Soil water and temperature in chemical versus reduced-tillage fallow in a mediterranean climate

A 2-year rotation of winter wheat (Triticum aestivum L.)-summer fallow is a dominant cropping system in the dryland region of the Pacific Northwest United States. Traditional, tillage-based summer fallow relies on a soil mulch to disrupt capillary continuity to conserve seed-zone water for early establishment of winter wheat. However, tillage to create the soil mulch and to subsequently fertilize and control weeds often results in unacceptable levels of wind erosion due to the burial of crop residues and the exposure of fine soil particles. Chemical (no-till) fallow (CF) and reduced-tillage fallow (RT) are two alternatives for reducing wind erosion. Our objectives were: (i) to assess the effects of CF and RT on seed- and root-zone temperature and water regimes; and (ii) to test the Simultaneous Heat and Water (SHAW) model for simulating management effects on soil temperature and water. Weather data, soil temperature, and water content were monitored in paired CF and RT treatments during April 2003-March 2004. The RT treatment was observed to retain more seed-zone water over summer compared to CF, consistent with relevant literature for Mediterranean environments and of critical importance to farmers. During the wet winter, CF gained more water than RT because of later planting of winter wheat, and thus less water use. Observed soil temperatures were higher in the CF due to its lower dry soil albedo, higher bulk density and thermal diffusivity than in the RT. SHAW-simulated water contents followed the general trend of the field data, though it slightly under-predicted soil water content for CF and over-predicted for RT. SHAW under-predicted soil temperature during the dry summer and over-predicted for the wet (November-December) period yet the overall trend was properly described with differences between simulations and observations decreasing with soil depth. Overall, SHAW proved adequate in simulating seed-zone and whole-profile soil water and temperature, and therefore may serve as a useful modeling tool for tillage and residue management.