APPENDIX G HYDROLOGY IMPACT ASSESSMENT MODEL

Transcription

1 APPENDIX G HYDROLOGY IMPACT ASSESSMENT MODEL

2 Hydrology Impact Assessment Model Prepared for: Town of Halton Hills Prepared by: SCHROETER & ASSOCIATES January 24, 2003

3 SUMMARY Schroeter & Associates undertook this study for Credit Valley Conservation to determine the hydrologic characteristics of the Silver Creek Subwatershed (Subwatershed 11). A hydrologic model of the Black and Silver Creek watershed (Subwatersheds 10 and 11) was developed using the GAWSER (Guelph All-Weather Storm-Event Runoff) model. In total, this model has 18 subcatchments, 10 channel routing reaches, one reservoir (Fairly Lake), and two sewage treatment plants. A distinct feature of this model is its accounting for hummocky topography, and interactions between the surface and groundwater flow systems caused by the unique geology and topography of the Black and Silver Creeks. The monthly parameter adjustment table established in recent applications of GAWSER for Credit River watersheds was used directly in this study. The use of this parameter adjustment table was confirmed through additional performance testing by comparing model output with daily flows at two Water Survey of Canada gauges (Black Creek below Acton 02HB024, and Credit River West Branch at Norval, 02HB008) for the period November 1988 and October The model was further validated using hourly flows for the same two gauges, and a temporary gauge located on Black Creek at Highway 7, for a total of 15 events occurring between May 1974 and June All three gauges were in operation for four events in Six of 15 events involved snowmelt computations. The agreement between observed and simulated hydrographs was entirely satisfactory for the purpose of this subwatershed study. The formulated hydrologic model was used to make estimates of flood flows resulting from return period (2, 5, 10, 25, 50 and 100 year) and the Regional Storm events. Estimates for flood flows were made for three main scenarios: existing, interim and ultimate conditions. The return period flood flows were compared with estimates derived from regional analyses (flood index method), and the agreement between the different methods was reasonable. Long-term water balance quantities, extreme flows (high and low) and flow-duration tables were computed by applying the model in continuous simulation mode for each scenario over the period November 1, 1960 to October 31, For existing conditions and a mean annual precipitation total of 822 mm, the model computed 504 mm of actual evapotranspiration, and 321 mm of total streamflow (89 mm as surface runoff, and 232 mm as baseflow), with minimal contributions to deeper groundwater storage. The long-term modelling results indicate that the impact of future development on the mean annual streamflow in Silver Creek could be significant. For Scenario 2 (Interim) conditions in Springbrook, surface runoff volumes increased by 3%, with an increase baseflow of 6%. For Scenario 3 (Ultimate) total runoff will increase by 33%, while the baseflows will increase by 10% primarily due to larger Sewage Treatment Plant effectives expected in the future. Appendix G Hydrology Impact Assessment Model G -i

4 ACKNOWLEDGEMENTS The principal writer of this technical appendix was Dr. Harold Schroeter, P.Eng. of Schroeter & Associates, Simcoe, Ontario. Guidance and suggestions during the preparation of this technical appendix, including the procurement of additional input data, were provided by the following individuals: Mrs. Hazel Breton, P.Eng., Credit Valley Conservation Brian Morber, Credit Valley Conservation Jackie Thomas, CET, Credit Valley Conservation Loveleen Grewal, Credit Valley Conservation John Perdikaris, M.Sc (Eng.), Credit Valley Conservation Daron Abbey, Water Hydrologeologic Inc. James Luce, and Mariette Prent, Aquafor Beech Limited Sam Bellamy, Grand River Conservation Authority Dr. Hugh Whiteley, P.Eng., School of Engineering, University of Guelph Hourly streamflow data for the Black Creek at Highway 7 gauge were made available by Burnside Environmental Ltd., whereas the streamflow data (both daily and hourly) for the Black Creek below Acton and Credit River West Branch at Norval gauges were supplied by the Water Survey of Canada. Historical climate information (daily maximum and minimum air temperature, rainfall and snowfall depths, and hourly rainfall depths) was obtained from the Environment Canada s Atmospheric Environment Service (AES), and Grand River Conservation Authority. Additional hourly rainfall data, and snow survey information were furnished by Schroeter & Associates. The assistance of all these individuals and agencies is gratefully acknowledged. Appendix G Hydrology Impact Assessment Model G -ii

5 TABLE OF CONTENTS ABSTRACT. ACKNOWLEDGEMENTS TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES.. Page II III IV VI VII 1.0 INTRODUCTION WATERSHED MODELLING OVERVIEW OF MODELLING PROCEDURES MODEL SET-UP Delineation of subcatchment boundaries Soil and land cover characteristics Special treatment of aggregate extraction areas Response unit drainage characteristics Subcatchment characteristics Stream channel data Treatment of detention ponds and marshes Treatment of special groundwater seepage and discharge Special reservoir routing procedures Consideration of Sewage Treatment Plant Flows Sensitivity analysis Schematic representation MODEL VALIDATION Procedures Meteorological and streamflow information Meteorological input data adjustments Streamflow data adjustments Snowmelt input data Initial conditions Parameter selection and seasonal adjustments Assessment of event modelling results Assessment of continuous modelling results. 28 Appendix G Hydrology Impact Assessment Model G -iii

6 TABLE OF CONTENTS CONTINUED Page 2.4 RESULTS AND DISCUSSION Continuous simulation Event Modelling MODEL APPLICATON FOR IMPACT ANALYSIS OUTLINE OF PROCEDURES MODIFICATIONS FOR FUTURE CONDITIONS FLOOD FLOW ESTIMATES Return period flood flows Regional storm flood flows Comparison of flood flow estimates: results and discussion LONG-TERM SIMULATIONS Water balance, extreme flows and durations Comparison of 7-day low flows and durations DISCUSSION OF IMPACT ANALYSIS RESULTS CONCLUSIONS REFERENCES 68 ATTACHMENTS A Additional simulated and observed hydrograph plots 71 Appendix G Hydrology Impact Assessment Model G -iv

7 LIST OF FIGURES Figure Number Description Page Number 2.1 Subcatchment divisions for hydrologic modelling purposes from the 4 Credit River Model Update Study Revised subcatchment divisions for the Silver Creek model Observed and simulated hydrographs at Norval for Oct. 1 to 24, Schematic representation of Black Creek watershed model Schematic representation of Silver Creek watershed model Comparison of observed hourly flows with the 24 fill-in mean daily 25 values for the Silver Creek at Norval gauge for the period September 10 to 15, Measured and modelled mean monthly flow volumes for the 10 yr run Observed and simulated flow duration curves for the 10 year run Comparison of observed and simulated hydrograph volumes and peak 34 flows for the 15 validation events Observed and simulated hydrographs for the February 2000 event Measured and modelled hydrographs for the May 2000 event 37 Appendix G Hydrology Impact Assessment Model G -v

