Modeling Seasonal Risk of Deep Drainage for Different Regions of Ontario: Implications for Source Protection Guideline Development
University of Guelph
Deep drainage (DD) is the main driver for leaching and potential groundwater contamination from non-point sources such as farm fields; therefore, DD should be considered when developing source water protection strategies across a large geographic area such as the Province of Ontario, Canada. To investigate this issue, the DD for 12 different regions of Ontario was estimated using the model DRAINMOD. The model was run on a daily basis using climate data from each region over the 1954-2001 time period. Soil textures selected to represent each region ranged from sand to clay, and along with large differences in climatic regime between regions, resulted in a large range of estimated average annual DD from about 20 mm to over 300 mm. The month with the greatest risk of drainage across all regions was April. In Ontario this is the month with the most snowmelt and, in general, soil conditions are changing from frozen to thawed, leading to a high probability of wet soil conditions and drainage. The two regions with the greatest probability of drainage occurring on a single day over the 48-year model period had the greatest amount of snowfall and the coarsest-textured (sand) soil. For the region with the sandy soil the greatest probability of drainage of 0.94 occurs on Apr. 6 for any given year. For the region with the highest snowfall, the greatest probability of drainage of 0.96 occurs on both Apr. 11 and Apr. 16 for any given year. In terms of guiding policy for source water protection, there are two main issues that arose from this study beyond the spring-season timing of the greatest risk of deep drainage. First of all, some regions have peak risk as low as 0.17 and most regions have peak risk less than 0.5. Although soil texture does influence DD probability, it appears that climatic differences between regions have an even greater impact. In summary, the model estimates of DD from this study indicate that policy on source water protection in relation to the amount and timing of DD should not be developed at a Province-wide scale across Ontario; including regional climatic variability and to a lesser extent different soil textures should be considered when developing source water protection strategies.