Whitemans Creek Tier Three Local Area Water Budget, Risk Assessment, and Drought Impact Study
Client: Grand River Conservation Authority, 400 Clyde Road, Cambridge Ontario
Contact: Stephanie Shifflett, M.Sc., P. Eng.; Water Resources Engineer
Key Personnel: E.J. Wexler, S. Malott, D. Kassenaar
The Whitemans Creek subwatershed is located in southwestern Ontario immediately south of the Region of Waterloo (left). The subwatershed drains an area of approximately 400 km² and discharges to the main branch of the Grand River. Agricultural activity comprises over 75% of the land use in the watershed. The area is also home to a number of Provincially Significant Wetlands (PSW’s) and sensitive coldwater fisheries. This important subwatershed has been the focus of water management studies since the 1950’s, as total permitted irrigation rates (primarily from drought sensitive shallow sand aquifers in the lower watershed) exceed 80,000 m³/d (21,100 Mgal/d).
An integrated groundwater/surface water model of the area surrounding the subwatershed was constructed using the GSFLOW code developed by the U.S. Geological Survey. The model includes several adjacent subwatersheds to capture cross-boundary flows and to simulate cumulative effects of water takings in key aquifers. A detailed geologic conceptual model was developed for the study area (below) incorporating data from the Ontario Geological Survey and was represented in the groundwater submodel at a fine (30 m) scale. The hydrologic submodel represents all hydrologic processes including runoff, ET, soil moisture, and snowmelt at a 60-m resolution. All mapped streams, ponds, and wetlands were represented within the integrated model code. The model was calibrated to match transient streamflow, groundwater, evaporation, and snowpack observations at numerous locations.
Understanding the cumulative effects of pumping, irrigation, drought and groundwater/surface water interaction is central to the agricultural water management in the subwatershed. Assessing all of these complex processes within a single modelling framework is difficult because of the dynamic interaction between climate, soil moisture and irrigation patterns. To address this issue, Earthfx modified the GSFLOW code to simulate demand-driven irrigation based on dynamic soil moisture conditions and crop requirements. Irrigated water, from wells or surface water diversions, is applied to farm parcels by adding the pumped volume above or below the plant canopy based on the irrigation method.
Return flows are simulated directly as overland runoff or interflow and are routed to streams or recharge the groundwater system. Additional modifications were made to PRMS to account for discharge to tile-drains under saturated soil conditions. Irrigation model parameters were calibrated by matching reported water takings for each crop type and subwatershed under average and dry year conditions. Results of the model will be utilized in the future to better manage water allocation under drought conditions.