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1. 1
Integrated Surface Water and
Groundwater Interaction Modelling using
GSFLOW
Watertech 2012
Dirk Kassenaar
Earthfx Inc.

2. 2
Presentation Overview
► Integrated GW/SW Modelling
 It’s not as simple as you think is it
 When is it needed?
► GSFLOW Overview
 Model design by committee: who won..
► GSFLOW capabilities - illustrated through applications
 Water budgeting and permit allocation:
► GW/stream linkage, total flow routing
 Eco-hydrology, wetlands, lakes and reservoirs
► GW interaction with lakes and wetlands, reservoir control structures
 Land use change and Low Impact Development
► Hydrology, soil and overland flow processes details

3. 3
Do you dream in polygons,
lines or cells?
 Polygons and HRUs: You’re a catchment hydrologist
 Lines and Sections: You’re a hydraulic engineer
 Cells and Layers: You’re a born groundwater
modeller
 Integrated Modelling= How do we move water
between these geometric shapes??
 GW/SW/SW modelling?

4. 4
SW and GW Model Representation
► Catchment Modelling (Hydrology):
 Basic unit: Hydraulic Response Unit or HRU
 Calibration focus: Flow at a gauge
 Strong: Simulation of climate, soil processes and storage
reservoirs
 Weak: GW represented as a bucket with a decay term
► Hydraulic Modelling:
 Basic unit: 1-D channel reach and section
 Calibration focus: River level stage (flood levels)
 Strong: Flood wave and peak flows
 Weak: GW?? (too slow to consider) Climate?
► Groundwater Modelling:
 Basic unit: Layers of interconnected cells
 Calibration focus: GW levels
 Strong: 3D distributed detail and levels
 Weak: inflows/outflows (recharge??, baseflow separation??)
► Integrated Modelling: Both Flows and Levels

5. 5
Model Zones
Soil water
Unsaturated
zone
Precipitation
Evapotranspiration
StreamStream
Evaporation
Precipitation
Infiltration
Gravity drainage
Recharge
Ground-water flow
Zone of aeration
Zone of saturation
Soil-zone base
Zone 1: Hydrology (Vegetation, Snow and Soil)
Zone 2: Hydraulics
(River Channels,
Wetlands and Lakes)
Zone 3: Groundwater (Unsaturated and Saturated aquifer layers)
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6. 6
When is Integrated Modelling NOT Necessary?
► Hydrology: Small catchments with limited GW
 Poor aquifers: Little chance of cross-basin flows,
limited losses to GW
 Flashy catchments/run-off dominated systems
► Hydraulics: Peak flow, storm and flood modelling
 Time frame in minutes: Will that peak flow take out
the bridge?
► Groundwater:
 Long term transient or steady state issues
 Short term pumping test analysis (no recharge events)

7. 7
When is Integrated Modelling
REALLY Necessary?
► Whenever there is significant movement of water between the zones:
Hydrology (soil), Hydraulics (channel), GW (aquifer)
 Significant individual or cumulative stress in one zone such that water may
move between zones
 Time frame of days to years
 When detail is necessary (sub-catchment or site level)
 When SW and GW watersheds diverge
► Typical integrated model applications
 Water budgeting, cumulative impact, permit allocation, drought impact
 De-watering, mine pit re-filling, tailings pond analysis
 Eco-Hydrology, wetland and fisheries impact assessment, low flow analysis
 Land use change, land development

8. 8
Ideal Integrated Model
► Distributed, cell based, detailed where needed to represent
engineering issues and stresses
► Physically based processes, but, more important, processes that match
the scale, resolution and available data
 Larger than the pore scale, but smaller than a lumped catchment
► Strong representation of the geometry and processes that interconnect
the systems
 Clear and direct interconnection
 Capability to represent shallow subsurface layer geometry
► Calibration emphasis on measureable flows and levels
 Precipitation, GW Levels, total stream flow at the gauge
► Simulation processes and time steps that represent real world stresses

