GWA Advisory Committee April 24, 2019
Agenda 1. Approval of Minutes of April 10, 2019 2. Sustainability Indicators • Land Subsidence • Depletion of Interconnected Surface Water 3. Monitoring Network 4. Groundwater Dependent Ecosystems Approach 5. Inter-basin Coordination 6. Next Steps and Key Decisions for the GWA 7. May Agenda Items 2
Sustainable Management Criteria Definitions 4
Objectives for SMC Discussion Sustainable Management Criteria Discussion Objectives: • Review approach for the sustainable management criteria • Review policy decisions related to minimum thresholds, measurable objectives, and monitoring network, to be brought to the Board in May 4
Sustainability Indicators: 5. Land Subsidence
Subsidence has not been Observed Historically in the Subbasin Monitoring Stations (USGS) DRAFT 6
Using GW Levels as a Proxy • The use of groundwater levels as a proxy metric for this sustainability indicator is justified by the significant correlation between groundwater levels and land subsidence and is necessary given the lack of extensive monitoring for land subsidence. DRAFT 7
Justification for Using Levels as a Proxy • Land subsidence requires two conditions – dewatering of subsurface materials and that those dewatered subsurface materials be compressible. • Historical declines in groundwater levels are not known to result in subsidence. • If the basin were to operate within the margin of operational flexibility proposed for GW levels, future dewatering would take place in similar geologic units to those currently dewatered. • It is therefore anticipated that additional declines in groundwater levels are unlikely to cause subsidence, as dewatered materials are expected to behave consistently with historical dewatering, which resulted in no known subsidence. Thus, the groundwater level minimum thresholds are protective against additional subsidence. 8
Geologic Composition Area of deepest proposed GW levels threshold (-130 ft MSL) 9
B- B’ Cross -Section Dewatering areas are primarily from within the Modesto/Riverbank Formation both under current conditions and at the MT condition for GW levels. 10
Action – Land Subsidence Action Needed: Recommendation on Sustainable Management Criteria for Land Subsidence. Policy decision will go to the Board in May. Sustainable Management Criteria Summary – Land Subsidence Criteria Narrative Description Consistent with groundwater levels minimum thresholds Minimum Threshold Consistent with groundwater levels measurable objectives Measurable Objective Consistent with groundwater levels interim milestones Interim Milestone Consistent with groundwater levels definition of Definition of Undesirable Result undesirable result 11
Sustainability Indicators: 6. Depletion of Interconnected Surface Waters
Two Approaches Approach 1 – Set minimum threshold and measurable objectives using stream modeling estimates Approach 2 – Use groundwater levels as a proxy, and monitor for depletion of interconnected surface water at selected monitoring locations 13
Depletions of Interconnected Surface Waters • Depletions are the additional losses or reduced gains in surface water bodies caused by groundwater production. • Quantification of depletions is relatively challenging and requires significant data on both groundwater levels near streams and stage information. • Quantification of depletions would require an estimate of losses and gains without groundwater production, which is difficult to estimate accurately, in addition to the estimates of losses and gains with groundwater production. 14
Approach 1: Set Thresholds Using Stream Modeling Estimates Losses may be used with the understanding that these losses are correlated with depletions, even if they are also correlated with higher streamflows. Quantify modeled stream losses under non-wet conditions and establish thresholds to protect against significant and unreasonable stream depletion 15
Approach 1: Set Thresholds Using Stream Modeling Estimates Maximum losses occur within the wettest years, based on the San Joaquin Valley Water Year Hydrologic Classification DRAFT 16
Approach 1: Set Thresholds Using Stream Modeling Estimates Removed all wet years as outliers with losses driven by high river stage and wider river conditions Historical Simulation Maximum of Non-Wet Years: 167,300 AFY Historical Simulation Minimum Historical Simulation Total of Non-Wet Years: 78,100 AFY Range in Losses: 89,200 AFY DRAFT 17
Approach 1: Set Thresholds Using Stream Modeling Estimates Added a buffer based on 100% of the historical range Proposed Minimum Threshold: 256,500 AFY (5-yr average of non-wet years) Historical Simulation Maximum of Non-Wet Years: 167,300 AFY Historical Simulation Total Range in Losses: 89,200 AFY DRAFT 18