8 LIST OF TABLES Table Number Page Number Description Level of modelling detail in each watershed model Comparison of drainage area estimates between modelling studies Hydrologic response unit drainage characteristics Subcatchment characteristics for existing conditions Channel element characteristics Observing climate stations available for study Characteristics of the Validation Events Blocks of equivalent snow accumulation in each ZUM Monthly parameter adjustment factors Water balance summary for Hydrograph Imperviousness values for each subcatchment and scenario Percentage hummocky area for each subcatchment and scenario Hour Rainfall Volumes by Return Period Temporal rainfall distribution patterns used in this study Summary of flood flow estimates: Scenario 1 - existing conditions Summary of flood flow estimates: Scenario 2 Interim conditions Summary of flood flow estimates: 45 Scenario 3 Ultimate conditions Regional Storm Rainfall Factors for Black and Silver Creeks Summary of flood flow estimates: 47 Regional Storm for Scenario 1 Existing conditions Summary of flood flow estimates: 48 Regional Storm for Scenario 2 Interim conditions Summary of flood flow estimates: 49 Regional Storm for Scenario 3 Ultimate conditions Return period flood flow estimates for gauged locations Comparison of flood flow estimates with other methods Water Balance Summary for Existing Conditions (Scenario 1) Water Balance Summary for Interim Conditions (Scenario 2) Water Balance Summary for Ultimate Conditions (Scenario 3) Extreme Flows and Durations Summary for Existing Conditions Extreme Flows and Durations Summary for Interim Conditions Extreme Flows and Durations Summary for Ultimate Conditions Return period 7-day low flows for gauged locations Flow duration table for gauged locations Comparison of return period 7-day low flows at gauged locations Comparison of flow duration table for gauged locations 62 Appendix G Hydrology Impact Assessment Model G -vi

9 1.0 INTRODUCTION This technical appendix provides more in-depth discussion about the approaches and procedures used in the hydrologic modelling activities conducted in support of the Silver Creek (Subwatershed 11) Subwatershed Study. Chapter 2 of this appendix documents the hydrologic modelling activities, including model set-up and validation. Chapter 3 outlines the application of the hydrologic model for impact analysis. Other technical appendices in the Phase I and II study reports show how the hydrology results are integrated with other study components. The specific tasks related to the hydrology component were as follows: 1. Develop hydrologic models of the Black and Silver Creek watersheds based upon information supplied by the Credit Valley Conservation (CVA) from the Credit River Watershed Management Study (Phase I), other watershed studies, field reconnaissance, and existing mapped information to determine the streamflow in the watershed resulting from observed sequences of meteorological inputs for existing and future development conditions. 2. Validate the existing conditions hydrologic model using available discharge measurements for the study area. 3. Compute the 2, 5, 10, 25, 50 and 100 year return period flood flows using the SCS Type II 24 hour storm pattern for selected locations in the study area, as well as the peak flows resulting from the application of the Regional Storm, for existing and proposed (future) watershed conditions. 4. Generate the 2, 5, 10, 20, 50 and 100 year return period high and low flows for selected locations in the study based on 'continuous' simulation using a time-series of meteorological data for at least 30 years or more for existing and future development conditions. 5. Perform water budget analyses for selected locations in the study area in response to existing and proposed watershed conditions. We are confident that the results reported here and in the main study report (Phase I and II) are informative and useful to the process of developing appropriate watershed management strategies for Silver Creek (Subwatershed 11). It should be recognized, however, that much of the work was not carried out at the level of detail of research studies; the results should therefore be interpreted with this in mind, and considered more valuable as indicators of direction and priorities than absolute predictions Appendix G Hydrology Impact Assessment Model G -1

10 2.0 WATERSHED MODELLING 2.1 OVERVIEW OF MODELLING PROCEDURES The development and validation of the Black and Silver Creek (Credit Valley Subwatersheds 10 and 11) watershed model is summarized herein. This model, based upon the GAWSER (Guelph All-Weather Sequential-Events Runoff model) format, is not a completely new setup, but rather it has evolved through successive revisions made in previous hydrologic studies conducted within the Credit Valley, each one adding new input data, and application experience, but all utilizing essentially the same methodology for setting-up and validating the watershed model as outlined briefly below. In , Triton Engineering Ltd., as part of the Credit River Water Management Strategy Phase I study, developed a comprehensive watershed model of the entire Credit River watershed using the GAWSER (Guelph All-Weather Storm-Event Runoff model) software package (Version 5.X). The Black and Silver Creek portion of that original GAWSER model was divided into a total 7 subcatchment elements, four in Black Creek, and three in Silver Creek. This model was calibrated and verified using observed flow data at the Norval gauge for four events (May 1974, May 1975, September 1986, July 1987). In 1992, Schroeter and Associates adapted this model for real-time flood forecasting, further tested it s performance against Norval gauge data for several additional events (August 1992, November 1992, and January 1993). As part of the Subwatershed 16 and 18 Study (1996 to 1999), Schroeter and Associates completely revised the Upper Credit River watershed model (Subwatershed 19, 17, 18, 16, 15, 13 and 20) using newer modelling techniques incorporated within GAWESR to handle surface to groundwater interactions, resulting from hummocky topography and aggregate extraction activities. This study involved an extensive calibration/validation effort involving streamflow data from three gauges for 13 events, and daily flow data for a continuous 7 year period. Later in 1999, Schroeter & Associates developed new models for Huttonville and Springbrook Creeks as part of the Credit Valley Secondary Plan (or Subwatershed 7 and 8b). In conjunction with Burnside Associates, Schroeter and Associates also re-built the Black (Subwatershed 10) and Silver Creek (Subwatershed 11) models as part of an investigation to assess irrigation storage requirements and water taking for a proposed golf course east of Acton, Ontario. This model comprising 10 subcatchment elements (7 in Black Creek, 3 in Silver Creek) was further validated with measured streamflows at two gauge locations for some eight events, and a seven-year continuous simulation period. By mid year 2000, approximately 85% of the original Triton GAWSER model of the Credit River watershed had been re-built through various studies involving the model author, Dr. H.O. Schroeter of Schroeter & Associates. At this time, Credit Valley Conservation had several comprehensive studies underway that required a revised watershed model of the Credit River watershed. They contracted Schroeter & Associates in December 2000 to reassemble the Credit River model based on the GAWSER format using the newest GIS information available to the CVC, in terms of topographic maps, quaternary geology and land cover classifications, and making use of modelling experience applied within the Credit River Appendix G Hydrology Impact Assessment Model G -2