9. 9
USGS-GSFLOW
Soil water
Unsaturated
zone
Precipitation
Evapotranspiration
StreamStream
Evaporation
Precipitation
Infiltration
Gravity drainage
Recharge
Ground-water flow
Zone 1: Hydrology (PRMS)
Zone 2: Hydraulics
(MODFLOW SFR2 and
Lake7)
Zone 3: Groundwater (MODFLOW-NWT)
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► GSFLOW is a new USGS integrated GW+SW model
 Based on MODFLOW-NWT and USGS PRMS (Prepitation-Runoff Modelling System)
 Both models fully open source, proven and very well documented
► GSFLOW Model Integration- design by expert committee:
 First author is a SW modeller, but there is strong evidence that the GW modellers
were quite persuasive (2 of the 3 zones are based on MODFLOW)…

10. 10
GSFLOW Hydrogeology:
MODFLOW-NWT
(aka MODFLOW-2011)

11. 11
GSFLOW Hydrogeology
► Based on MODFLOW-NWT (MODFLOW-2011)
 New Newton-Raphson matrix solver – all new engine
 Designed for complex variably saturated and topographically complex systems
 Designed for wet/dry converting layers
► ie. The shallow subsurface where GW and SW interact
 Uses variable cell size MODFLOW FD grid
► GW Inflows: GW Recharge (PRMS discharge to Unsaturated Zone Flow package)
 Either 1-D Richard’s equation or simple plug flow
► Selectable on a cell by cell basis within the model
► Very fast – use allows advanced UZF to be simulated only where necessary
► GW Outflows: New discharge processes supported:
 Groundwater discharge (including ET) to the soil zone (and subsequent interflow)
 Groundwater discharge to streams, lakes and wetlands using the SFR2 package
Unsaturated
zone
Ground-water flow
Evapotranspiration
StreamStream
Gravity drainage
Recharge
Water table
Water table
Ground-water flow

12. 12
GSFLOW Dual-Grid Design
► GSFLOW can use two different grids for the SW and GW processes
► GW Mesh:
 Uniform or variable cell sized MODFLOW-style grids
 Allows refinement around the wells or significant geologic features
 Used for aquifer layers, lakes and wetlands
► SW Mesh:
 Polygon catchments (to keep the hydrologists happy), or, uniform or
variable cell sized MODFLOW grids
► Benefit: Add cells and resolution only where needed
 High resolution DEM for surface processes, runoff and focused recharge.
 Use variable cell GW mesh for refinement around well, drawdown
prediction,

13. 13
GW Feedback: Dunnian Runoff
► Runoff that occurs off fully saturated soils
 Occurs when the water table is at or near surface
 Not sensitive to surficial material K
► Can create runoff from saturated gravels
 Spatially controlled:
► Tends to occur in stream valley areas
 Seasonally controlled:
► Tends to occur in spring when water table is higher
► Not sensitive to rainfall intensity or model time step
Unsaturated
zone
StreamStream
Gravity drainage
Recharge
Ground-water flow

14. 14
Dunnian Runoff
► Likely occurs where depth to water table is less than 2 m
 Stream valleys and slopes where flowing wells, springs and headwater seeps
are present

15. 15
GSFLOW HydroG Conclusions
► Limits: Yes, it is still MODFLOW
 But… not the MODFLOW that your father used
 GSFLOW does build on the extensive industry knowledge of MODFLOW
► MODFLOW portion of GSFLOW can be run independently
► First GSFLOW time step is simply a MODFLOW Steady State simulation
► GSFLOW GW features:
 New solver designed for complex shallow geometry and wet/dry layers
► Ideal for rewetting problems such as mine filling
 Variable cell-sized GW mesh can be defined independently of the SW mesh
 New processes: GW discharge to soil zone (and interflow)
► Benefit:
 High resolution representation of
 Full simulation of Dunnian (saturation excess) runoff (GW feedback)

