1

Assessing Sensitivity to Drought and Climate Change

with an Integrated Surface Water/Groundwater Model

at the Subwatershed Scale

MODFLOW and More 2015 E.J. Wexler, P.J. Thompson,

M.G.S. Takeda, Dirk Kassenaar

Earthfx Incorporated, Toronto, Ontario

Special thanks: Shelly Cuddy and Katie Howson

Lake Simcoe Conservation Authority

2

Lake Simcoe Restoration and Protection

► Fourth largest lake in Ontario and has history of environmental stress.

► Watershed is under pressure from increased development

► Lake Simcoe Protection Act (2008) requires that every subwatershed be studied to assess:

current water demand,

effect of future land-use change and increased water demand

response to drought

response to future climate change

► Approach: develop fully integrated GW/SW models using USGS GSFLOW code

Lake Simcoe Watershed

3

GSFLOW - Integrated GW/SW Model

► GSFLOW combines MODFLOW-NWT with the PRMS hydrologic model

PRMS code can be used as a distributed (cell-based) model

Cascade method used to route overland runoff between cells

Runoff can re-infiltrate downslope

High water table contributes to Dunnian runoff and higher ET

UZF module for unsaturated flow and GW ET

LAK and SFR2 simulate GW interaction with lakes and streams

4

Oro Moraine Study Area

Oro Moraine

Study sub- watersheds

► Study focused on three subwatersheds on NW shore of Lake Simcoe

Oro Creeks North

Hawkestone Creek

Oro Creeks South

► Oro Moraine is a high-recharge surficial deposit that feeds many headwater streams.

► Model encompassed all catchments fed by the Oro Moraine.

Model Boundary

5

Surficial Geology

► The Oro Moraine

sits on top of regional till plains

► Tunnel Channels - tills have been eroded by sub-glacial flow

► Sands plains are remnants of glacial Lake Algonquin

► Best viewed in section

Oro Moraine

Tunnel Channel

Sand Plain

Till Plain

6

Hydrogeologic Model

► A complex 3-D geologic model was available from the OGS

► Provide very detailed mapping of shallow aquifer system

► Formed basis of the groundwater sub-model layers

► Shows the Oro Moraine, regional till plains, and infilled tunnel channels

7

Surface Water System

► Study area has numerous streams and wetlands

► Flow routed through all stream segments as shown

► 85 lakes and wetlands also represented

► Four gages to calibrate GSFLOW model

7

Stream Gage

8

Hydrologic Model Inputs/Outputs

► PRMS Sub-model Inputs

Daily Climate Data

► Rainfall (NEXRAD)

► max/min temp.

► Solar radiation

Topography (DEM)

Land cover (% imperv)

Soil properties (n, fc,wp)

► Model computes daily water budget components Net P, Snowmelt, Interception,

RO, Infiltration, ET, Recharge

8

PRMS Flow Chart

9

► Results for Coldwater River (02ED007)

9

Calibration to Daily and Monthly Flows

Observed (blue) Simulated (red)

10

Simulated Recharge

► PRMS run using uniform grid with 50 m cells

► Results show average annual recharge from a 32-yr simulation

► Shows high recharge on Oro Moraine.

► Results dominated by soil properties

► Results from PRMS passed to MODFLOW

10

11

Simulated Heads: Layer 1 and 7

11

► Shallow system show influence of topography and streams. Deeper system (below regional tills) is more subdued

DROUGHT ASSESSMENT

12

13

10-Year Historic Drought Period

13

1953-1967

► Used daily climate data from 1956-1967 drought to analyze subwatersheds response

► Three prior years used for model start-up

14

Drought Impact on Streamflow

► Limited drought impact in Oro North

► Moderate change in Hawkstone tribs

► Large change in Oro South tribs and main branch

► Similar patterns seen in wetland response

► Drought sensitivity depends on whether streams are linked to Oro Moraine or recharged locally

% change in average monthly streamflow at height of drought

15

Pathline Analysis

► Endpoints from forward tracking confirm that Oro Moraine feeds headwater streams and wetlands along flank

► Deep flow path emerge far from the Moraine in North Oro

► South Oro has little connection to Moraine compared to North Oro and Hawkestone

15

End Points

Pathlines

16 16

Section line is through the two watersheds. Differences in the till thickness and the aquifer continuity affect the behavior of the streams in Hawkstone and Oro South Hawkstone Creek runs along the base of the moraine cutting off flow to South Oro

CLIMATE CHANGE ASSESSMENT

17

18

GCM models of Climate Change

► Climate predictions are done with Global Circulation Models (GCM).

► Many different GCMs with different assumptions.

► Predictions of annual Temp and Precip change cover a wide range

► Most show 1.5 - 4 C increase by 2070 for Southern Ontario

► Most show increase in winter Precip and decrease in summer/fall Precip

► GW/SW models can be run with a range of GCM predictions to bracket range of likely outcomes

Increase in Mean Annual Temperature (C)

% C

han

ge

in

Me

an

An

nu

al

Pre

cip

ita

tio

n

Selected by Percentile Method

Modelled for this Study

19

Change Field Method - Baseline versus CGCM3T63

Shift in Month Temperature Values shifted by 1 to 5 C

Percent Change in Monthly Precipitation Values scaled by -45 to 45%

► Many methods for downscaling GCM outputs for local-scale models

► Change Field method selected for this analysis

Shift observed Temp by predicted monthly increase for each GCM scenario

Multiply local observed Precip values by monthly scale factor

► Selected approach does not change frequency or intensity of storms

20

Comparison of Low Flow Change – North vs South Oro

• CGMC3T63 scenario shows more flow in winter months. Spring freshet is earlier

• Reduction in summer flows due to lower rainfall and longer recession period.

• Summer flow change in South Oro more pronounced due to poor connection

Shellswell Creek (South Oro)

Bluffs Creek (North Oro)

21 21

Change in Total Streamflow – Oro South

• Ensemble of models show consistent results

• Log scale highlights significant reduction in summer flows due to lower summer rainfall and longer recession period.

22

Climate Change: Conclusions

► Climate effects in Southern Ontario:

More recharge and baseflow discharge in the winter

Spring freshet earlier due to earlier snowmelt

Drought sensitive reaches will be further stressed in summer

► Understanding the underlying geology is essential

Shallow geology is important

Interconnection of streams to recharge feature is key factor

Oro Moraine also provides high storage

► GSFLOW proved extremely useful for analyzing GW/SW response under a variety of climatic conditions.

23

Baseline Click for animation CGCM3T63

Thank you! Questions or Comments?