11 over the past 10 years. Additional stream valley cross-sectional information was collected in 20 new locations, building on the original data set assembled in the Triton 1991 Study. This revised Credit River watershed model has a total of 180 subcatchment elements, and has been further validated using observed streamflow data from nine gauges maintained by the Water Survey of Canada, and another eight temporary gauges installed in several subwatershed studies for more than 60 gauge-events. The Black/Silver Creek portion of this overall Credit River model comprises 17 subcatchment elements as depicted in Figure 2.1, of which 10 are in Black Creek, and six in Silver Creek formed the basis for the model applied in the present Subwatershed 11 study. The actual steps taken to set-up the model are outlined briefly in this chapter, together with some additional commentary where ever the methodology differs from the previous work. Consequently, the sub-section headings used in the following reports Appendix H for the Caledon Credit River Subwatershed Study (Subwatersheds 16 and 18) (CVC, 2000) Technical Appendix A, Black Creek Golf Course Course: Hydrologic Modelling Assessment (Schroeter and Associates, 2001) Credit River Watershed Hydrology Model: Revisions (Schroeter and Associates, 2001; 2002), and Credit Valley Subwatershed Study and Servicing Plan (Huttonville Creek and Watercourse 8a) Technical Appendix Hydrology (Schroeter and Associates, 2002) are given here so the interested reader can readily find further details on the model set-up, validation, and application procedures. These four reports will be referenced throughout this and the next chapter, and for easy reference purposes, will simply be noted as the Subwatershed 16 and 18, Black Creek GC, Credit River Model Update, and Subwatershed 7 and 8a Studies. Where information that is contained in other Subwateshed 11 Study reports, they will be simply referred to as the Phase I or Phase II Reports. Appendix G Hydrology Impact Assessment Model G -3

12 Figure 2.1 Subcatchment divisions for hydrologic modelling purposes from the Credit River Model Update Study Appendix G Hydrology Impact Assessment Model G -4

13 2.2 MODEL SET-UP Delineation of subcatchment boundaries As shown in Figure 2.2.1, the Silver Creek watershed has been divided into 13 subcatchment elements for hydrologic modelling purposes. These subcatchments were chosen to have stream crossings at all flow monitoring stations, significant points of interest (e.g. damage centres, golf courses), and to reflect the spatial variations in soil type, land use and meteorological inputs. Other subcatchments were delineated to improve modelling results based upon: i) according to large changes in longitudindal slope of major tributary streams within the subcatchment, ii) the need to have subcatchment shapes such that a single overland flow path length is representative, and iii) the degree of imperviousness (e.g. can it be classed rural or urban?). The subcatchment boundaries were marked by hand on 1:45,000 scale topographic map sheets, from which drainage areas and stream lengths were measured. Information on the surficial geology (quaternary) were transferred to the base map so they could be overlaid on the subcatchment boundaries, from which soil types and land cover (e.g. forest) areas were determined. The total drainage area of Silver and Black Creek upstream of Norval was found to be km 2, which is an increase of 8.3 km 2 from the Triton model (1991), and 1.1 km 2 more than the value published by Water Survey of Canada (WSC). Table summarizes the level of modelling detail for each subwatershed model, in terms of number of subcatchments and channels, their mean size and range. The mean subcatchment size is 5.12 km 2, whereas the mean channel reach is 3090 m in length. This level of modelling detail, in terms of mean subcatchment size and channel lengths, is comparable to other recent GAWSER applications (see Schroeter et al., 2000a). The results of comparing the measured drainage areas with those published in previous studies for several key locations are summarized in Table One can see there is considerable agreement (within +10%) between the measured drainage areas and those published previously. The greatest discrepancies (as high as 20%) between those measured in this study and previous work are for the flatter areas, where the contour interval used in the different scale maps results in shifts in the watershed boundary. Appendix G Hydrology Impact Assessment Model G -5

14 Figure Revised subcatchment divisions for the Silver Creek model Appendix G Hydrology Impact Assessment Model G -6

15 Table Level of modelling detail in each watershed model a) Subcatchment details Watershed Total Area (km 2 ) Number Mean Subcatchment Area (km 2 ) Subcatchment Size Range (km 2 ) Black Creek (Subwatershed 10) to 9.63 Silver Creek (Subwatershed 11) to 13.7 Total for modelling to 13.7 Watershed b) Stream channel details Total Length (m) Number Mean Channel Length (m) Channel Length Range (m) Black Creek (Subwatershed 10) to 8390 Silver Creek (Subwatershed 11) to 5100 Total for modelling to 8390 Table Comparison of drainage area estimates between modelling studies Point of Interest This Study S & A 2001# WSC 1991 Triton Study Black Creek below Acton* Black Creek North Tributary at Highway 7* Black Creek u/s Silver Creek Confluence Silver Creek d/s Snows Creek Silver Creek u/s of Black Creek Confluence Silver Creek d/s of Black Creek Confluence Credit River West Branch at Norval* Notes: 1. All areas are given in km 2 Streamflow gauges denoted with * 2. WSC (Water Survey of Canada) areas taken from streamflow summary reports 3. # S & A 2001 refers to the Black Creek GC Study. Appendix G Hydrology Impact Assessment Model G -7

16 2.2.2 Soil and land cover characteristics To account for the wide variation in runoff generation response attributed to the different land cover features and soil types (e.g. source areas), the subcatchment elements were further subdivided into nine 'hydrologic response units' (HRUs); one impervious and eight pervious. The HRUs defined below were originally established in the Subwatershed 16 and 18 Study, and were adopted for use throughout the Credit River watershed in the Model Update Study. Hydrologic Response Description (vegetation/soil type) Unit (RU) 1 Impervious Surfaces (includes open water) 2 Wetlands Low Vegetative Cover (includes pasture and row crops) 3 Peat and muck (not included in wetlands HRU) (exposed bedrock) 4 Silt Till, Silty clays 5 Silty sands 6 Sand 7 Gravel High Vegetative Cover (Forests) 8 Low infiltration (includes soils in RU3 to RU5) 9 High infiltration (includes soils in RU6 and RU7) In the Subwatershed 16 and 18 study, there was very little exposed bedrock in the area, and so it was included in the HRU 1 or impervious category. However, the amount of exposed bedrock in Subwatersheds 10 and 11 was significant (greater than 5% for each subcatchment involved), and so adding it to the impervious surfaces HRU would produce too much runoff, although fractures present in these areas would reduce the amount of runoff. Because the amount of peat and muck (HRU 3) was low (much less than 10%) relative to some of the other HRUs, the exposed bedrock area was added to HRU 3. Open areas have low vegetal growth, like pastures, cropped fields, fallow and grasses. They are grouped together because they change from year-to-year. 'Low vegetative cover' is a more stable term for use in long-term modelling. The wetlands response unit permitted a reasonable accounting of the evapotranspiration from these areas. Soil type areas were measured from the quaternary geology maps for the area, the same information used in the hydrogeologic investigations. The land cover and soil type maps were overlaid on the subcatchment boundary map, from which the area contributing to each response unit group within a subcatchment could be measured directly. For rural subcatchments, the impervious areas include roads and adjoining shoulders, lanes, ditches and stream channels. The total impervious area in a given subcatchment can be determined by measuring the length of the roads and streams from topographic maps, and multiplying by a representative width. In previous applications of GAWSER in southern Appendix G Hydrology Impact Assessment Model G -8

