DEVELOPMENT OF THE ROOT ZONE WATER QUALITY MODEL (RZWQM) FOR OVER-WINTER CONDITIONS

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Soil temperature and water conditions through the winter and early spring drive many important physical,chemical and biological processes. Impacts of management practices on these complex processes are often difficult topredict. The primary objective of this study was to incorporate routines for snow, soil heat, and soil freezing from theSimultaneous Heat and Water (SHAW) model into the process based RZWQM to extend its applicability to winterconditions. Routines from SHAW for simulating transfer through flat and standing residue layers were also included. TheRZWQMs solution of the Richards equation was retained, making it necessary to decouple the SHAW modelssimultaneous solution of the heat and water equations. The modified RZWQM was applied to varying tillage and residueconditions using data from Pullman, Washington, and Akron, Colorado, and compared to the original SHAW model.Statistical comparisons indicated that the two models simulated soil temperature similarly for most plots, showingsuccessful implementation of the SHAW routines. Differences in simulated soil temperatures and ice contents between thetwo models were related to differences in computing soil water transfer and solution of Richards equation. Modelefficiency for soil temperature simulated by the modified RZWQM, defined as the fraction of variability in measuredtemperature accounted for by the model, ranged from 0.71 to 0.92 within the top 25-cm at the Pullman site; simulatedsnow and soil frost depths were similar to previous simulations from the SHAW model. Model efficiency for simulatedtemperature at the Akron sites ranged from 0.87 to 0.98. Dynamic response of soil water potential was simulatedreasonably well, with model efficiencies ranging from 0.61 to 0.86 for the Akron site. This modified version of theRZWQM, that includes frozen soil and boundary conditions representative of varying surface conditions, makes the modelmore responsive to management of soil and water resources in northern latitudes.