GROUNDWATER MODELING

-- April 4, 2019

OUTLINE

1. Why Model?

2. Groundwater Flow.

3. Input Data.

4. Adaptability and Maintenance.

5. Available Models and Applications.

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1. WHAT MODELS CAN AND CANNOT DO

Can quantify groundwater conditions:

- Water level changes.

- Direction and rate of flow.

- Storage volume changes.

Can project future changes.

Cannot make decisions.

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1. WHY MODEL IS NEEDED FOR THE GROUNDWATER

SUSTAINABILITY PLAN (GSP)?

Quantify “current” conditions and project changes over the next 50 years:

Water level thresholds and objectives.

- Cones of depression.

- Increased pumping lifts

Water inflows, outflows and volume in the basin (called the ‘water budget’).

- Overdraft (storage depletion)

- Additions and losses associated with interconnected surface water

Help select projects that prevent undesirable results.

Determine data needed to monitor plan effectiveness.

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2. GROUNDWATER FLOWBegin with a hydrogeological

conceptual model (HCM)

of how water enters,

leaves, and moves

through the basin.

Many factors affect the

movement of water across a

basin over time (variable

geology, recharge, pumping,

etc.).

2. AN AQUIFER IS KIND-OF LIKE A SPONGEWater Storage and Transmitting Properties

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Aquifer’s water storage and

transmitting capability is

determined by its geology –

specifically porosity,

permeability, sediment

compressibility and ability

to release water.

2. PORE SIZE IS RELATED TO SEDIMENT SIZE (TEXTURE)

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Water is stored in the pore

space between sediment grains.

Water

Sediment

Grain Pore Space

2.EXAMPLES OF SEDIMENT TEXTURE

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GravelSandSiltClay

2. SEDIMENT DEPOSITION

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Zone #1 - High energy (e.g., gravel).

Zone #2 - Shallowing of topographic relief (e.g., sands and silt).

Zone #3 - Low energy (e.g., silt and clay).

2. STREAM CHANNEL MIGRATION AND SEDIMENT

DEPOSITION OVER GEOLOGIC HISTORY

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2. STREAM CHANNEL DEPOSITS

12Source: DWR Bulletin No. 118-3, 1974

2. FLOW (Q) = VELOCITY (V) x AREA (A)

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HYDRAULIC CONDUCTIVITY OF SOILS

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2. DARCY’S LAW CALCULATES FLOW RATE THROUGH SEDIMENT

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𝑙

H1 H2

Q = Discharge (L3/T)A = Cross Sectional Area (L2)K = Hydraulic Conductivity (L/T)H = Water level elevation or “head” (L)𝑙 = Distance Between Wells (L)

𝑄 = 𝐴(𝐾 ×𝐻2− 𝐻1

𝑙)

3. INPUT DATA –

Building a model in some ways is analogous to

constructing a house

Construct the frame - mesh/grid, layering, boundaries

Plumb and wire – define aquifer properties

Attach exterior – recharge, pumpage, water deliveries, etc.

Paint and landscape – calibration

Move-in – implementation

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3. “Construct the Frame:” Mesh/Grid

Model domain divided spatially into elements/cells representing areas of

constant properties and stresses

3. “Construct the Frame:” Mesh/Grid

Rectangles approximating a curve:

Narrower

rectangles

More accurate

approximation

3. “Construct the Frame:” Mesh/GridCells approximating an area:

Smaller cell

area

More accurate

approximation

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3. “Construct the Frame:” Layering

Basin vertically divided in to

layers based on stratigraphy

Includes thin shallow layers to

improve simulation of

SW/GW interaction

Stratigraphy inferred from cross-section F-F’ in DWR (1974) Evaluation of Ground Water

Resources: Sacramento County. Bulletin No. 118-3. Reprinted April 1980.

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3. “Construct the Frame:” Boundary Conditions

Types of Boundaries

“No flow”

“Specified flow”

“Specified head” or “Constant head”

“Head-dependent flow”

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3.“Construct the Frame:” Boundary Conditions“No flow” = No groundwater crosses boundary.

Basin

“A”

Basin

“B”

Groundwater conditions in Basin “A” are independent of Basin “B.”

