Modeling effects of contrasting tillage and management on hydrological processes in selected soils of the Pacific Northwest US

Soil erosion by water is detrimental to fertility and crop yield as well as soil biological and physical properties. Soil erosion could be affected by winter precipitation, intermittent freezing and thawing of soils, steep slopes, and improper management practices as well as combination of these factors. The tillage practices play an important role on infiltration, winter runoff and erosion, and seed-zone water storage. Understanding of hydrological processes is crucial to developing land-use and management plans for reducing runoff and erosion and for conserving seed-zone water. Adequate understanding of hydrological processes is also essential to develop models that can serve as effective predictive tools. The objectives were as follows: 1. to assess the suitability of WEPP, a physically-based erosion model with a newly implemented energy-budget-based winter routine, for quantifying field-observed winter processes; 2. to evaluate winter hydrological and erosion processes as affected by two contrasting tillage practices; 3. to assess the effects of Chemical Fallow (CF) and Reduced-tillage Fallow (RT) on seed- and root-zone water and temperature regimes; 4. to test the Simultaneous Heat and Water (SHAW) model's ability to simulate management effects on soil water and temperature distribution. Long-term erosion research plots (2003-07) subject to continuous tilled bare fallow (CTBF), and continuous no-tillage (NT), were established at the USDA-ARS Palouse Conservation Field Station near Pullman, WA. The plots were monitored for runoff, erosion, soil temperature and water content, and depths of snow and freeze-thaw as well as climate data. The study with paired CF and RT treatments was conducted in 2003-04 at the Dryland Research Station at Lind, WA. Field data showed that NT plots generated negligible runoff and erosion compared to CTBF. Frost occurred more frequently and frost depth was deeper in the CTBF compared to the NT. The modified WEPP model could reasonably reproduce major winter processes. Yet it cannot represent all the complicated winter phenomena observed in the field. The RT treatment retained more seed-zone water during the summer compared to CF. In general, soil temperatures in the CF were higher than the RT. Overall, SHAW proved adequate in simulating seed-zone and whole-profile soil water and temperature