Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 1, 1999
Publication Date: N/A
Interpretive Summary: Effective agricultural management requires an understanding of complex interactions between chemical, physical, hydrological, biological and meteorological processes. Land managers need to address these important interactions in the near surface, but lack the means necessary. An efficient way to evaluate the effectiveness of different management strategies is with the use of modeling tools. The Root Zone Water Quality Model (RZWQM) was developed to help manage the use of agrochemicals and tillage/no-tillage practices by assessing environmental impacts of alternative management strategies. However, snow and soil freezing components were not included in the first RZWQM model, which has restricted its application to non-winter periods. One of the more detailed models of snow and freezing soil is the Simultaneous Heat and Water (SHAW) Model. The primary objective of this study was to couple the RZWQM with winter routines of snow and soil freezing from the SHAW model. The model was applied to sites with varying residue configurations to illustrate the ability of the model to simulate soil temperature, snow depth, and soil frost. Comparisons of model simulations from SHAW and the modified RZWQM indicated that the two models simulated soil temperature similarly, showing successful implementation of the SHAW routines. This modified version of the RZWQM, that includes frozen soil and boundary conditions more representative of varying surface conditions, makes the model more responsive to management of soil and water resources in northern latitudes where snow and frozen soils can influence management decisions.
Technical Abstract: Temperature and soil water conditions through the winter and early spring drive many important physical, chemical and biological processes. Impacts of management practices on the interactions of these complex processes are often difficult to predict. The primary objective of this study was to incorporate routines for snow, soil heat, and soil freezing from the Simultaneous Heat and Water (SHAW) model into the RZWQM to extend its applicability to winter conditions. The RZWQM's solution of the Richard's equation was retained, making it necessary to decouple the SHAW model's simultaneous solution of the heat and water equations. The modified RZWQM and the SHAW model were applied to varying tillage and residue conditions using data from Pullman, Washington and Akron, Colorado. Statistical comparisons indicated that the two models simulated soil temperature similarly for most plots, showing successful implementation of the SHAW routines. Model efficiency for soil temperature simulated by the modified RZWQM, defined as the fraction of variability in measured temperature accounted for by the model, ranged from 0.71 to 0.92 within the top 25 cm at the Pullman site; simulated snow and soil frost depths were similar to previous simulations from the SHAW model. Model efficiency for simulated temperature at the Akron sites ranged from 0.87 to 0.98. Dynamic response of soil water potential was simulated reasonably well, with model efficiencies for the RZWQM ranging from 0.61 to 0.86. This modified version of the RZWQM, that includes frozen soil and boundary conditions more representative of varying surface conditions, makes the model more responsive to management of soil and water resources in northern latitudes.