Approach 1: Measurable Objectives Measurable Objective based on average from Sustainable Simulation Proposed Minimum Threshold: 256,500 AFY (5-yr average of non-wet years) Proposed Measurable Objective: 123,900 AFY DRAFT 19
Approach 2: Use GW Levels as Proxy GSP regulations allow GSAs to use groundwater levels as a proxy metric for any sustainability indicator, provided the GSP demonstrates that there is a significant correlation between groundwater levels and the other metrics. One possible approach for this is: 1) Demonstrate that the minimum thresholds and measurable objectives for chronic declines of groundwater levels are sufficiently protective to ensure significant and unreasonable occurrences of other sustainability indicators will be prevented. In other words, demonstrate that setting a groundwater level minimum threshold satisfies the minimum threshold requirements for not only chronic lowering of groundwater levels but other sustainability indicators at a given site. 2) Identify representative groundwater elevation monitoring sites where minimum thresholds and measurable objectives based on groundwater levels are developed for a specific sustainability indicator. In other words, the use of a groundwater level minimum threshold is not intended to satisfy the minimum threshold requirements for chronic lowering of groundwater but is intended solely for establishing a threshold for another sustainability DRAFT indicator. 20
Approach 2: Details on Modeling Process • Historical depletion of interconnected surface water is not known to be significant or unreasonable. • Proposed groundwater level minimum thresholds and undesirable results have an associated level of additional depletions. • Depletion above that volume is not likely, as groundwater levels below undesirable results would be required • The current groundwater level minimum thresholds (draft, pending final confirmation and calls) were evaluated to check for groundwater level undesirable results (non-dry year pairings where 25% or more of wells fall below their minimum thresholds) based on existing future simulations. 21
Approach 2: Results in Context with Streamflows • The sustainable simulation does not result in groundwater level undesirable results. • The projected conditions simulation does result in undesirable results. • The additional stream losses that occurred in the projected simulation compared to the historical simulation are estimates of depletions - they can be linked to increased groundwater pumping. • Projected conditions simulation additional depletions over the historical simulation are 70,000 AFY - approximately 1.4% of total stream outflows. • An additional 70,000 AFY of stream depletion is proposed to not be considered significant and unreasonable. • Depletions greater than an additional 70,000 AFY require groundwater levels that would be classified as undesirable results under the GWL indicator. Therefore, groundwater level thresholds are protective of the depletion of interconnected surface water. 22
Action – Depletion of Interconnected Surface Water Action Needed: Recommendation on Sustainable Management Criteria for Depletion of Interconnected Surface Water. Policy decision will go to the Board in May. Sustainable Management Criteria Summary – Interconnected Surface Water Consultant Recommendation – Approach 2 (GWE as Proxy) Criteria Narrative Description Consistent with groundwater levels minimum thresholds Proposed Minimum Threshold Consistent with groundwater levels measurable objectives Proposed Measurable Objective Consistent with groundwater levels interim milestones Proposed Interim Milestone Proposed Definition of Undesirable Consistent with groundwater levels definition of undesirable result Result 23
Sustainability Goal 4
Sustainability Goal The sustainability goal succinctly states the GSAs’ objectives and desired • conditions of the Subbasin. The proposed Sustainability goal description for the Subbasin is: to maintain an economically-viable groundwater resource for the beneficial use of the people of the Eastern San Joaquin Subbasin by operating the basin within its sustainable yield or by modification of existing management to address unforeseen future conditions. • The exact wording of the sustainability goal for Eastern San Joaquin Subbasin is still under development. A discussion measures and an explanation of how the goal will be achieved in 20 years will be presented at a later time. 25
Monitoring Network 4
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