17 Ontario, the imperviousness of rural watersheds usually represents about 1.5 to 3% of the area (Schroeter & Associates, 1996). The values used here (see Table 2.2.3) are comparable. For urban subcatchments (e.g. 1005, 1123 and 1125), the impervious area was estimated by assigning a representative impervious percentage to the area measured from the land use maps. For the subcatchments with predominately residential areas within them, the impervious value was set at 35%. For subcatchments containing reservoirs or lakes, with surface area greater than 3% of the drainage area, the impervious total includes the surface area of the reservoir (or lake) under normal operating conditions. The classification scheme for response units outlined here has been utilized in most GAWSER application hydrology studies (see Schroeter and Boyd, 1998; Schroeter et al., 2000a) Special treatment of aggregate extraction areas As done in the Subwatershed 16 and 18 Study, aggregate extraction areas were treated separately in the runoff generation calculations as recharge ponds, where effectively all the runoff water is directed to ground water storage. One aggregrate extraction area was included in Black Creek subcatchment Response unit drainage characteristics Because there were essentially no changes in the response unit definitions (see Section 2.2.2), there was no need to alter the response unit drainage characteristics. However, because the exposed bedrock areas are now included in HRU 3 (peat and muck), the effective hydraulic conductivity (KEFF), the maximum speepage and percolation rates (CS and D) were reduced accordingly (by about half). Minor adjustments were made to some of the characteristics following consultation with the hydrogeologist on our study team. The actual the response unit drainage characteristics applied in the model are summarized in Table They represent mid-summer (around July 30th) values Subcatchment Characteristics Essentially, the procedures for computing the subcatchment characteristics, particularly the length, width, and overland runoff lag constant were left unaltered for this study. The new cross-section information gathered as part of the Model Update Study was a welcome addition to the model input dataset, eleminating the need to borrow cross-sections or use the same cross-section for many subcatchment elements. Subcatchment areas (A), lengths (L), and main channel slopes were measured from the available topographical maps within the CVC Appendix G Hydrology Impact Assessment Model G -9

18 GIS database. A full description of how the subcatchment lengths and widths, as well as the other overland runoff lag and subsurface recession flow constants were chosen is given in the Subwatershed 16 and 18, and the Model Update Study reports, as well as the GAWSER Training Guide and Reference Manual (Schroeter and Associates, 1996). The Subwatershed 7 and 8a Study report supplies detailed information about setting the specific characteristics for urban subcatchments, those defined has having an imperviousness of greater than 10%. For the two subcatchments that contain the greatest portions of the town of Georgetown, namely subcatchment 1123 and 1125, the existing conditions impervious area percentage was calibrated by applying the model to several small rainfall events occuring between October 2001 and June The philosophy here is that for small rainfall events (those with rainfall volume less than 15 mm) most of the observed runoff will come from the impervious surfaces only (see Ecologistics, 1988, Schroeter and Watt, 1989). In Figure 2.2.5, we see a comparison between the observed and simulated hydrographs for the Norval gauge during the period October 1 to October 24, Initially, this simulation period was applied to the model with an impervious percentage of 12.3% for two lower Georgetown subcatchments (1123 and 1125) as found in the Black Creek GC Study, and the agreement between the measured and modelled hydrographs was very poor. However, when the impervious percentage was found to be 18.5 and 18.2% for subcatchments 1123, and 1125, respectively, through calibration against streamflow data for the Norval gauge from three 6-day events (late October 2001, early November 2001, and mid-june 2002), the agreement between the observed and simulate hydrographs was greatly improved as illustrated by the excellent results displayed in Figure The groundwater factor, GWFACT, which tells the program how much infiltrated water generated will not return as baseflow at the subcatchment outlet, is usually set to zero for most applications. However, for this study (see Table 2.2.5), GWFACT=1 for all the Silver Creek subcatchments (e.g to 1113) that are situated above the Niagara Escarpment. For the Black Creek subcatchments, the GWFACT values were determined in the Black Creek GC Study from a review of information given Halton Aquifer Management Plan Phase 2 report (Holysh, 1997). Table lists the drainage areas, the response unit percentage, cross-section assignment, overland runoff lag and subsurface recession constants, as well as the percentage hummocky area and the GWFACT values for each subcatchment under existing conditions. Appendix G Hydrology Impact Assessment Model G -10

19 Figure Observed and simulated hydrographs at Norval for October 1 to 24, 2001 Appendix G Hydrology Impact Assessment Model G -11

20 Table Hydrologic response unit drainage characteristics Symbol Description Units Imp Wet Lands Low Veg. Peat Muck Low Veg. Silt Clay Low Veg. Silty Sand Low Veg. Sand Low Veg. Grav. High Veg. Low Perm High Veg. High Perm Response Unit Number DS Maximum depth of depression Storage (mm) KEFF Infiltration into 1 st soil layer (mm/h) CS Infiltration into 2 nd soil layer (mm/h) D Infiltration out of 2 nd layer (mm/h) SAV Average suction at the wetting front (mm) X Groundwater Contribution Indicator: =SS, 0=GW FATR Groundwater Fraction (not used in this model, set=1) INC Maximum depth of interception storage (mm) First Soil Layer HI Soil layer thickness (mm) SMCI Saturated soil-water content (porosity) (vol/vol) IMCI Initial soil-water content (vol/vol) FCAPI Field capacity soil-water content (vol/vol) WILTI Wilting point soil-water content (vol/vol) Second Soil Layer HII Soil layer thickness (mm) SMCII Saturated soil-water content (porosity) (vol/vol) IMCII Initial soil-water content (vol/vol) FACPII Field capacity soil-water content (vol/vol) WILTII Wilting point soil-water content (vol/vol) Appendix G Hydrology Impact Assessment Model G -12

21 Table Subcatchment characteristics for existing conditions Silver Creek Watershed Model: Existing Conditions UNITS=2 Revised: August 12, 2002 C ===== Response Unit Percentages ============== Overland Routing Information Baseflow Other Characteristics NHD AREA LENGTH WIDTH IMP MCVS MCQR OCVS OCQR FTB FTLO KSS KGW %HUM QGWI POND GWFACT GWON (ha) (m) (m) % Black Creek Subcatchments * * Silver Creek Subcatchments * * * * * * * Appendix G Hydrology Impact Assessment Model G -13

