Fallow Tillage Effects on Evaporation and Seedzone Water Content in a Dry Summer Climate

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Abstract
Early fall establishment of winter wheat is essential for high yields and erosion control following fallow in the Pacific Northwest, USA. Successful early stands require tillage management that minimizes water loss from the seedzone during the relatively dry summer. A numerical model was developed to investigate the dynamic aspects of heat and water flow in the upper soil layers with emphasis on effects of conventional tillage fallow and no‐till (chemical fallow) on these processes and ultimately the seedzone water content. After 15‐day simulations used to verify the model with measured values of meteorological variables and soil properties, predicted and measured water content distributions across the seedzone generally agreed within 0.01 kg/kg. Exclusion of thermally induced vapor flow from the model gave a higher predicted evaporative water loss and poorer agreement between measured and simulated soil water content distributions. The values for characterizing the atmospheric boundary layer resistance were also critical for correctly simulating soil temperature and water content profiles. Evaporative water loss for no‐till over a 90‐day simulation period using measured meteorological inputs was 70% higher than for conventional tillage, resulting in water contents in the seedzone that were too low for successful stand establishment under chemical fallow. Thus, for the climatic conditions and the soil studied here, some tillage may be necessary for retention of adequate seedzone water for early fall‐winter wheat establishment.