Tillage Effects on Fallow Water Storage in the Eastern Washington Dryland Region

Early stand establishment of winter wheat (Triticum aestivum L.) is essential for efficient production and erosion control in much of the northwest U.S. dryland wheat region. Stand success depends on efficient storage and retention of winter precipitation and thus, on soil management practices which maximize infiltration and minimize evaporative loss during fallow. In this study fall chiseling (25 cm deep) and disking (13 cm deep) were pared with no tillage with respect to the effect on overwinter water storage in the low precipitation (24 cm annually) region of eastern Washington. Tillage effects were evaluated during a mild winter with minimal soil freezing and during a cold winter with runoff conditions from rain and snowmelt on frozen soil. Three spring tillages, sweep cultivation (13 or 20 cm deep) or disking (13 cm deep), were superimposed on each of the fall tillages to evaluate effects on oversummer soil water retention, both deep in the profile and in the seed zone. In addition, these tillages were compared with a chemical fallow treatment with no tillage. During the cold winter, fall chiseling increased water storage markedly and disking to a lesser extent as compared with no tillage (8.7 cm increase' with chiseling, 2.3 cm with disking), but tillage had no effect during the mild winter. September to May storage as percentage of precipitation for this period was 87, 60, and 50% for chiseling, disking, and no tillage, respectively, for the cold winter, and 62% irrespective of tillage during the mild winter. The increase in water stored during the cold winter by chiseling was attributed to improved infiltration properties of the frozen layer associated with this tillage. Storage efficiency with chiseling was greater during the cold winter as compared with the mild winter largely because of differences in the nature of the precipitation. In the mild winter most precipitation fell as intermittent showers, whereas in the cold winter precipitation events were fewer but each was greater in amount. Oversummer soil water loss (May to September) was not influenced by type of spring tillage; amounts lost ranged from 4 to 9% of the total profile water stored in May. Although oversummer loss increased with increases in total stored water, after the cold winter the amount of stored water in the profile at seeding time was still considerably greater for the plots which received fall tillage. Seed zone water at the end of fallow was generally increased with increase in total profile water content, and was higher where spring tillage was used as compared with chemical fallow.