Board Workshop Discusses Methodologies to Determine Groundwater Overdraft Responsibility within Kings Subbasin, Among Additional GSP Topics
At the June 27, 2018 North Fork Kings GSA Board Workshop, technical consultant Kevin Johansen of Provost & Pritchard provided a detailed update on Groundwater Sustainability Plan (GSP) elements within the context of SGMA-mandated subbasin coordination. Because the State requires sustainability at the subbasin level, the seven Kings Subbasin GSAs are seeking agreement on the methodologies used to develop their individual GSPs.
The coordination group is working towards an overdraft yield estimate within the Kings Subbasin, and are seeking agreement on a methodology to assign appropriate responsibility among each GSA for that overdraft amount. Work to determine unconfined aquifer (unconfined aquifer, as opposed to confined, exists in the absence of a clay layer) storage change and groundwater boundary flows is currently underway.
Using best available monitoring well data, contour maps from the Spring of 1999 and the Spring of 2011 are used to determine aquifer storage change and boundary flows within the Kings Subbasin. Contour maps utilize groundwater elevation (feet above mean sea level) to illustrate the direction that groundwater moves. The map below (click image to enlarge) indicates groundwater elevation levels with numbers along the blue contour lines in the year 2011 in the Kings Subbasin. Water flows perpendicular to these contour lines from higher elevations to lower. The map indicates Kings Subbasin groundwater generally flows from East to West. Please note that the elevation numbers are preliminary and subject to change pending improvements in well monitoring data.Preliminary numbers for unconfined aquifer storage change are calculated for each GSA in the subbasin using groundwater elevation data from 1999 to 2011. The subbasin as an entirety has experienced an estimated average annual storage change of 206,000 AF. Please note these numbers are preliminary and subject to change pending data improvements.
A methodology for answering the important question of how much overdraft each GSA is responsible for must be agreed upon by the seven GSAs. Johansen outlined five potential methodologies for determining the apportionment of storage change responsibility.
Methodology 1: The approach of this methodology is simply to prorate the subbasin total change in storage (the estimated 206,000 AF) amount based on GSA acreage – in other words, to equally distribute the total change amongst the GSAs by acreage. This method lacks causal explanation for change in storage.
Methodology 2: The second proposed methodology utilizes only change in storage within the boundary of each GSA. This method lacks information on groundwater flow in and out of the GSA.
Methodology 3: The third alternative takes into account the GSA’s change in groundwater storage and its groundwater boundary flows. This method looks at the estimated change in storage in a GSA, together with net boundary flow (water flowing in and water flowing out) to assign responsibility.
Methodology 4: Identical to the third method, except that it accounts for historical flow patterns and its associated benefits or losses to a GSA over a period of time.
Methodology 5: The final methodology utilizes the water budget concept. This method uses calculated water demands for each GSA depending on land use, and assumes that whatever demand not met by surface water delivery or precipitation is met by groundwater pumping.
The current goal is to coordinate amongst the seven Kings Subbasin GSAs on an agreed methodology from the list above. No formal agreement has been made, but methodology 4 represents a holistic and equitable approach in calculating distribution of overdraft responsibility. Although a key component, change in storage is not enough to assign overdraft responsibility to each specific GSA because it excludes additional causal information such as groundwater flows and historical flow impacts. Assigning responsibility is key for each GSA’s individual GSP, as it sets a target for mitigation that can be achieved by projects and management actions.
The presentation included preliminary calculations, and covered a broad range of GSP elements including Kings Subbasin Coordination outline in the above article, Water Budget, Sustainable Management Criteria, Preliminary Water Quality Characterization, Management Area Considerations, and Next Steps.