22 2.2.6 Stream channel data Stream channel data are necessary inputs to both the overland flow (runoff) and channel routing calculations in GAWSER. Consequently, representative cross-sections are required inputs to the routing procedures, where the parameters are computed directly by the program using the channel length, bed slope and a characteristic rating curve developed for the section. Typical off-channel sections were developed for rural (e.g. 1001, 1101) and urban subcatchments (e.g. 1123, 1125) in the Subwatershed 16 and 18 Study. Additional field measurements collected in the Model Update Study added more than 20 cross-sections to the model input database originally assembled in the Triton (1991) study, more than doubling the number perviously available. Profile plots for these sections are summarized in the Model Update Study report. Table below summarizes the characteristics for the channel routing reaches (elements) considered in the model. Table Channel element characteristics Channel Length Slope Cross-Section Watercourse Number (m) (m/m) Number Black Creek Black Creek Black Creek Black Creek North Branch Black Creek North Branch Black Creek Black Creek Black Creek Silver Creek Snows Creek Snows Creek Snows Creek Silver Creek Silver Creek Silver Creek Silver Creek Silver Creek Appendix G Hydrology Impact Assessment Model G -14

23 2.2.7 Treatment of detention ponds and marshes Distinct hydraulic features within the subwatershed were isolated, and considered as diversion of flow, or reservoir (pond) elements. Special seepages to groundwater storage are described in the next section. Storage-outflow characteristics were established one distinct reservoir elements (Fairy Lake) from information given in previous modelling reports. Method of accounting for the hummocky areas through the use of a hypothetical recharge pond at the outlet of the subcatchment was applied initially to the Credit River watershed in the Subwatershed 16 and 18 Study, and has been described fully in that report. The exact same procedures were applied in this revised Silver Creek model Treatment of special groundwater seepage and discharge Each subcatchment element in the normal usage of GAWSER is considered to be a total selfcontained hydrologic unit. This means that all the precipitation falling on a given subcatchment is accounted for in the computations. Infiltrated water returns as baseflow, so the total outflow becomes the sum of computed runoff, subsurface and baseflow components. Although this is an idealized situation that facilitates hydrograph calculations, it is not always true in nature; infiltrated water may reappear as baseflow at some other point downstream in the watershed, or flow to another watercourse entirely. The GWFACT factor, noted earlier, accounts for deeper groundwater contributions, that can be re-directed to a groundwater storage array in program memory. At some other point downstream in the drainage network, water can be released from this storage array, usually at a rate specified as a percentage of the water remaining in the groundwater storage array. Spot baseflow measurements are useful in determining which subcatchments contribute to the groundwater storage array, and where the releases (discharges) from this array are considered in the model. From discussions with our study team hydrogeologist it was decided to allow 60% of the water available in the groundwater storage array re-enter Silver Creek downstream of subcatchment 1119, which is located below the Niagara Escarpment, and the remaining 40% would re-enter Silver Creek at Node 4248, immediately downstream of the Black Creek outlet. For Black Creek, 10% of the flows stored in the groundwater storage array are returned to Black Creek immediately downstream of channel 3125 below Acton, whereas the 100% of the remainings re-enter Black Creek at Node 4230 near Stewartown. The methodology for setting the seepage and discharge rates is described fully in the Subwatershed 16 and 18 Study report Special reservoir routing procedures Since the completion of the Subwatershed 16 and 18 study, GAWSER s new ROUTE SPECIAL POND routine has been used for most complete storage routing applications, primarily because it handles direct evaporation from the lake/reservoir surface, handles Appendix G Hydrology Impact Assessment Model G -15

24 recharge to the groundwater storage array, and accounts for some diversions. This routine was used to handle the water balance computations for Fairy Lake in Acton.. Information about the elevation-outflow-storage-surface area tables was given earlier Consideration of Sewage Treatment Plant Flows Direct specification of sewage treatment plant (STP) effluents can be specified as a table of mean monthly discharges, or could be read from a disk file directly to account for the complete time series of STP discharges. In the present analysis, the effluent from the Acton STP was specified in the computations for subcatchment 1005, and Georgetown STP outflows in subcatchment 1125, all of which are entered as a table of mean monthly discharges taken directly from the MOE discharge report (MOE, 1989). The mean monthly discharge table for each STP entered in the model was established from the available information for the 1985 to 1994 period. The mean annual discharge for the Acton STP was found to be 2590 m 3 /d (or 0.03 m 3 /s), and m 3 /d (or m 3 /s) for the Georgetown STP Sensitivity analysis A sensitivity analysis was not undertaken as part of this project, because the model was setup in exactly the same manner as described in the Subwatershed 16 and 18, and Subwatershed 7 and 8a Studies, with the same parameter values for representative response units. Hence, such an analysis would provide no new information about the sensitivity of model output to changes in the input parameters as outlined in these previous reports Schematic representation The schematic representation of the Black and Silver Creeks hydrology model, showing the linkage of subcatchment, channel and reservoir elements, is displayed in Figures and A consistent numbering scheme was adopted in the Model Update Study to help identify points of interest within each subwatershed. Subcatchments in each major tributary (e.g. Subwatershed 11) utilize four digit identification numbers; the first two signify the subwatershed, and the last two denote the subcatchment number (e.g. 1001, 1105) assigned in the order of occurrence as they are added to the flow in the major tributary channels. For example, subcatchment 1113 would contribute flow to Silver Creek downstream of subcatchment Channel elements are numbered in the 3000 s sequentially (e.g. 3120, 3135, 3168) as they occur in the model. Hydrograph addition points are numbered in the 4000 s, and reservoir elements (e.g. Fairy Lake) are numbered in the 5000 s. Within the sequences of hydrograph numbers assigned to various elements, some numbers are missing, which is to facilitate introduction of new elements in any future model updates (notice, in Figure 2.1 that Silver Creek has 6 subcatchments in the Model Update Study, and 13 in the present study, as shown in Figure 2.2.1). Appendix G Hydrology Impact Assessment Model G -16

25 Figure Schematic representation of the Black Creek watershed model Appendix G Hydrology Impact Assessment Model G -17

26 Figure Schematic representation of the Silver Creek watershed model Appendix G Hydrology Impact Assessment Model G -18