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3.“Construct the Frame:” Boundary Conditions?No flow

No cross boundary flow

along “aquifer” and

“bedrock” contact.

Bedrock

No FlowNo-flow

Boundary

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3.“Construct the Frame:” Boundary Conditions“Constant Head” = Cross boundary flow calculated to

keep water level constant along boundary

Basin

“A”

Basin

”B”

GW Flow

Groundwater conditions in Basin “A” and Basin “B” influenced by

specified water level.

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3.“Construct the Frame:” Boundary ConditionsConstant Head

Basin

”B”

GW Flow

Flow in or out of lake in

response to changes in

groundwater system

and/or lake levels.

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3.“Construct the Frame:” Boundary Conditions“Specified Flow” = Cross boundary flow specified by user

Basin

“A”

Basin

”B”

GW Flow

Groundwater conditions in Basin “A” and Basin “B” influenced by

quantity and direction of specified flow.

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3.“Construct the Frame:” Boundary ConditionsSpecified Flow – darcy’s law

Basin

”B”

GW Flow

𝑙

H1 H2

𝑄 = 𝐴(𝐾 ×𝐻2− 𝐻1

𝑙)

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3.“Construct the Frame:” Boundary Conditions“Head-Dependent Flow” = Cross boundary flow proportional to

water level differences

Basin

“A”

Basin

”B”

GW Flow

Groundwater conditions in Basin “A” and Basin “B” determined by

relative water level changes within basins.

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3.“Construct the Frame:” Boundary ConditionsHead-Dependent Flow

Basin

”B”

GW Flow

Monitoring well location.

Flow calculated based on

difference between adjacent

external water level and basin

model-water calculated water

level.

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3.“Construct the Frame:” Boundary ConditionsHead-Dependent Flow

River/Stream is a special case

of head-dependent flux

boundary.

Source: Harbaugh, 2005, USGS Techniques and Methods 6-A16

Low permeability

streambed material

Low permeability

streambed material

Head at the node

in the cell

Head at the node

in the cell

Head at the bottom of the riverbed is equal to

the head at the node in the cell

Head at the bottom of the riverbed is equal to

elevation of bottom of riverbed

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3.“Construct the Frame:” Boundary Conditions

Uncertain boundary

conditions can be located

large distances from basin to

correspond with physical

boundaries or reduce

influence on basin model

results.

Model Boundary

Basin Boundary

Eastern San Joaquin Water Resources Model

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3. “Construct the Frame:” Boundary ConditionsLinked/nested models

Uncertain boundary conditions can be determined by output from other (e.g., Linked/Nested Models).

Specified water level or flux

along/across Local Model

boundaries linked to simulated

values from Regional Model.

3.“Construct the Frame:” Boundary ConditionsLinked/nested models

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EXAMPLE: regional

model-calculated water

levels used by local

model.

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3.“Construct the Frame:” Boundary ConditionsEvaluating cross boundary flows

Flow directions should be the same

Outflow from one basin should be an inflow to the

adjacent basin

The magnitude of flow to/from adjacent basins

need not be exact but should agree within

reason

Differences should not materially change conclusions

based on model output

The inferred source/sink for the water should be

the same

The water should be entering/leaving the same primary

aquifer, surface water feature, etc.

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3.“Plumb and Wire:”

Aquifer Properties

Water storage and transmitting

properties

-Aquifer tests

-Published values

-Inferred from geology and lithology

Laudon and Belitz, 1989, Texture and depositional

history of near-surface alluvial deposits in the central

part of the western San Joaquin Valley, California, USGS

Open-File Report 89-235

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3.“Attach exterior:” Inflows and outflows

Recharge

R = P + D – RO – CU + ΔS

R = recharge

P = precipitation

D = deliveries (surface water supply)

RO = runoff

CU = consumptive use (ET, urban use)

ΔS = change in storage

Precipitation

(P)

Evapotranspiration

(CU)

Runoff (RO)

Groundwater

Recharge

(R)

Surface Water

Deliveries

(D)

UNSATURATED

SOIL

WATER TABLE

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3.“Attach exterior:” Inflows and outflows

Pumping

Urban, Agricultural and Residential Pumping

- Metered

- Estimated

Land use

Typical use

Precipi-

tation

(P)

Total

Crop

Demand

(D)

Surface Water

Deliveries (D)

Groundwater

Basin

Pumpage (P)

3.“Paint and Landscape:” Calibration

An acceptable model calibration:

Employs reasonable aquifer properties (e.g. hydraulic

conductivity).

