Impact of Land Use on Soil Hydrology in Winter

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Climate change is having a profound impact on winter-season soil hydrology in many regions of Canada. For instance, mid-winter thaws, which are becoming commonplace, can generate a significant amount of runoff (RO) and deep drainage (DD) of snowmelt or rain. The impacts of land use and other factors on the amounts of RO and DD generated during mid-winter thaws are not well understood. Measuring the amounts of RO and DD directly in the field is not straightforward especially during short-term mid-winter thaws. And estimating RO and DD through soil hydrology modelling requires measurements or at least estimates of soil hydraulic properties, namely the soil water characteristic (SWC), soil freezing characteristic (SFC) and hydraulic conductivity (K). These three key relationships are best measured in the field to accurately reflect field soil hydrological conditions. Nevertheless, they have mainly been measured in the lab or less frequently in the field during the growing season and only very rarely during the winter season. In particular, K has received very little, if any, investigation in the field during freezing and thawing conditions and only a few laboratory studies have been conducted. To address these knowledge gaps, the objectives of this research are: (i) to determine the impacts of common land use practices on SWC, SFC and K of soils during freezing and thawing conditions in the field and (ii) to incorporate temporal variability of these properties into modeling of soil hydrology in winter. The proposed research will take place at field sites in southern and northern Ontario representing two different winter-season climatic regions for a five-year period to capture some year-to-year variability in winter climatic conditions. Field sites will be chosen in each region to represent common, local land-use practices such as agricultural row crop (including tillage and no-tillage), grass, and forest settings. To maximize the likelihood of success and widespread adoption of the research methods, the instrumentation used in the field will be selected from existing, commercially available equipment. To measure SWC, SFC and K under freezing and thawing conditions, soil temperature, water content, potential, and soil water flux must be measured. There are numerous commercially available sensors that measure both soil temperature and water content, but measuring soil water potential under freezing and thawing conditions is more challenging. Traditionally, water-filled tensiometers are used to measure soil water potential but they only operate under unfrozen soil conditions; however, dielectric-based sensors are now available that operate under both unfrozen and frozen conditions. The proposed research is novel in the following ways: (i) K has not been measured in the field under temporally-variable freezing and thawing processes; (ii) hysteresis, although noted to occur during freezing and thawing in previous studies, has not been investigated extensively using field data, especially for K and (ii) it will develop new models to include temporal variability in SWC, SFC and K as a first step in improving winter-season soil hydrology models. Immediate implications of this research are in improving accuracy of flood forecasting during mid-winter thaws, aid in the interpretation of mechanisms to explain spikes in greenhouse gas production during spring thaw periods, enhance source water (both surface and groundwater) protection from quantitative and qualitative perspectives, and provide guidelines for developing crop management strategies for climate change adaptation. Future work will then focus on incorporating spatial variability and scaling-up of these phenomena to improve modeling of watershed-scale winter season hydrology.