27 2.3 MODEL VALIDATION Procedures In any hydrologic modelling exercise, it is generally assumed that if a given model reproduces an observed or measured sequence of quantities (e.g. streamflow volume, reservoir water levels) that confidence can be placed in its predictive capability, from which management options or decisions are often made. Obviously, if additional comparisons between model output and measured quantities are made and their agreement is deemed to be acceptable, then more confidence can be placed in predictions from the model, particularly for impact analyses.. Consequently, an important step in any hydrologic modelling exercise is to establish the level of confidence in the predictive results, or validating the model. This important confidence building or validation step in the modelling procedures is often referred to as calibration, although the term calibration has been used interchangeably with verification, validation and confirmation. This is unfortunate, because calibration is a unique step in the modelling procedures, apart from validation, verification or confirmation. Model calibration is a process of adjusting model parameters, variables or other inputs in order to reduce the differences between simulated and observed flows (or other hydrologic quantities) to levels that are deemed acceptable (see Watt et al., 1989; James and Burgess, 1982). The adjusted or calibrated parameters or variables are then verified or validated by applying the model to an independent data set that was not used for calibration. According to James and Burgess (1982), model calibration involves a trial-and-error procedure to achieve optimum parameter levels that produce a reasonably good match between model results and observed data. The parameters, whose values are based on field measurements or are well-established from previous studies, remain fixed. Those to be calibrated are adjusted based on a goodness of fit criterion using visual or statistical comparisons between measured and simulated results (see James and Burgess, 1982; Schroeter and Boyd, 1998). A model is said to be robust if its parameter settings can be transferred from one watershed to another (Schroeter and Watt, 1989). A simple comparison of model output with any observed values does not constitute a calibration exercise, unless the parameters are adjusted to improve the agreement between observed and simulated results. On the whole, any comparison between measured and modelled results is always considered part of the model confirmation or validation procedures. In summary then, the Black and Silver Creeks watershed model has been adequately confirmed or validated. In this regard, the following validation checks have been made. 1. The GAWSER (Guelph All-Weather Sequential-Events Runoff) model has been extensively calibrated, verified and validated in more than 35 watershed modelling studies within the last 16 years were conducted for Ontario watersheds. These applications Appendix G Hydrology Impact Assessment Model G -19

28 constitute model comparisons with observed flow data from more than 120 gauges for more than 1600 gauge-events. For continuous simulation work, the model has been compared with long-term streamflow data from more than 40 gauges for some 600 gaugeyears of application. For urban runoff modelling, the model has been tested with data from more than 12 gauges for more than 60 gauge-events. This massive amount of experience gained in applying the model over Ontario in the last 16 years was utilized directly in formulating the revised Black and Silver Creeks watershed model. The monthly parameter adjustment table has proved to be an extremely robust tool in applying GAWSER throughout Ontario. 2. Of particular relevance to the present work, GAWSER was applied in Phase I of the Credit River Watershed Management Study (Triton, 1990), and the Subwatershed 16 and 18, Black Creek GC, Subwatershed 7 and 8a, and the Model Update Studies. The montly parameters adjustment table applied in the Model Update Study was utilized with no changes in the present study. 3. Mean annual evapotranspiration amounts estimated by the physically-based GAWSER model were well within acceptable ranges reported in numerous southern Ontario climatology documents and maps (e.g. Brown et al., 1974; OMNR, 1984). 4. The modelling results were reviewed by other members of the study team and the computed numbers were found to make sense, or reasonable. Hence, the collective experience of the entire study helped to validate the results of the hydrologic modelling. In this study, the main objective of the validation procedures was to ensure that the level of performance provided by the revised Silver Creek watershed model was at least as good, if not better than the previous model(s), without collecting, assembling and processing additional meteorological and streamflow data (as was done in the Subwatershed 16 and 18 Study). Consequently, only readily available meteorological and streamflow data were used for validating the revised hydrology model. For this purpose, continuous streamflow measurements have been available at two gauge sites maintained by the Water Survey of Canada (WSC) in the Black and Silver Creek watersheds. Additional streamflow measurements were collected by Burnside and Associates Ltd. as part of the Black Creek GC Study for a gauge installed on the North Branch of Black Creek at Highway 7 for the period December 1999 to July All these gauges are listed below. Name of Station WSC Station ID Operational Periods North Branch of Black Creek at Hwy 7 N/A Dec to July 2000 Black Creek below Acton 02HB024 June 1987 to present Credit River West Branch at Norval 02HB008 October 1960 to present Appendix G Hydrology Impact Assessment Model G -20

29 The two WSC gauges have been in operation simultaneously since June 1987, but for testing the model in event mode, hourly flows were available initially for eight events at the Norval gauge only from previous work (as mentioned at the start of this chapter). However, as noted above, hourly flows were later obtained for all three gauges for the period December 1999 to July Most of the validation procedures focussed on continuous simulation applications during the period November 1988 to October 1997, and for event modelling of 15 events between May 1974 and June 2002 for the Norval gauge. Of these 15 events, hourly flows were available for the Below Acton gauge for 7 events. All three gauges listed above were in operation for four events in This amount of streamflow data were believed to be sufficient to ensure that the model was reasonably valid for the present applications. The model validation procedures were divided into two parts. First, the model was applied in continuous simulation mode for the 10 year period November 1, 1988 to October 1997 to confirm the monthly parameter adjustment factor table. Secondly, the model was applied to total of 15 individual events and the results compared to the observed hourly flows available for all three gauges. This exercise provided further confirmation on the parameter settings checked during the 10 year simulations, but also permitted the routing calculations in the different hydrologic elements (e.g. overland flow, channel and reservoir) to be assessed, in terms of hydrograph timing and peak flows estimates Meteorological and Streamflow Information As mentioned earlier, an objective of the model validation procedures was to utilize as much as possible existing meteorological and streamflow data taken from previous studies that involved GAWSER applications for the Credit River watershed. Daily discharges where necessary were extracted from WSC s HYDAT CD-ROM. All the meteorological data assembled and processed during four previous studies mentioned at the beginning of this chapter (see Table ) were utilized directly. Some new meteorological was obtained from the GRCA (Grand River Conservation Authority), and Dr. Schroeter s own raingauge located in Guelph. For the 10 year continuous simulation period applied to the Silver Creek hydrologic model, the mean annual precipitation was 907 mm, and the mean annual discharge at the Norval gauge was 1.42 m 3 /s (or expressed as a depth, 349 mm). Table summarizes the meteorological inputs and streamflow characteristics for the validation events, and includes the peak flows for each gauge. The average duration for the event simulation periods was 10 days (240 hours), which meant that in event mode, the model was being tested for continuous simulation in small pieces. As will be shown later, the four observed events for the gauge at Highway 7 on the North Branch of Black Creek could be considered 1.25 to 2 year events. For the Norval gauge, some of the observed events (e.g. May 1974; September 1986) had return periods on the order of 5 to 10 years. Six of the 15 events involved snowmelt. Appendix G Hydrology Impact Assessment Model G -21

