Modelling of Environmental Flow Regimes for the East Holland River Subwatershed, York Region, Ontario.

Client: Lake Simcoe Region Conservation Authority: Mr. David Lembcke, Manager, Environmental Science and Monitoring, 120 Bayview Pkwy, Box 282, Newmarket, ON L3Y 4X1 (905.895.1281)
Key Personnel: Spencer Malott, E.J. Wexler

The Lake Simcoe Region Conservation Authority (LSRCA) has adopted the Environmental Flow (E-Flow) approach to meet the requirements of the 2009 Lake Simcoe Protection Plan. The objective of this study was to assist the LSRCA in characterizing the E-flow regimes for ten key stream reaches within the East Holland River Subwatershed.

The study involved creating a “cutout” of a larger integrated groundwater/surface water flow model developed for York Region and the City of Toronto. The local model was refined and re-calibrated to better match flows at three streamflow gauges within the East Holland River subwatershed. The focused re-calibration presented many challenges due to the wide hydrologic and hydrogeologic diversity of the study area. The southern upland area of the model consists of the Oak Ridges Moraine, an important recharge feature, which contributes baseflow to headwater streams. The central lowland area of the model consisted of low-recharge clay plains overlain by the highly urbanized areas of Aurora and Newmarket.

The re-calibrated model was then applied to simulate fourteen different scenarios, including different land use, conditions, various implementation levels of low impact development (LID) measures, and future climate change. Earthfx extracted 20-year streamflow hydrographs at the ten-points of interest for E-Flow analysis for each of the fourteen scenarios (see Figure 1).

Land use scenarios consisted of: 1) pre-settlement conditions with all anthropogenic features removed, 2) current land use, and 3) future land use. Water budget and streamflow analyses showed significant changes have occurred between pre-settlement and current conditions, owing largely to the removal of forest cover and the addition of significant impervious area. Changes between current and future land use were relatively small and were generally restricted to local reductions in recharge within the future development areas.

Three scenarios with varying degrees of LID implementation included: 1) 100% retrofitting of urban areas; 2) 50% retrofitting of commercial and institutional areas and 50% of residential roadway right of ways; and 3) 10% retrofitting of large parking lots and 10% of residential roadway right of ways. The LID features were represented in the model by a depression storage reservoir that captured and stored overland runoff and allowed for overflow, evaporation, and direct infiltration to groundwater. All three scenarios had an impact on the water budget and streamflow within the subwatershed. LID features were successful at reducing surface runoff and increasing groundwater recharge, with the scale of the impact related to the scale of the LID implementation (Figure 2).

The long-term performance of the LID features was analysed by post processing each scenario. Regionally, the LID features completely retained about 80% of precipitation and snowmelt events of less than 10 mm, 70% of events between 10 and 25 mm, and 42% of events greater than 25 mm. Runoff retention efficiency was related to the infiltration rate at the site. Sites with higher permeability allowed more infiltration and, in turn, faster replenishment of available storage.

The final model simulations consisted of 10 climate change scenarios. The “change field” approach was selected, which involves calculating mean monthly changes in future climate (temperature and precipitation) based on output from the GCM models. The change fields were generated with a baseline period of 1971-2000 and a future climate period of 2041-2070. Temperature was higher year-round for all climate change scenarios, resulting in higher rates of PET and AET. Precipitation varied by month and by scenario resulting in a range of predicted hydrologic effects. Warmer and wetter winters with more mixed rain-snow events consistently resulted in higher winter streamflows and often an earlier spring freshet. This is illustrated through the average simulated monthly snowmelt shown in Figure 3. Warmer summers also often resulted in less groundwater recharge and lower summer stream baseflow.