Impact of Land Use on Soil Hydrology in Winter

气候变化正在对加拿大许多地区的冬季土壤水文产生深远影响。例如，仲冬融化正变得司空见惯，它可以产生大量的径流(RO)和深排水(DD)的积雪融化或降雨。土地利用和其他因素对仲冬融化期间产生的RO和DD数量的影响尚不清楚。在田间直接测量RO和DD的数量并非易事，尤其是在冬季中期的短期融化期间。通过土壤水文模型估算RO和DD需要测量或至少估算土壤水力特性，即土壤水分特性(SWC)、土壤冻结特性(SFC)和导水率(K)。这三个关键关系最好在田间测量，以准确反映田间土壤水文条件。然而，在生长季节，它们主要是在实验室测量或在田间较少测量，只有在冬季很少测量。特别是，在冻结和解冻条件下，K几乎没有接受过实地调查，也只进行了几项实验室研究。为了弥补这些认识空白，本研究的目标是：(I)确定在田间冻融条件下，常见土地利用方式对土壤SWC、SFC和K的影响；(Ii)将这些特性的时间变异性纳入冬季土壤水文模型中。拟议的研究将在安大略省南部和北部代表两个不同冬季气候区域的实地地点进行，为期五年，以捕捉冬季气候条件的一些年际变化。将在每个地区选择田间地点，以代表当地常见的土地利用做法，如农业行作物(包括耕作和免耕)、草地和森林环境。为了最大限度地提高成功的可能性和广泛采用研究方法，现场使用的仪器将从现有的商业设备中挑选出来。在冻融条件下，要测量土壤的SWC、SFC和K，必须测量土壤的温度、含水率、势能和土壤水分通量。市场上有很多传感器可以同时测量土壤温度和水分，但在冰冻和融化条件下测量土壤水势更具挑战性。传统上，充水张力计被用来测量土壤水势，但它们只在未冻结的土壤条件下工作；然而，基于介质的传感器现在可以在未冻结和冻结条件下工作。这项研究的创新之处在于：(I)在冻结和解冻过程中尚未在野外测量到K；(Ii)滞后现象，尽管在以前的研究中注意到在冻结和解冻过程中会发生，但还没有被广泛地利用野外数据进行研究，特别是K；(Ii)它将开发新的模型，将SWC、SFC和K的时间变异性包括在内，作为改进冬季土壤水文模型的第一步。这项研究的直接影响是提高仲冬融化期间洪水预报的准确性，有助于解释解释春季融化期间温室气体产生激增的机制，从定量和定性的角度加强对水源(地表水和地下水)的保护，并为制定适应气候变化的作物管理战略提供指导。今后的工作将侧重于纳入这些现象的空间变异性和放大，以改进流域尺度冬季水文学的模拟。

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.