16. 16
GSFLOW Hydraulics:
Stream Routing

17. 17
GSFLOW
Hydraulics
► Streams can pick up precipitation,
runoff, interflow, groundwater and
pipe discharges
► Stream losses to GW, ET, channel
diversions and pipelines
► GW Leakage/discharge is based
on head difference between
aquifer and river stage elevation
 An extra stream bed conductance layer
exists under each river reach
 Similar to MODFLOW rivers, but the
head difference is based on total flow
river level
River Loss
River Pickup

18. 18
(Markstrom et.al., 2008)
GSFLOW: Stream Channel Geometry
► The Stream Flow Routing package (SFR2) represents stream channels using an
8-point cross-section in order to accommodate overbank flow conditions
► Streamflow depths are solved using Manning’s equation
► Different roughness can be applied to in-channel and overbank regions
► SFR2 incorporates sub-daily 1D kinematic wave approximation if analysis of
longitudinal flood routing is required

19. 19
GSFLOW Application: Water Budget
► Water budgeting, permit allocation and cumulative impact
assessment
 Sample question: What is the contribution of stream leakage to the
aquifer system during the summer?
► GSFLOW Simulation:
 Detailed analysis of various components of the stream/aquifer
interaction
 Sample animations showing summer stream flows and leakage

20. 20
Total Streamflow and Hortonian Runoff
► Gradational Stream Color:
Total stream flow
accumulation
► Background Blue Pulses:
Runoff from rainfall events
► Animation shows headwater
tributaries flowing after a
storm and then drying up
during the summer months
► Animation Link

21. 21
GW Leakage to Streams
► Blue stream reaches:
Streams that pick up water
from the GW system
► Red stream reaches:
Streams that loose water to
the GW system
► Red Pulses: Runoff from
storm events raise water
levels in the stream and
drive water into the aquifer
system
► Note reversals in GW/SW
gradient
► Animation Link

22. 22
Baseflow Discharge
► Gradational Stream Color:
Total baseflow discharge to
the streams
► Note: During storm events
the stream levels rise and
reverse the GW/SW gradient
(baseflow discharge stops
when the stream levels rise)
► Animation Link

23. 23
GSFLOW Stream Routing Conclusions
► GSFLOW features:
 Streams can be incised in the GW system layers
 Interaction is conceptually similar to MODFLOW Rivers, but with total flow routing
 Streams can dry up and later rewet
 Every component of the stream flow can be identified and visualized
► Limitations: Stream routing simplified when compared to storm water models
 Timing and channel flow representation not ideal for peak flow or flood modelling
 (However, GW interaction is likely not significant during peak flow analysis)
► Overall benefits for water budgeting and cumulative impact:
 Full accounting of gains and losses to the stream network
 Ideal for simulation of impact during low flow conditions
 Allows calibration to total measured streamflow at the gauge
► Much more direct than trying to calibrate to a baseflow estimate

24. 24
GSFLOW Eco-Hydrology Application:
Wetlands, Lakes and Reservoirs

25. 25
GSFLOW Application: Eco-Hydrology
► Eco-hydrology broadly includes the assessment of wetlands, streams
and fisheries issues
 Existing catchment and hydraulic models cannot represent the GW
discharge dominated low flow conditions that are essential to
understanding the hydroperiod of a wetland
 Existing GW models can simulate discharge to wetlands, but without
simulating total flow and stage (GW and SW) they may over-estimate
► Issues:
 Spring storage and leakage to GW (fill, spill and leak)
 Hydroperiod assessment: preservation of temporal water level patterns
 GW connection: many lakes and wetlands are both gaining and loosing
 Reservoir control structures and water management
 Baseflow discharge

26. 26
GSFLOW Lakes and Wetlands
► Separate water balance done for
each lake to determine y:
► QIN + P – E – QLEAK(y) = QOUT(y)
► Wetlands and lakes can penetrate
multiple aquifer layers
► SFR2 handles lake
inflows and outflows.
► Outflow can be a fixed rate or
determined by stage-discharge
► Multiple inlets and outlets are
allowed

