by Rob Gailey and Jay Lund
Domestic wells serve sizable potable water demands in California and much of the world. These wells tend to degrade and fail with declining regional groundwater levels. In areas of irrigated agriculture, impacts to shallower domestic wells may occur from ongoing groundwater use and worsen during drought when agricultural pumping increases to compensate for diminished surface water supplies. Impacts on domestic wells include increased pumping lift, pump cavitation, well screen clogging, and wells running dry. Our recent work examines the potential for managing these impacts in part of the San Joaquin Valley (shown in Figure 1) where groundwater sustainability plans were completed in 2020 as required by the Sustainable Groundwater Management Act.
As groundwater levels decline during drought, we consider mitigation actions and costs for additional pumping energy, lowering pumps, cleaning well screens, and replacing dry wells with deeper wells. These actions allow continued domestic well use. Our analysis estimates: 1) the range and magnitude of mitigation actions, 2) their likely costs, 3) where and when impacts are likely to occur, and 4) labor and time needed to resolve problems for those who depend on domestic wells for drinking water. These actions and costs are driven by how drought declines in groundwater are likely to affect existing domestic wells of varying depths.
Estimated total mitigation cost for groundwater level declines to planned management targets (Measurable Objectives defined under the Sustainable Groundwater Management Act) ranges from $42 to $96 million for this part of the San Joaquin Valley, depending on well retirement age – information known only approximately (Figure 2). If groundwater levels decline further during drought to defined limits below the management targets (Minimum Thresholds defined in SGMA plans) allowed during periods of shortage, costs increase to $120 to $249 million. Costs for groundwater levels declining to the Measurable Objectives are comparable to those that would occur from a repeat of the 2012 to 2016 drought. Costs for declines to the Minimum Thresholds are somewhat more than those estimated for twice the groundwater level decline of the 2012 to 2016 drought. The highest costs are in the northern and central study area where well densities are greatest (Figure 3). The deeper Minimum Threshold groundwater levels increase the area and depth of mitigation needs. Including older wells significantly increases costs.
Although these costs are large for domestic well owners and users, they are quite small compared to the economic benefits to agriculture from additional pumping during droughts. This remains true, even considering that the additional water pumped must be replenished after the drought, because the Sustainable Groundwater Management Act prohibits long-term groundwater overdraft.
Prices for agricultural water in this region during non-drought and drought periods are approximately $250 and at least $1,000 per acre-foot, respectively. Because additional groundwater extraction during drought must be replaced after the drought (from additional water purchases and recharge or future reductions in non-drought pumping), the value of agricultural pumping during drought may be estimated as the avoided cost of purchasing water during drought (the difference between drought and non-drought prices conservatively estimated at $750 per acre-foot). For this cost savings and considering the estimated change in storage from the Measurable Objectives to the Minimum Thresholds, the value of drought pumping for agriculture in the study area is estimated at $9 to $27 billion, or approximately $2 to $5 billion per year over a five-year drought. This compares to $34 billion in agricultural revenue during 2019 for the eight counties in the San Joaquin Valley. Domestic well costs are less than two percent of the value to agriculture from managing to the Minimum Thresholds during droughts.
Despite uncertainty in estimating specific impacts (due to incomplete records on well construction, well retirement and groundwater hydrology), it seems clear that domestic well mitigation needs and costs from agricultural pumping are likely to be large. These include thousands of pumps being lowered in wells and hundreds of kilometers drilled for well replacement. The scope of mitigation may be still greater since 1) the study area includes only 59 percent of domestic well construction records for critically-overdrafted groundwater subbasins in the San Joaquin Valley and 2) additional mitigation work would be needed for shallow agricultural wells. Although other mitigation actions, such as expanding and consolidating centralized community water systems, would often be best, maintaining well supplies is often the best or only near-term option.
The estimated labor for the largest cost (well replacement) indicates the level of effort required to mitigate domestic well impacts. For the Measurable Objective, approximately 50 to 132 km of drilling would be required depending on assumed retirement age. The requirements increase to 148 to 347 km for drawdowns to Minimum Thresholds. The unknown future timing and magnitude of droughts, and therefore potential departures from Measurable Objectives to the Minimum Thresholds, creates uncertainty in the timing and intensity of needed mitigation. Given the substantial effort and scarce skilled labor to accomplish mitigation actions for domestic wells, there may be insufficient capacity (funds and skilled labor) to complete this work as impacts occur, so pre-mitigation for the most vulnerable areas should be addressed preventatively before impacts occur.
Given the high costs to agriculture of making groundwater management plans more stringent, preventative mitigation should be undertaken for vulnerable, high-impact areas. Such measures could greatly reduce drought damages and interruptions for domestic well supplies, and reduce the cost and response time of mitigation. Assuming that mitigation measures do not create additional water quality problems, the cost of such mitigation is much less than the benefit to local agriculture from pumping additional groundwater during a multi-year drought. Domestic well mitigation in advance of droughts is a cheap way to build drought storage for agricultural water supply.
Rob Gailey is a practicing hydrogeologist in California. Jay Lund is a Professor of Civil and Environmental Engineering at the University of California, Davis, where he is also Co-Director for its Center for Watershed Sciences.
California Department of Food and Agriculture (2020) California agricultural statistics review 2019-2020
Gailey, R.M. and Lund, J.R., “Managing Domestic Well Impacts from Overdraft and Balancing Stakeholder Interests,” CaliforniaWaterBlog.com, May 20, 2018.
Gailey RM (2020), California supply well impact analysis for drinking water vulnerability webtool, Community Water Center, January 2020.
Hanak E, Lund J, Arnold B, Escriva-Bou A, Gray B, Green S, Harter H, Howitt R, MacEwan D, Medellín-Azuara, Moyle P and Seavey N (2017) Water stress and a changing San Joaquin Valley, Public Policy Institute of California, March 2017.
Lund JR, Medellin-Azuara J, Durand J, and Stone K (2018), “Lessons from California’s 2012-2016 Drought,” J. of Water Resources Planning and Management, Vol 144, No. 10, October 2018.
State of California (2021), Household water supply shortage reporting system.
Stone K and Gailey R (2019), “Economic Tradeoffs in Groundwater Management During Drought,” CaliforniaWaterBlog.com, June 10, 2019.