ClimateChange Assessment Guide.pdf - University of Waterloo

E-22Guide for Assessment of Hydrological Effects of Climate Change in Ontario10.0SimulatedObservedm3/s1.00.10 10 20 30 40 50 60 70 80 90 100% of time flow is less thanFigure 3-8: Comparison of flow frequency distributions of simulated and observed streamflow forSubwatershed 19, 2001-2004.The correspondence in the long and short term confirms the model’s reliability in simulating a wide variety ofconditions and the overall water balance. In this calibrated form, the model results can be used for scenariocomparisons and sensitivity type assessments wherein the simulated absolute instream flow rates are secondary tothe overall sensitivity of the system to relative change. The estimation of water balance terms (i.e., groundwaterrecharge, runoff and evapotranspiration) are applications of the model that can be made with more accuracy thanshort-term event-based conditions. The MODFLOW grid encompasses Subwatershed 19 within CVC, as well as portions of the Humber RiverWatershed, Nottawasaga River Watershed and the Grand River Watershed (Figure 1-5). The model domain isapproximately 17 km in a north-south direction, and 18 km east-west, resulting in a model area of approximately306 km 2 .The model domain was designed to encompass the entire upper Subwatershed 19 area, and to extend to the naturalboundaries of the groundwater flow system. The model domain lies largely outside the subwatershed boundary tominimize potential boundary effects on model predictions within the study area.The finite difference grid has a total of 415 rows and 405 columns within the model area for a total of 168,075 cells.The model subdivides the subsurface vertically into 9 layers; each layer generally represents a hydrostratigraphic unitsuch as a sand aquifer, or a till aquitard. The top and bottom elevation of each layer are assigned in MODFLOW toeach of the model cells. The hydrostratigraphic layers are assigned to be consistent with the regional conceptualhydrogeologic model and represent three overburden layers and five bedrock layers.The HSP-F model was calibrated to estimate recharge rates (specified flux) for input into the top layer of theMODFLOW model. HSP-F calibration efforts were focused on the low flow regime to be able to better constrain thegroundwater recharge estimates. The resulting calibrated average annual recharge estimates ranged from a low of0 mm/yr where groundwater discharges to some wetlands to a high of 317 mm/yr on the Orangeville Moraine.