27. 27
GSFLOW Application: Eco-Hydrology
► Example Application: Evaluate the role of a network of
vernal pools, wetlands, quarries and reservoirs in
maintaining stream flow across a wellfield
 Vernal pools (sloughs): fill in the spring, gradually dry up through
the summer

28. 28
Example: Surface
Water Features
► 475 km of mapped
streams
 Many reaches are
actually riparian
wetland complexes
► 338 Wetlands
► 12 Lakes and ponds

29. 29
Surface Water
Features
► 2 Reservoirs with
multiple structures
 Gates, stop logs,
intakes, and spillways
► 1 Diversion
► 1 Quarry Discharge
Point
► Surface Water
Takings from permit
and water use
databases
Quarry
Diversion
Reservoirs
Wellfield

30. 30
Simulated Stage
in Lakes and
Wetlands
Some Long
Riparian wetlands
broken into linked
chain

31. 31
Simulated GW
Seepage
from Lakes and
Wetlands
Seepage In (red)
Seepage Out (blue)

32. 32
Simulated Seepage
from Lakes and
Wetlands
Seepage In (red)
Seepage Out (blue)
Most wetlands
show upgradient
GW inflows and
downgradient GW
outflows

33. 33
Seepage In (red)
Seepage Out (blue)
Wellfield pumping
enhances leakage
from reservoir
Cross section through
reservoir/wellfield
Wellfield
Quarry
Reservoirs

34. 34
Simulated Stage in Reservoir (as per Operation Rules)
Shows Release from Outlets for Flow Augmentation
Actual Operations differ from “Operating Rules”
Constant
Head
No-Flow

35. 35
GSFLOW Application: Eco-Hydrology
► Conclusions:
 GSFLOW can simulated wetlands, lakes and reservoirs that cross-
connect multiple aquifer layers
► New MODFLOW-NWT solver can simulate the complex variably
saturated wet/dry cells in and around the lakes
 Seasonal storage in the wetlands is significant: fill in the spring and
drain through the summer

36. 36
GSFLOW Hydrology Application:
Soil Zone Processes and Recharge

37. 37
GSFLOW Cell-based
Hydrology Model
► Fully distributed cell-based model – each cell has unique parameters
 Land use, surficial geology, slope, aspect, elevation, etc.
► Interception storage, depression storage and percent imperviousness are all
distributed according to the land use mapping
► Snowmelt is handled using a 2-layer energy balance approach
 Treats snow pack as a porous medium allowing mass redistribution and refreezing.
 Spatial distribution of the snow pack is handled by a locally-derived snow curve.
 Frozen soils are modelled using an infiltration limiting rate
► Two forms of runoff generation are modelled:
 Infiltration rate capacity (Hortonian flow)
 Saturation excess (Dunnian flow)
Soil water
Evapotranspiration
StreamStream
Surface runoff
Precipitation
Infiltration
Surfacerunoff
Interflow

38. 38
GSFLOW Hydrology

39. 39
GSFLOW: Sub-Cell Processes
Rooftop
Impervious areas &
Depression storage
Pervious area
Tree canopy
interception
Micro-topographic
depressions
• Sub-cell components
• Impervious area
• Impervious depression storage
• Direct runoff
• Option to route water from impervious
to pervious areas
• Pervious area
• Pervious area depression storage
• Canopy interception Parking
Grass
(1 model cell)

40. 40
GSFLOW: Soil Zone
• Soil zone is essentially
three integrated
reservoirs that fill, drain
and spill
• Multiple algorithms
available for runoff
partitioning
•Linear and non-linear
contributing area
infiltration routines
•SCS) Curve Number
(CN)
•Green and Ampt (new
– hourly option)
Fast & slow
interflow
(Tension storage)
Groundwater recharge
(Markstrom et.al., 2008)

