Merced Subbasin GSA Joint Technical and Advisory Committee Meeting - - PowerPoint PPT Presentation

Merced Subbasin GSA Joint Technical and Advisory Committee Meeting November 14, 2018

Undesirable Results

• “Significant and Unreasonable” negative impacts that can
• ccur for each Sustainability Indicator
• Conditions that we do not want to occur
• Used to guide and justify GSP components
• Monitoring Network
• Minimum Threshold
• Projects and Management Actions

Brainstorming: What Undesirable Results Are We Trying to Avoid?

Chronic Lowering of Groundwater Levels Reduction in Groundwater Storage Seawater Intrusion Degraded Water Quality Land Subsidence

Depletion of Interconnected Surface Water

Chronic Lowering of Groundwater Levels Reduction in Groundwater Storage Seawater Intrusion Degraded Water Quality Land Subsidence

Depletion of Interconnected Surface Water

Undesirable Results – Comments from Stakeholder Committee

• 1. Cost of pumping water
• 2. Harder to recharge (with decline in

levels)

• 3. Energy requirements increasing
• 4. Shallow wells going dry
• 5. Well replacement costs
• 6. Decline in yields

Groundwater Elevation

• 1. Human consumption
• 2. Reduced crop yields
• 3. Soil impacts
• 4. Public health + sanitation

Degraded Water Quality

• 1. Loss of storage
• 2. Infrastructure impacts
• 3. Irreversible system impacts
• 4. Flood flow impacts
• 5. Planned projects impacts

Subsidence

• 1. SED impacts
• 2. Environmental quality + habitat

Interconnected Surface Water

Minimum Thresholds

Minimum Thresholds Need to be Developed for All Six Sustainability Indicators

Chronic Lowering of Groundwater Levels Reduction in Groundwater Storage Seawater Intrusion Degraded Water Quality Land Subsidence

Depletion of Interconnected Surface Water

Minimum Thresholds – Updated Approach

• Added 18 monitoring wells for threshold analysis
• Merced County domestic wells database
• Active wells
• Omits wells that do not meet County annular seal requirement
• Filtered for other outliers
• Minimum threshold is defined as the shallowest of either
• Historical low groundwater elevation at the monitoring well, minus a

buffer (range of min & max GWLs from 2008-2018) – this assumes that over the next 20 years, GWE will decline at approximately half the max rate seen over the past 10 years

• UNLESS this would dewater the shallowest nearby domestic well

– in this case, threshold was increased to protect nearby wells

Voluntary Wells Added

Minimum Thresholds Example: Well 31916

Well 11

Minimum Thresholds Example: Well 31916

Example:

Buffer

Minimum Thresholds Example: Well 31742

Well 11

Minimum Thresholds Example: Well 31742

Example:

(Buffer not used)

Minimum Thresholds Example: Well 32342 (new voluntary well)

Well 11

Minimum Thresholds Example: Well 32342 (new voluntary well)

Example:

Buffer

Water Budgets

Water Budgets: Defining Timeframes

Historical Water Budget

Uses historical information for hydrology, precipitation, water year type, water supply and demand, and land use going back a minimum of 10 years.

Current Conditions Baseline

Holds constant the most recent or “current” data on population, land use, year type, water supply and demand, and hydrologic conditions.

Projected Water Budget

Uses the future planning horizon to estimate population growth, land use changes, climate change, etc.

Water Budgets: Merced Integrated Water Resources Model

Key Model Features

• Hydrologic Period: WY 1965-

2015

• Detailed Stream

Configurations

• Includes Ag Conveyance

and Distribution System

• Land Use and Cropping

Patterns

• Ag Demand Estimation

Verified by Remote Sensing

• Detailed Surface Water

Delivery System

• Ag Pumping Data for Ag

Districts

• GW Pumping Estimates for

private Pumping

• Municipal Well Data

Land and Water Use Budget

[Historical Simulation]

Land and Water Use Budget

[Historical Simulation]

Land and Water Use Budget

[Historical Simulation]

Land and Water Use Budget

[Historical Simulation]

Land and Water Use Budget

[Historical Simulation]

Land and Water Use Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

Groundwater Budget

[Historical Simulation]

(25 Year Historical Average)

Inflows Outflows

Projected Water Budget – Modeling Inputs

• Hydrologic Period: Water Years 1969-2018 (50-Year

Hydrology)

• River Flows
• Merced: MercedSIM
• San Joaquin: CalSim
• Local Tributaries: Historic Records
• Land Use and Cropping Patterns:
• 2013 CropScape modified based on discussions with GSAs
• Urban Water Use:
• General Plan Buildout Conditions
• Basin Average GPCD: 300
• Surface Water Deliveries provided by local purveyors

Projected Water Budget Uses 50 Years of Historical Hydrology

• 25
• 20
• 15
• 10
• 5

5 10 15 20 25 30 35 5 10 15 20 25 30 1968 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2018

Cumulative Departure (in) Precipitation (in)

Water Year Precipitation (in) Long Term Average Cumulative Departure

Land and Water Use Budget

[Projected Conditions Baseline]

Groundwater Budget

[Projected Conditions Baseline]

Groundwater Budget

[Projected Conditions Baseline]

Inflows Outflows

Going from Water Budgets to Quantifying Sustainable Yield

• What is sustainable yield?
• “the maximum quantity of water, calculated over a base period

representative of long-term conditions in the basin and including any temporary surplus, that can be withdrawn annually from a groundwater supply without causing an undesirable result.”

• How do we develop this?
• Developed through a groundwater model scenario, modifying

conditions to balance out the change in stored groundwater over time

• How do we work toward balance?
• Implement projects and management actions to increase recharge
• r decrease production

Sustainable Yield – Modeling Analysis

• Modeling Approach
• Lower groundwater production through reduced agricultural and

urban demand across the model domain

• Assumptions
• 25-Year Implementation Period: operations will remain consistent,

and groundwater levels will continue to decline until 2040

• Inter-Subbasin Flows: adjoining subbasins will operate similarly to

Merced, whereas subsurface flows will remain similar to long-term average historical conditions

DRAFT Results: Initial simulations only address subbasin yield, analysis is needed to gauge effect on ensure minimum thresholds.

25-Years 50-Years

Basin Storage

Land and Water Use Budget

[Sustainable Yield Analysis]

Groundwater Budget

[Sustainable Yield Analysis]

Groundwater Budget

[Sustainable Yield Analysis]

Groundwater Budget

[Sustainable Yield Analysis]

Inflows Outflows

So What Does This Mean?

• Merced Subbasin will need to reduce groundwater pumping

by approximately 25% overall

• In order to meet demands, additional projects and

management actions will need to be implemented

Total Water Use

Surface Water

OVERDRAFT

Sustainable Groundwater Surface Water

Projects and Mgmt Actions

Sustainable Groundwater Yield

Projected Condition Sustainable Condition

Public Workshops

MERCED SUBBASIN PUBLIC WORKSHOPS

• Tuesday, December 6,

2018

• 6:00 - 8:00 PM

Planada Community Center, Main Hall

• 9167 Stanford Avenue,

Planada

• Thursday, December 13,

2018

• 6:00 - 8:00 PM

Franklin Elementary School, Multipurpose Room

• 2736 Franklin Road,

Merced

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