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Integrated Modelling with GSFLOW in a Complex Watershed on the

Niagara Escarpment

E.J. Wexler1, Jacek Strakowski2, Dirk Kassenaar1, Mason Marchildon1, Pete Thompson1, Richard Niswonger3

1Earthfx Incorporated Toronto, ON, Canada 2Conservation Halton, Milton, ON, Canada

3U.S. Geological Survey, Carson City, Nevada

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Presentation

•GSFLOW Overview

•Study Area

•Model Development and Calibration

•Insights

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Why GSFLOW?

•Integrated modelling required in systems with complex processes and interactions

•GSFLOW is:

•Fully-coupled GW/SW model

•Fully distributed

•Open source, well documented, free!

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GSFLOW is a combination of two USGS codes: MODFLOW-NWT and PRMS

PRMS handles climate and soil water processes MODFLOW simulates GW flow, unsaturated flow (UZF),

streamflow and lakes (SFR2/LAK3)

GSFLOW Processes

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Study Area

•Source Water Protection study conducted for Conservation Halton

•270 km2 study area

•Study area straddles Niagara Escarpment

•Three wellfields

•Several large quarries

•Complex reservoir operations

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Topography

•Bedrock plain above Niagara Escarpment and Halton Till plain below •50-100 m elevation change. Some near vertical slopes

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Surface Water •Model represented 475 km of streams •275 natural lakes, wetlands, reservoirs and quarry ponds

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Reservoirs

•Two managed reservoirs: Hilton Falls Reservoir and Kelso Lake •Flow into Hilton Falls controlled by upstream diversion and quarry discharge •Water is released from Hilton Falls to help maintain high summer stage in Kelso Lake •Minimum outflow from Kelso Lake needed for waste assimilation

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PRMS Submodel Inputs

Climate NEXRAD rainfall EC station at Georgetown (snowfall) McMaster U. and Local Sites (all have different PoRs) Solar Radiation

Soil Properties Estimated from Surficial Geology and Ag. Soils Maps

Land Cover from Airphotos by CH Estimate vegetative cover and imperviousness

Topography Slopes from 5-m DEM

Study area

NEXRAD Coverage

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Halton Till to east, Wentworth

Till in west

Exposed bedrock and thin drift in

centre

Outwash in re-entrant valley

Outwash and bedrock high

recharge areas

Surficial Geology Halton Till

Wentworth Till Bedrock

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Land Use – Milton area

Hwy 401 and Maplehurst Correctional Complex Land Use Class assigned to 20-m Grid

% Impervious Based on

Land Use Class

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Cascade Flowpath Generation

20-m DEM analyzed to generate “many-to-one” network Cascade flow paths mimic dendritic pattern of mapped streams

Need to route runoff from cells

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Net Precipitation August 25, 2007

Actual ET

Runoff Infiltration

PRMS Daily Flows

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MODFLOW Submodel

Inputs

•Variable grid (matched to PRMS 20-m uniform grid)

•5-m cells in wellfields

•Maximum 80-m cell

•375 rows and 310 columns

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Geologic Analysis

•Over 13,000 wells (mostly water wells) used to pick bedrock surface and tops of other formations. •Picks kriged to form continuous surfaces

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MODFLOW Layers

•Model layers follow stratigraphy

•Upper weathered bedrock treated as separate layer

•Layers adjusted to preserve continuity.

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Stream and Lake Interaction

•PRMS directs runoff to lakes and streams, kinematic wave used to route streamflow

•Lake stage/area stage/volume relations developed by mapping lake to grid

•Reservoir operations incorporated

•Groundwater leaks in/out of each lake cell and stream segment depending on stage and aquifer head.

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Calibration Targets

•High quality wells with continuous WL data

•Water Well data

•16-Mile Creek gauge

•Estimated outflows from reservoirs

•Reservoir stage

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Regional Calibration

•Simulated heads in (Sept 2009) (blue) •Interpolated Static Water levels from Water Well data (brown)

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MODFLOW-NWT stable even when modelling steep gradients Shows daily, seasonal, and year-to-year response in heads Click for Animation

Limestone Creek

Milton Outlier

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Simulated Stage in Reservoirs

OBSERVED PREDICTED

Kelso Lake

Hilton Falls

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Simulated Heads in Wells

OBSERVED PREDICTED

PGMN Well W00008

Quarry Well BH-112

Mohawk Raceway Well – Opsteen Farm

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Simulated Heads in Wellfield Monitors

OBSERVED PREDICTED

Campbellville – MW2/05B

Kelso TW3/67

Kelso TW4/67

Kelso wells out of phase with normal seasonal response. Shows influence of high leakage from lake at summer stage

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Insights and Conclusions

• Model calibration successful at regional and local scales

• Could not match groundwater heads without good

representation of recharge and surface flows

• Could not match surface flows without representing

complex geology in re-entrant valley

• Model useful for region water budget analysis as well as for

analyzing wellfield and reservoir operations.

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Click for Animation Questions? Special thanks to Conservation Halton and USGS

staff for input and help in this study

Limestone Creek

Milton Outlier