Model-calculated water levels and flows are reasonable.

- Water levels in wells generally agree.

- Seasonal and multi-year changes generally agree.

- Flow directions consistent with observed conditions.

- Volumetric water budget is consistent with independent estimates.

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3.“Paint and Landscape:” Calibration

“History matching” compares model-calculated water levels with corresponding measured values.

Adjust model input within reasonable ranges to improve match.

Ideally the differences are small and random at different locations and times.

Source: GRA WSB model presentation

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3.“Move in:” implementation

Extract water budget and estimate sustainable yield.*

Develop scenarios to represent land use changes, climate

changes, and proposed projects.*

Identifying management areas.*

Identify impacts of projects and management actions.*

Identify data gaps and monitoring networks improvements.*

* GSP Regs §354.18, §354.18(d), §354.20, §354.38, and §354.44

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4. MODEL ADAPTABILITY AND MAINTENANCE

Don’t have to be replaced

or redone

Can be updated and

improved as more and

better data becomes

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4. MODEL ADAPTABILITY AND MAINTENANCE

The level of effort for updates depend on the quality of existing input

data and the questions model is designed to answer, which usually evolve

with time.

Model maintenance is most economic and efficient when:

- There is consensus among stakeholders

- The number of owners is maximized

Model reliance is maximized.

Number of investors is maximized

Physical Models Mathematical Models

Analytical

Numerical

5. AVAILABLE MODELS – WHICH MODEL TO USE?

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𝐾 ×∆𝐻

𝑙

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5. AVAILABLE MODELS -The model platform is the numerical solver for the mathematical

model

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5. AVAILABLE MODELS -The application employs the platform to represent a specific site/basin

Example applications relevant to the Cosumnes Subbasin - the Subbasin is contained entirely within the geographic extent of the model

Central Valley Groundwater-Surface Water Simulation Model (C2VSim-FG)

Central Valley Hydrologic Model (CVHM)

Eastern San Joaquin Water Resources Model (ESJWRM)

Sacramento Area Integrated Water Resources Model) (SacIWRM)

- Updated version Cosumnes South American North American Model (CoSANA)

Sacramento Valley Groundwater-Surface Water Simulation Model (SVSim)

EXAMPLE ApplicationCentral Valley Groundwater-Surface Water

Simulation Model (C2VSIM-FG)

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EXAMPLE ApplicationCentral Valley Hydrologic Model (CVHM)

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EXAMPLE ApplicationsSacramento Area Integrated Water Resources Model

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5. SGMA STANDARDS - PLATFORM AND APPLICATION

California Department of Water Resources Sustainable Groundwater Management Program, “Best Management Practices for the Sustainable Management of Groundwater Modeling BMP,” December 2016.

California Department of Water Resources Sustainable Groundwater Management Program, “Best Management Practices for the Sustainable Management of Groundwater Hydrogeologic Conceptual Model

BMP,” December 2016.

DWR Sustainable Groundwater Management Program, “Groundwater Sustainability Plan (GSP) Emergency Regulations Guide,” 2016.

Standard Notes

Publicly available documentation IWFM or MODFLOW

Peer reviewed mathematical foundation

and model code IWFM or MODFLOW

Public domain open-source software IWFM or MODFLOW

Covers entire basin (at a minimum) Model Grid/Mesh Specifications

Boundary conditions consistent between

adjacent basin models South American and Eastern San Joaquin Valley

Based on detailed HCM In Progress

Sensitivity tests and uncertainty analysis Model Development

Model adaptability (e.g., accommodate

additional data and/or refined HCM) IWFM or MODFLOW

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TAKE AWAY

Models support decision makers – they don’t make decisions.

Expectations - No model is perfect.

Model’s can be updated and improved over time.

- Quantity and quality of existing input data.

- Questions model is intended to answer evolve.

Maintenance and improvement is most efficient when ownership is maximized:

- There is consensus among stakeholders.

- Distributes costs.