1. Hydrologic modeling of conservation farming practices on the Palouse Joshua Van Wie a, Jennifer Adam a, Jeff Ullmanb a Department of Civil and Environmental Engineering, b Department of Biological Systems Engineering, Washington State University AGU Fall Conference 2009 Introduction Conservation of water is essential to successful dryland farming in the Palouse region. The Palouse is under the combined stresses of scarcity and timing, receiving an average of only 22 inches of precipitation annually with 60 percent occurring between November and March when crops require a minimal amount of moisture. Water is the most limiting factor to crop yield in most years (Pannkuk, 1998), creating a strong necessity to capture and store winter precipitation in the soil. Conservation techniques leave a greater amount of crop residue on the surface after harvest that provides a physical barrier against runoff and evaporation (Fuentes, 2003) in addition to increasing infiltration by regulating soil temperature during cold weather (Singh, 2009). The purpose of this study is to investigate no-till methods and to simulate the over-winter water savings that may occur under widespread adoption of these methods using hydrologic modeling. Methods Tillage effects are represented in DHSVM by adjusting soil and vegetation parameters according to the intensity of tillage. The model is calibrated and verified using field scale runoff data from land under conventional and no-till management. Results Calibration at the basin scale without considering tillage effects over a 10-year period resulted in a Nash-Sutcliffe coefficient of 0.76 and a volume error of 2.3%. Field scale calibration results will be used in watershed scale simulations to quantify the widespread impacts of no-till adoption. Soil moisture in the root zone at the beginning of the growing season will be compared for conventional and no-till. Increased retention of water through the winter months and a higher soil water content at the beginning of the growing season may have the potential to increase crop yields in this arid region. Frozen Soil Surface residue insulates the soil during the winter resulting in fewer soil freezes that contribute to increased runoff through the reduction of hydraulic conductivity. An algorithm is added to DHSVM that accounts for the insulating effect of residue and by determining hydraulic conductivity separately for conventional and no-till fields. Soil Physical Properties Tillage implements disrupt the soil, altering hydraulic and structural properties in addition to incorporating residue. The effect of porosity on basin streamflow is shown: • Conventional tillage ρ = 0.50 • No-till ρ = 0.40 • Model • The Distributed Hydrology Soil Vegetation Model (DHSVM) (Wigmosta et al. 1994) is applied for this research. • The model is first calibrated and validated at the field scale and then at the watershed scale. • Model inputs include soil, vegetation, and elevation data along with a stream network and climate forcing data. Conclusions • DHSVM has the potential to distinguish some of the key differences between the impacts of no-till farming and conventional methods through the adjusting of soil and vegetation model parameters. • A considerable amount of water may be retained in the seed-zone during the winter months by decreasing evaporation and runoff through the correct use no-till farming and residue management. • Contact • Joshua Van Wie • Dept. of Civil and Environmental Engineering • PO Box 642910, Pullman, WA 99164-2910 • joshvanwie@hotmail.com Evaporation Crop residue provides a physical barrier against evaporation and significantly reduces evaporation losses compared with bare soil. Evaporation is reduced by a factor based on the mass of plant residue remaining on the soil after harvest. Stream network Vegetation Elevation