Hydrologic modeling of conventional and conservation farming practices on the Palouse

Joshua Bernard Van Wie

The production of dryland crops, such as spring and winter wheat, in a semi-arid region requires a reliable and adequate water supply. This supply of water available for crop use is of heightened importance in areas such as the Palouse region where the majority of annual rainfall occurs during the winter months and must be retained in the soil through the dry summer growing season. Farmers can increase water conservation at the field and watershed scales through the adoption of best management practices that incorporate tillage practices and crop residue management. This research analyzed conventional tillage (CT) and no-till (NT) cropping practices by representing them in a watershed-scale hydrologic model in order to determine whether either practice would effectively increase water storage prior to the growing season. The Distributed Hydrology Soil Vegetation Model (DHSVM) was applied and calibrated to represent the physical changes to infiltration, evaporation, and runoff that result from changes in the amount of residual crop cover after harvest brought on by management practices. The model was calibrated with field observations at (1) the basin scale using streamflow observations followed by (2) the field scale using runoff observations from individual plots that are under CT and NT management. DHSVM was calibrated to produce a Nash-Sutcliffe (N.S.) model efficiency of 0.69, which falls within the range of efficiencies for DHSVM reported in literature (0.57 - 0.91). By adjusting transmittance factors and albedo for crop residue versus bare ground, the model was modified to incorporate the effects of CT and NT practices on the surface energy balance and infiltration and runoff on frozen soil. DHSVM was found to predict surface temperature with an N.S. efficiency of 0.60, and the model was able to predict the soil state, either frozen or unfrozen, 81% of the time. In order to simulate evaporation dynamics CT was modeled as bare soil while NT is modeled as a dead-vegetation understory. It was found that modeled soil moisture was approximately 50% lower during the majority of the winter months in CT management than NT. Predicted volumetric soil moisture content on April 1st was 0.29 in conventionally tilled farmland, while no-till farmland had a moisture content of 0.34. This difference in winter and spring soil moisture was caused primarily by decreased evaporation under NT, with minimal effects of decreased infiltration into frozen ground. Two methods of crop yield estimation indicated that the increased spring soil moisture of NT may result a 19% increase in wheat yield. It was concluded that NT farming has the potential to increase the capture and retention of winter precipitation in the root-zone that may result in higher yields of spring and winter wheat in the Palouse. Furthermore, DHSVM was found to be suitable for investigating regional agricultural management issues and demonstrates the potential to address additional scientific questions pertaining to sustainable farming.

Hydrologic modeling of conventional and conservation farming practices on the Palouse

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