30 Table Observing climate stations available for study Station Name Station Code Owner Available Period Of Record* Data Collected Above Erin CVCA003 CVC RG Blue Springs Creek IHD AES P,T,RG,E Boston Mills CVCA004 CVC RG Cataract CVCA002 CVC RG Fergus Shand Dam AES P,T,RG Georgetown AES P,T Georgetown WWTP AES P,T Guelph Arboretum AES P,T,RG Guelph OAC AES P,T,RG,E Guelph Lake Dam GRCA003 GRCA P,T,RG Guelph Grandridge Schroeter S & A P,RG Norval CVCA008 CVC RG Shand Dam GRCA001 GRCA P,T,RG Toronto Pearson Int l A AES P,T,RG Notes: P daily precipitation (rain and snow) T - daily maximum and minimum air temperature RG - Recording raingauge (tipping bucket) E - Pan evaporation estimates Meteorological input data adjustments In the Black and Silver Creek watersheds, meteorological inputs can vary significantly with location. To account for these variations, the GAWSER program accepts inputs on the basis of separate Zones of Uniform Meteorology (ZUM). A ZUM (Schroeter and Whiteley, 1992) is defined as a portion of a watershed throughout which one set of meteorological measurements can be used to calculate snowmelt and runoff. Typically, several subcatchments are covered by one ZUM. Usually, ZUM boundaries are made to agree with the drainage areas at streamgauge locations, so that the meteorological inputs can be adjusted or confirmed directly with discharge data. For this study, two ZUMs have been defined, one for the Black Creek watershed, (e.g. subcatchment 1001 to 1030), and the second one for the Silver Creek watershed (with subcatchments 1101 to 1125). For some events, the meteorological inputs were distributed even further according to the areas in subwatershed above and below the Niagara Escarpment. For meteorological inputs to the model, generally the closet available recording raingauge, climate station or snow course to a given ZUM provides direct input for that ZUM. In this study, CVC s Norval raingauge would supply input for the Silver Creek ZUM, and the above Erin gauge for the Black Creek ZUM. This allocation procedure was applied to the initial snowpack information, as well as the daily climate data. Appendix G Hydrology Impact Assessment Model G -22

31 Table Characteristics of the Validation Events Model Mean Inputs Hydrograph Event Dur. Rain Snowmelt Gauge Vol. Peak TP (d) (mm) (mm) Name (mm) m^3/s (h) ================================================================================ May 15 to May 21, Norval May 4 to May 10, Norval Sep 10 to Sep 16, Norval Jul 19 to Jul 25, Acton Norval Mar 9 to Mar 21, Acton Norval Apr 3 to Apr 15, Acton Norval Jan 3 to Jan 9, Acton Norval Nov 1 to Nov 7, Norval Feb 21 to Mar 4, Acton Hwy Norval Apr 19 to Apr 25, Acton Hwy Norval May 9 to May 21, Acton Hwy Norval Jun 10 to Jun 22, Acton Hwy Norval Oct 22 to Oct 28, Norval Nov 1 to Nov 7, Norval Jun 4 to Jun 28, Norval ================================================================================ Mean Characteristics: Acton Hwy Norval Appendix G Hydrology Impact Assessment Model G -23

32 During the historic event modelling, the appropriate recording raingauge information was not always available (e.g. gauge malfunctioning), and had to be estimated from other sources. For instance, rainfall records from some of the other gauges (e.g. Boston Mills, Guelph Arboretum/Dam or Fergus Shand Dam) were used in place of any missing data for the primary gauges. When Norval gauge data were not available, records for the CVC Boston Mills or Cataract raingauges were used as back-up. Daily climate records for the Georgetown WPCP (AES ) were used as checks on daily rainfall totals. Recording raingauge data from the Toronto Pearson International Airport were used for winter simulation events (e.g. February 2000), because the AES hourly rainfall data are not available for the November 1 st to March 31 st period. For long-term simulations, the Guelph Arboretum/OAC, Fergus Shand Dam, Georgetown WPCP and Pearson Airport 36-year datasets ( ) prepared during the Eramosa River Watershed Hydrology (Schroeter and Boyd, 1998) were used for both Black and Silver Creek ZUMs. Because of uncertainty in which data were the most applicable for the Silver Creek watershed, all four data sets were applied in the present study. These datasets represent the closet available climate stations with complete records for 30 plus years. As part of recent work for the Grand River Conservation Authority (GRCA) (see Schroeter et al., 2000a,b), both of these datasets have been expanded to include the 1997 to 2001 observations. Because Pearson Airport is situated below the Niagara Escarpment and is several km east of the study area, long-term meteorological data were deemed to be unrepresentative of the climate occurring in the Silver Creek watershed Streamflow data adjustments The streamflow comparison data were adjusted to account for missing values in the records caused by ice conditions or gauge malfunctions. When only a few flow values were missing in the records, the missing flows were estimated by interpolation from the observed values. For some events, where the hourly discharges were not available for complete days, the published mean daily flows for the same period were entered to fill-in the 24 missing values, and hence provide hydrograph volumes for model assessment purposes. In these instances, it s difficult to compare hourly simulated flows with the mean daily values, and some imagination is required to make a qualitative assessment. Figure illustrates the difficulty in qualitatively comparing the hourly and mean daily flows for the Silver Creek at Norval gauge. Appendix G Hydrology Impact Assessment Model G -24

33 Figure Comparison of observed hourly flows with the 24 fill-in mean daily values for the Silver Creek at Norval gauge for the period September 10 to 15, 1986 Appendix G Hydrology Impact Assessment Model G -25

34 2.3.5 Snowmelt Input Data Snow accumulation and melt in different land cover units within a watershed are accounted for in GAWSER by defining 'blocks of equivalent accumulation' (BEAs). For the Silver Creek watershed, six EABs were identified and considered: two types of field blocks (ploughed and grass/pasture/grains), forests, and three edge blocks (e.g. road easements, fence lines and forest edges). Edge blocks are areas with significant capacity to store snow during blowing snow conditions. See Schroeter and Whiteley (1986) and Burkart et al. (1991) for further information about snow accumulation characteristics among differing landscape units in southern Ontario. Representative patterns for the study area are noted in the Phase I report. The BEAs were estimated from land cover information given in Table using similar relationships between blocks found in the Grand River (Schroeter & Whiteley, 1986). The snowmelt model parameters applied in the Subwatershed 16 and 18 Study were used directly with no adjustments, except for the percentage area assigned to each BEA, which are noted in Table by ZUM. Table Blocks of equivalent snow accumulation in each ZUM Subwatersheds ZUM Units Fields Ploughed Fields Grass Forest Roadway Easements Fence Lines Forest Edges Black & Silver (10 & 11) 7 (% Area) Initial conditions Initial watershed conditions are represented by three variables in the GAWSER program: initial soil-water content, initial streamflow at time zero (also called baseflow), and initial snowpack conditions (for snowmelt events only). The methodology outlined in the Subwatershed 16 and 18 Study was used directly and not altered beyond incorporating the new land use information Parameter selection and seasonal adjustments Previously published values were employed as first estimates for all model parameters. In this case, parameter values were taken directly from the Subwatershed 16 and 18 and Model Update Studies. Once the model is completely set-up, the number of parameters requiring additional adjustment during calibration, should that become necessary, are relatively few. As mentioned Appendix G Hydrology Impact Assessment Model G -26

