about 1 October, 1 May, and 1 October to divide the fallow season into three periods (first summer, fall-winter-spring, second summer). Precipitation was measured in the plot area and other meteorological conditions were measured at a weather station about 350 m from the plot area. Fallow PSE averaged 20% (range 8-34%) for CT and 35% (range 20-51%) for NT, much lower than previously reported for NT at Akron. PSE was not different between the two systems during the second summer fallow period. The largest PSE difference between the two systems was seen during the fall-winter-spring period (32 vs. 81%). Fallow soil water increased an average of 111 mm under CT and 188 mm under NT. PSE during the three fallow periods was related to tillage, precipitation, air temperature, and wind speed, but sometimes counter-intuitively. A simple linear regression using inputs of tillage system, percentage of fallow precipitation events with amounts between 5 and 15 mm, and percentage of fallow precipitation events with amounts greater than 25 mm can be used to accurately estimate PSE and fallow period water storage in this region.