Development of the root zone water quality model (RZWQM) for over-winter conditions

Soil temperature and water conditions through the winter and early spring d rive many important physical, chemical and biological processes. Impacts of management practices on these complex processes are often difficult to pre dict. The primary objective of this study was to incorporate routines for s now, soil heat, and soil freezing from the Simultaneous Heat and Water (SHA W) model into the process based RZWQM to extend its applicability to winter conditions. Routines from SHAW for simulating transfer through flat and st anding residue layers were also included. The RZWQM's solution of the Richa rds' equation was retained, making it necessary to decouple the SHAW model' s simultaneous solution of the heat and water equations. The modified RZWQM was applied to varying tillage and residue conditions using data from Pull man, Washington, and Akron, Colorado, and compared to the original SHAW mod el. Statistical comparisons indicated that the two models simulated soil te mperature similarly for most plots, showing successful implementation of th e SHAW routines. Differences in simulated soil temperatures and ice content s between the two models were related to differences in computing soil wate r transfer and solution of Richards' equation. Model efficiency for soil te mperature simulated by the modified RZWQM, defined as the fraction of varia bility in measured temperature accounted for by the model, ranged form 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. Mode l 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 ranging from 0.61 to 0.86 for the Akron site. This modified version of the RZWQM, that includes frozen soil and boundary conditions representative of varying surface conditions, makes the model m ore responsive to management of soil and water resources in northern latitu des.