35 early, the model comparisons made in this report did involve any model calibration. Previously published values were used throughout. The program adjusts the specified parameters for all response units and subcatchments in a similar manner, as shown here for effective hydraulic conductivity (KEFF). [2.3.7] KEFF(i) used = FKEFF * KEFF(i) specified where FKEFF is the effective hydraulic conductivity adjustment factor, the subscript used denotes the value of KEFF actually used in the runoff calculations for response unit (i), and the subscript specified represents the value of the parameter (e.g. KEFF in Table 2.2.4) for response unit (i) actually entered in the input files during model set-up. In previous applications of GAWSER, the most commonly adjusted parameter factors have been the following: FDS Maximum depth of depression storage factor FKEFF Effective hydraulic conductivity factor (for surface infiltration) FCS Maximum seepage rate (movement of water from layer 1 to 2) FD Maximum percolation rate (movement of water out of layer 2) FKO Overland runoff lag factor FKMF Combined refreeze/snowmelt factor FIMCI Initial soil-water content adjustment factor for soil layer 1 FIMCII Initial soil-water content adjustment factor for soil layer 2 FEDAY Potential evapotranspiration adjustment factor FINS Interception storage adjustment factor Values of unity for any of the above factors means that the 'as set-up' values specified in the watershed files are used directly in the calculations. The rationale for adjusting these factors is given in the Subwatershed 16 and 18 and Subwatershed 7 and 8a Study reports. The monthly parameter adjustment factors used in the present applications are given in Table below. This table represents parameter factor values at the midpoint of each month (say the 15 th ). The actual parameter adjustments used in the calculations are then linearly interpolated from the monthly table depending on the Julian date. The values for FEVAP were updated using pan evaporation values from the Blue Spring Creek IHD climate station. Appendix G Hydrology Impact Assessment Model G -27

36 Table Monthly Parameter Adjustment Factors Symbol JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC FDS FKEFF FCS FD FKO FKSS FKMF FNEW FEVAP FINS Assessment of event modelling results The methodology for assessing the event modelling results has been outlined in detail in the Subwatershed 16 and 18 Study. In this report, the event modelling results are not assessed using any statistical measures, such as the Nash-Sutcliffe (1970) model efficiency or the goodness of fit index proposed by Schroeter and Boyd (1998). The approach taken here is strictly qualitative, in which observed and simulated hydrographs for all three gauges are presented for some representative events, and commentary on what the comparisons mean in terms of model performance are given in the text. This approach has been adopted here (as in the Model Update Study) because, as you recall, no new meteorological or streamflow data were assembled and processed for this study. Consequently, the model applications are made with the available meteorological data set, which may not be representative for the drainage areas immediately upstream of each gauge. In the Phase I report, it was noted that the presence of the Niagara Escarpment exerts great influence on the distribution of climate variables, and hourly rainfall in particular. It is recognized that the further improvements in model performance would require a significant effort in assembling and processing additional meteorological and streamflow data beyond the requirements for the present study Assessment of continuous modelling results The procedures for assessing the continuous simulation results were presented in the Subwatershed 16 and 18 Study. As mentioned for the event modelling, the method of assessment adopted here is strictly qualitative. In this regards, comparisons will be made between observed and simulated mean monthly flow volumes, and the measured and modelled flow duration curves. Comments are given in the text to help interpret what the results mean in terms of model performance. Appendix G Hydrology Impact Assessment Model G -28

37 2.4 RESULTS AND DISCUSSION Continuous Simulation The hydrologic model was applied for the period November 1, 1988 to October 31, 1997, the period when both long-term gauges were in operation. The initial or starting conditions (e.g. initial snowpack, soil-water contents, and river flows) were estimated in the same manner as outlined in section A first check on the results for the 10-year simulationd is a water balance table produced by GAWSER. Table gives a sample of the mean annual water balance quantities and how they are distributed by month throughout the year for the Silver Creek at Norval gauge. These quantities represent the areal average for the drainage area upstream of the Norval gauge. Table WATER BALANCE SUMMARY FOR HYDROGRAPH 4250 ========================================= Location: Silver Ck at Norval gauge Scenario File: Existing conditions Period: 1987/11/01 to 1997/11/01 Area: km^2 Water Balance Quantities (in mm) Infiltration Total Month Precip ET Runoff (Baseflow) (Losses) Flow JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Total Appendix G Hydrology Impact Assessment Model G -29

38 The individual quantities in Table can be expressed in a water balance [2.4.1] Precip = ET + Runoff + Baseflow + Losses where Precip represents the total precipitation (rainfall plus snowfall), ET is the combined evapotranspiration and sublimation total, Runoff is the mean annual runoff, Baseflow is the portion of the infiltrated water that returns to the stream, and Losses signifies the amount of infiltrated water that does not return to the receiving stream. The Losses total also includes water stored in the system, and is sometimes referred to as the net storage term. For instance, the positive totals for Losses during the winter months (e.g. December to March) represents snow on the ground, whereas the negative values during the summer months (e.g. July to August) denotes water pulled from soil-water storage. Total Flow is the sum of Runoff and Baseflow. Tables like can be produced for any point of interest in the watershed model. Water balance quantities for other points of interest will be shown in the next chapter on Impact Analysis. From Table 2.4.1, one can see that the mean annual precipitation for the water year period is about 907 mm. The average annual evapotranspiration (plus sublimation) total is about 517 mm, a reasonable value for this part of the Ontario according to Brown et al. (1980) and OMNR (1984). The mean annual runoff total is 111 mm, of which 39% is generated during the months of February to April. The mean annual total streamflow is 386 mm, of which 71% appears as baseflow. Although not shown, the observed mean annual streamflow for the same time period is 342 mm, which is only 12% lower than the simulated value listed in Table This particular run was made using the Shand Dam meteorological data set. Other datasets were employed (e.g. Guelph OAC/Arboretem/Dam, Georgetown STP, and Pearson International Airport), but on a consistent basis, the agreement between observed and simulated mean annual flow volumes was very good for the Shand Dam data set. Figure shows the measured and modelled mean monthly volumes for both gauges for the simulation period. In general, the distribution of monthly volumes has been preserved, meaning that high and low volume months follow the pattern we would expect. March and April are the highest months because of the spring freshet, whereas July, August or September are the lowest, as they should be. For Norval (bottom graph), the observed volumes for March have been underestimated. This discrepancy is attributed to two possible reasons: a) unrepresentative rainfall, and b) inaccurate flow measurements in the period just prior to ice cover breakup. Both reasons are plausible, although the first one is most likely. Recall, that hourly rainfall data are not usually available during the winter months, and is often estimated from data measured outside the watershed, in this case Pearson Airport or Fergus Shand Dam. For low flow months (e.g. July to September), actual discharges from the two STPs (Acton and Georgetown.) would improve the simulations for the downstream locations. At present, the same mean monthly flow table is applied to each year in the modelling period for the each STP. Moreover, additional spot flow measurements would help improve the summer low flow periods by providing more information on the distribution of baseflows throughout the watershed. Overall, the agreement between the observed and simulated monthly volume plots is very encouraging, notwithstanding the complexities cited earlier. Appendix G Hydrology Impact Assessment Model G -30

39 Figure Measured and modelled mean month flow volumes for the 10 year run. Appendix G Hydrology Impact Assessment Model G -31