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 occur 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 Chronic Lowering of Groundwater Levels Reduction in Groundwater Storage Reduction in Groundwater Storage Seawater Intrusion Seawater Intrusion Degraded Water Quality Degraded Water Quality Land Subsidence Land Subsidence Depletion of Interconnected Surface Water Depletion of Interconnected Surface Water

Undesirable Results – Comments from Stakeholder Committee Groundwater Elevation Degraded Water Quality 1. Cost of pumping water 1. Human consumption 2. Harder to recharge (with decline in 2. Reduced crop yields levels) 3. Soil impacts 3. Energy requirements increasing 4. Public health + sanitation 4. Shallow wells going dry 5. Well replacement costs 6. Decline in yields Subsidence Interconnected Surface Water 1. SED impacts 1. Loss of storage 2. Infrastructure impacts 2. Environmental quality + habitat 3. Irreversible system impacts 4. Flood flow impacts 5. Planned projects impacts

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 Current Projected Water Conditions Water Budget Baseline Budget Uses historical Holds constant the information for most recent or “current” Uses the future hydrology, data on population, land planning horizon to precipitation, water use, year type, water estimate population year type, water supply and demand, growth, land use supply and demand, and hydrologic changes, climate and land use going conditions. change, etc. back a minimum of 10 years.

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) Outflows Inflows

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 30 35 30 25 25 Cumulative Departure (in) 20 Precipitation (in) 20 15 10 15 5 0 10 -5 -10 5 -15 -20 0 -25 1968 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2018 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] Outflows Inflows

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 or 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 Basin Storage 25-Years 50-Years DRAFT Results : Initial simulations only address subbasin yield, analysis is needed to gauge effect on ensure minimum thresholds.

Land and Water Use Budget [Sustainable Yield Analysis]

Groundwater Budget [Sustainable Yield Analysis]

Groundwater Budget [Sustainable Yield Analysis]

Groundwater Budget [Sustainable Yield Analysis] Outflows Inflows

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 Sustainable Projected Surface Water OVERDRAFT Groundwater Yield Condition Sustainable Projects Sustainable and Mgmt Surface Water Condition Groundwater Actions Total Water Use

Public Workshops

MERCED SUBBASIN PUBLIC WORKSHOPS  Tuesday, December 6,  Thursday, December 13, 2018 2018  6:00 - 8:00 PM  6:00 - 8:00 PM Planada Community Center, Franklin Elementary School, Main Hall Multipurpose Room  9167 Stanford Avenue,  2736 Franklin Road, Planada Merced

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