Modelling Scenarios to Estimate the Potential Impact of Hydrological Standards on Nutrient Retention in the Tobacco Creek Watershed, Manitoba, Canada

Nutrient loading from agricultural drainage systems into downstream aquatic ecosystems, like Lake Winnipeg in the prairie province of Manitoba, Canada, represents a major challenge for water quality management. In order to improve water quality in downstream waterbodies, the Manitoba government is currently investigating the relationship between hydrological standard of agricultural drainage network and nutrient retention in the drainage systems. Briefly, oversized drains have more capacity to transport nutrients, which can increase nutrient loading to downstream waterbodies, especially during rainfall events. Currently, the hydrological standards of agricultural drainage design in Manitoba were mainly developed according to cost-benefit analysis without considering nutrient retention. The purpose of this study was to use computer modelling techniques to simulate the impact of drain size (based on different hydrological standards) on nutrient retention within an agricultural drainage network. The site chosen was the Tobacco Creek Watershed, an agricultural area which drains into the Red River, and thence into Lake Winnipeg. Suspended sediment, nutrient and flow data, from several locations along the Brown drain within this watershed, were used to calibrate a water quality model. Scenarios were then simulated with the model to estimate how different drain sizes affect nutrient transport and retention. Sampling took place during the spring and summer of 2013 starting with freshet and ending when the drains dried up near mid-summer. Study results indicated that the amount of nutrients transported was generally greater during freshet and summer rain storms. Occasionally, however, nutrients in summer discharge exceeded those transported during freshet. The water quality model was applied to the Brown drain to investigate the effects of different drain sizes for rainfall amounts under 2, 5, 10, 15, and 20 year return periods. Generally the results indicate that as the return periods became larger (in larger channels) lower nutrients concentrations were predicted downstream (higher decay rates). On average, the model predicted a 15%–20% decline in nutrient concentration with a 20-year return channel design compared to a 2-year return. The research from this study may provide an impetus to the policy-making process of drainage design.