1. Water levels and groundwater storage
Water levels or groundwater elevations are generally the most straightforward and commonly understood metrics that groundwater managers rely on because they are easily measured in wells, and the effect of chronic lowering of groundwater levels translates directly to failing pumps or dry wells. However, setting criteria such as minimum thresholds requires a process that includes various stakeholders and all affected beneficial users. Setting minimum thresholds for chronic lowering of groundwater levels requires the following:
- Identifying the appropriate representative monitoring wells
- Evaluating historical water levels at those wells
- Determining what past levels were too low and have resulted in significant and unreasonable impacts on beneficial users
- (Optional) Running simulation models to determine what the water levels may be in the future with projected conditions including land use changes and climate change.
DWR’s main concerns on chronic lowering of groundwater levels is that GSAs did not adequately take domestic well users into consideration when setting minimum thresholds. One challenge is that domestic well information such as location, depth, and screen intervals is not well documented. Nonetheless, an initial analysis can be done by intersecting the domestic well density and depth information from DWR with a contour map of groundwater elevation minimum thresholds. This approach provides an initial estimate of the percentage of domestic wells that would be impacted if all representative monitoring wells reach the minimum threshold at the same time, as a worst-case scenario. This provides a good starting point for beneficial users to understand what the minimum thresholds would represent, should they be reached, or worse, exceeded.
Groundwater storage can be directly correlated to groundwater elevations. Many GSPs used groundwater levels as a proxy for measuring change in groundwater storage, which is a convenient way to use the same measurement and representative wells for two different sustainability indicators. The challenge is to prove that the minimum thresholds set for water levels are adequate for groundwater storage and ensure that total groundwater pumping in the basin does not exceed the calculated sustainable yield.
2. Land subsidence
Land subsidence becomes an undesirable result if it affects land surface uses such as cracked levees, reduced canal conveyance capacity, or structural integrity of bridges and buildings. SGMA regulates subsidence that is due to groundwater pumping (and not due to other effects, such as from tectonics) and irreversible (or inelastic). Therefore, under SGMA, subsidence can be halted through management of groundwater withdrawals and thus, water levels, but it cannot be reversed. SGMA’s intent is to avoid or minimize land subsidence impacts on land surface beneficial uses, which has not been appropriately addressed in some GSPs, according to DWR’s reviews. DWR indicated the need for “additional analysis to understand the significant correlation between groundwater levels and land subsidence, particularly where groundwater levels will continue to decline” (DWR presentation to the Association of California Water Agencies) to avoid significant impacts on land users.
3. Interconnected surface water
Interconnected surface water (ISW) with groundwater is an important aspect of sustainable groundwater management, as it connects above and below the groundwater source and affects several different beneficial users of both groundwater and surface water. ISW can induce stream depletion when nearby pumping causes surface water to flow toward groundwater. ISW is also the most complex aspect of SGMA, the least understood, and the one that GSAs most struggle with in developing appropriate policies. It is not a coincidence that many GSPs failed to follow the regulations for ISW, which require an estimation of “the quantity and timing of depletion of interconnected surface water systems due to groundwater pumping.” This effect is very difficult to measure directly in the field, and modeling tools are often used to assess the following:
- Where surface water and groundwater are connected
- If surface water is losing or gaining relative to shallow groundwater, and
- The volume or rate of water exchange between surface water and groundwater.