Modifying the Soil Temperature Module in SWAT for Application in Atlantic Canada: Module Development, Validation and Impacts on Watershed Modelling

Abstract:

Accurate estimates of soil temperature are of particular importance in describing many hydrological and biological processes. Soil temperature predictions in the popular hydrological model, Soil and Water Assessment Tool (SWAT), are largely incorrect when applied to regions with significant snow cover in winter. In this study, a new physically-based formulation of soil temperature is developed as an alternative to the empirical soil temperature module currently used in SWAT. The physically-based formulation simulates soil temperature in different soil layers as a result of energy transfer amongst the atmosphere, snow, and soil layers. With the new soil temperature formulation, the only additional inputs for the modified SWAT are three new parameters, which need to be calibrated. Both the original and modified versions of SWAT are tested against field data collected from the Black Brook Watershed, a small watershed in Atlantic Canada. The results indicate that both versions of SWAT are able to provide acceptable predictions of temperature in different soil layers during the non-winter period of the year. However, the original SWAT severely underestimates soil temperatures in winter (within a range of -10 to -20˚C), while the new version produces results that are more consistent with field-based temperatures (within a range of -2 to 2˚C). Furthermore, water discharges, sediment and nutrient loadings estimated using the modified SWAT was compared against the original SWAT and field measurements for the same watershed. The results demonstrates that modified SWAT enhances the modelling accuracies on baseflow discharge, sediment, NO₃-N and Sol-P loadings in the watershed because the new soil temperature module improves soil temperature simulation accuracy in winter. Moreover, this study also investigates the differences between the original and modified SWAT in determining water flow paths and nutrients fates in the watershed. Compared with the original SWAT, the modified SWAT predicts less surface runoff, lower soil moisture content but more percolation and lateral flow, especially for the winter period. Similarly, the modified SWAT predicts less nitrate loading with surface runoff while more nitrate loading with lateral flow and percolation in winter but less in other seasons.