Impact of Land Use on Soil Hydrology in Winter

气候变化对加拿大许多地区的冬季土壤水文产生了深远的影响。例如，冬季中期的解冻变得越来越普遍，可以产生大量的径流（RO）和深度排水（DD）的融雪或雨水。土地利用和其他因素对冬中期融雪期间产生的RO和DD量的影响尚不清楚。在田间直接测量RO和DD的数量并不简单，特别是在冬季中期的短期解冻期间。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）和水力传导性(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，它们主要是在实验室测量的，在生长季节在田间测量的频率较低，在冬季很少测量。特别是，K在冻结和解冻条件下很少接受实地调查，只有很少的实验室研究进行了。为了解决这些知识空白，本研究的目标是：(i)确定在田间冻结和融化条件下，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化纳入冬季土壤水文模型。拟议的研究将在安大略省南部和北部两个不同冬季气候区域的实地进行，为期五年，以捕捉冬季气候条件的一些年际变化。将在每个地区选择代表常见的当地土地利用做法的场地，如农业行作物（包括耕作和免耕作）、草地和森林环境。为了最大限度地提高研究方法成功和广泛采用的可能性，该领域使用的仪器将从现有的商业设备中选择。为了测量冻融条件下的SWC、SFC和K，必须测量土壤温度、含水量、电位和土壤水通量。市面上有许多测量土壤温度和含水量的传感器，但在冻结和解冻条件下测量土壤水势更具挑战性。传统上，充水张力计用于测量土壤水势，但它们只能在未冻结的土壤条件下工作；然而，基于电介质的传感器现在可以在非冷冻和冷冻条件下工作。提出的研究在以下方面是新颖的：(i)在时间可变的冻结和解冻过程下，K没有在现场测量；（ii）迟滞，虽然在以前的研究中注意到在冻结和融化期间发生，但尚未使用现场数据进行广泛调查，特别是对K。（ii）它将开发新的模型，包括SWC， SFC和K的时间变化，作为改进冬季土壤水文模型的第一步。本研究的直接意义在于提高冬季中期融雪期间洪水预报的准确性，帮助解释春季融雪期间温室气体产生峰值的机制，从定量和定性角度加强对水源（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策略提供指导。未来的工作将集中于纳入这些现象的空间变异性和按比例放大，以改进流域尺度冬季水文的建模。

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.