Rob Gailey, Graham Fogg, Thomas Harter, Jay Lund, Helen Dahlke, Richard Frank, Tim Ginn, Richard Howitt, Mimi Jenkins, Bonnie Magnuson, Josué Medellín-Azuara, and Samuel Sandoval Solis

The Sustainable Groundwater Management Act of 2014 creates an opportunity to establish standards for the way California accounts for its stores of groundwater, which provide up to 60 percent of the state’s water supply during droughts.

The new law requires regional agencies to prepare Groundwater Sustainability Plans for “high” and “medium” priority groundwater basins, as designated by the California Department of Water Resources [1].

Earlier this year we suggested an outline for developing the plans in an orderly, scientific and transparent fashion. Central to each plan will be a water budget analysis, which catalogs the conceptual framework and available data on the hydrologic function of groundwater basins. The water budget serves as a summary of knowledge on a basin and a potent screening tool to evaluate approaches for sustainable management.

Water budget analyses may include 1) assessment of conditions during previous periods when current sustainability criteria would have been met and 2) consideration of options to adjust future basin inflows and outflows to achieve sustainability.

If time, financial resources and data availability are limited, the water budget analysis may be the only assessment performed for a Groundwater Sustainability Plan. In other cases, the water budget may be the starting point for more detailed modeling, which is often essential for reducing water budget uncertainties and ascertaining effects of various management options.

A water budget compares inflows and outflows to show change in storage:

Inflows – Outflows = Change in Storage

Because the left side of the equation equals the right side, the term “water balance” is also used. The change in storage will average close to zero over long periods of sustainable management.

Each term in the equation can have several components:

• Inflows: groundwater flow from neighboring groundwater basins; seepage from natural streams, lakes and wetlands; recharge from precipitation, stormwater runoff, and agricultural return flows; and intentional recharge from infiltration ponds or wells
• Outflows: well pumping; groundwater flowing to neighboring basins; seepage to springs, rivers, wetlands or lakes; uptake by plant roots
• Change in storage: gains and losses of water within aquifers, ranging from water stored in coarse sands and gravels to finer silts and clays (where losses are the main cause of land subsidence and are not reversible)

Although the groundwater balance concept is simple, estimating components of a water budget can be challenging. Most wells in California are not metered. Water flows are otherwise hard to measure because they generally occur over large areas and long periods of time. Underground flows, of course, cannot be measured directly because they are hidden. Also, the terms of water budget equations vary over time, making results depend on the period analyzed [2].

Here are some approaches for estimating water budget components:

Importantly, many of the water budget component estimates in the table have large uncertainties and are both time dependent and interdependent. In other words, a groundwater budget is transient, and the transient changes are interdependent among the components. Fortunately, construction and calibration of groundwater flow models can reduce uncertainties and represent transients and interdependencies.

Water budget components and approaches for estimating quantities will vary among basins because of differences in hydrogeologic conditions. For example:

• Types of recharge and their relative importance will vary among basins. In the Tulare Lake hydrologic region, recharge generally occurs 1) on eastside alluvial fans from precipitation and snowmelt, 2) throughout the basin from agricultural return flows and 3) at artificial recharge sites. In contrast, recharge in the North Coast and Central Coast regions generally occurs from localized agricultural return flows and from more broadly distributed precipitation and runoff.
• Groundwater inflow and outflow at basin boundaries will be easier to estimate for some basins more than others. For coastal basins, water-bearing formations are sometimes located in valleys cut into much less permeable rock. So it may be reasonable to estimate there is no flow across some basin boundaries. The task for some coastal basins will be evaluating flows at the boundary with the ocean. In the Central Valley and in some regions of Southern California, however, boundaries between basins are usually porous, requiring calculations of inter-basin flows.
• Evaluating changes in storage will depend on basin geology and variations in groundwater levels over time. Analysis would likely include 1) spatial interpolation of water levels in coarse sediments at different times and 2) quantifying contributions of water stored in different sediments [3]. This evaluation may be easier for less complex basins or basins with better characterization and monitoring.

Preparing Groundwater Sustainability Plans will involve uncertainties, as does most decision-making in life. Stakeholders and experts will differ in their opinions on the significance of the uncertainties in water budget analyses and in projections of future climate, land use, surface water deliveries and other conditions. However, concerns over uncertainties need not preclude action. Often there is sufficient information to proceed on some important actions, even as information to support additional actions is being developed.

A constructive approach for moving forward with the plans may be to 1) accept the inevitability of some uncertainty, 2) implement actions based on current information and 3) plan to adjust actions as new information becomes available.

Uncertainty analysis can play an important role in developing and evaluating management approaches. New data can come from additional basin characterization; monitoring groundwater system responses to management actions; refinement of groundwater budgets, models and ancillary calculations; and updated information on management plans and actions. Water budget analysis will prove useful in performing uncertainty analysis and accounting for new information as it becomes available.

The authors, all with UC Davis, have been examining implementation of California’s Sustainable Groundwater Management Act.

[1] Initial Groundwater Basin Prioritization under the Sustainable Groundwater Management Act

[2] Periods of analysis on the order of several years to decades are likely to be the most useful for evaluating issues related to sustainable management.

[3] Releases from storage occur over the full range of sediment textures in aquifer systems. The time scales of the releases can vary and is longer for finer sediments. Specific circumstances will dictate approaches used to estimate volumes of groundwater released from storage. In some cases, modeling may be used to integrate information about a groundwater system and help estimate storage releases.

Further reading

Lund, J, T. Harter, R. Gailey, G. Fogg, R. Frank, H. Dahlke, T. Ginn, S. Sandoval Solis, T. Young, A. Fisher, R. Langridge, J. Viers, T. Harmon, P. Holden, A. Keller, M. Kiparsky, T. Greene, S. Mehl, J. Gurdak, S. Gorelick, and R. Knight. 2015. “Creating effective groundwater sustainability plans.” California WaterBlog. March 4, 2015

Lund, J. 2015. ”When water counts, accounting matters.” Los Angeles Times. April 24, 2015