41. 41
…
Overland runoff
Interflow
To stream channel
GSFLOW: Overland Runoff
► Runoff pathways are defined by digital terrain model
► Both runoff and subsurface/interflow are routed
 Pathways represented by a distributed cascade of linear and/or
non-linear reservoirs, every cell having their own independent
reservoir.
 The cascade is continued until a stream or swale (e.g., surficial
depressions, hummocky topography) is reached
► Runoff from one cell can infiltrate in an adjacent cell

42. 42
GSFLOW Application: Urban
Development and LIDS
► Land use change and urbanization considerations:
 Need to understand and mitigate increases in runoff
► Aging infrastructure – old sewers cannot handle flows
► Urbanization in upstream portions of a catchment
► Climate change and storm intensity
► Preservation and restoration of urban rivers and wetlands
► Solution: Low Impact Development (LID)
 LIDs are used to reduce runoff through enhanced GW infiltration
 LID design options include:
► Bioswales, infiltration galleries, permeable pavers, green roofs, etc.

43. 43
Sample LIDs Assessment with GSFLOW
► Proposed new development for 70,000 residents
Proposed commercial area
Proposed Low Density Residential
Existing wetlands
GW fed streams and wetlands

44. 44
Runoff: Pre-development
► GSFLOW overland run-off and interflow simulated with
cascading inter-cell flow network
Till uplands

45. 45
Runoff and Interflow Animation
► Soil zone moisture and runoff patterns Animation Link

46. 46
Recharge: Pre-development
► GW Recharge is not one-dimensional but includes both re-
directed runoff and vertical infiltration
Higher recharge at the
geologic contact due to
re-infiltration of runoff
Till uplands
Coarser grained
beach deposits

47. 47
Recharge: Post-development (no LIDS)
► Simulations indicate local wetland and stream features
affected by both changes in runoff and recharge
Lost recharge due to land
use change

48. 48
Recharge: Post-development (no LIDS)
► Lower recharge and runoff from the residential lots
Lost recharge and wetland
discharge to due to both
development and runoff
changes

49. 49
Recharge: Post-development (with LIDS)
► Simulations of residential LIDs (roof leaders to yards) and larger scale
LID features (3rd pipe infiltration galleries and ponds)
Unlined ponds added to
enhance infiltration in
vicinity of wetlandsInfiltration gallery
under commercial
developments

50. 50
Regional Level LIDS Assessment
► Simulation of catchment scale GW discharge patterns
High baseflow discharge
GW discharge to
stream and wetlands

51. 51
Conclusions: Integrated Modelling
► When is integrated GW/SW modelling really necessary?
 Whenever there is significant stress that might cause water movement
between zones (soil/channel/aquifer)
► Integrated model calibration: both flows and levels
 From a GW perspective, integrated modelling actually simplifies the
calibration because it allows direct calibration to observed precipitation and
measured stream flow
► Choosing an integrated model:
 Is the simulation process scale and resolution (spatial and temporal)
consistent and balanced?
► Why choose a peak flow channel model coupled to a GW model?
 Need to think about the coupling zones:
► Shallow layer geometry and wet/dry layers
► Refinements in the areas of interest

52. 52
Conclusions: GSFLOW
► GSFLOW: An integrated model designed by hydrogeologists
 A groundwater model with surface water processes
 MODFLOW, but significantly adapted to handle shallow wet/dry problems
► Ideal for:
 Analysis of cumulative impact of GW takings on SW features
 Eco-hydrology, fisheries, drought and low-flow condition analysis
 Problems involving pits, lakes and wetlands that incise one or more
subsurface layers
► Not for:
 Surface water storm flows, peak flows, flood waves
 Detailed in-channel flow and level simulation
 “Flashy” catchments

53. 53
Integrated Modelling: Insights
► Infiltration and recharge are 3D processes
► Recharge is much more variable than you think
► Hydrologists now think that subsurface layer geometry
drives runoff
 Google “old water paradox” and the 2011 Birdsall-Dreiss Lecture
 You will need to rethink your shallow layer conceptual model
► You cannot independently calibrate the SW and GW
components and then “slap” them together
 If you could, you probably don’t need an integrated model