by Thomas Harter and Bill Brewster

California has a unique and highly variable climate in which drought reoccurs periodically. California began this century in a dry period from 1999 to 2005, and experienced droughts from 2007 to 2009, and 2012 to 2016.  Such wet-dry cycles can be seen in Figure 1, which shows total rainfall amounts per water year (water years run from October 1 to September 30). These dry cycles greatly affect the state’s groundwater basins.

Figure 1: California statewide annual precipitation. Source: DWR 2017

Despite the current storms, the 2018 water year is well below average, and that pattern may continue. But from a groundwater perspective, it’s clear that dry is the new norm.

Why do groundwater basins continue to suffer the impacts of drought long after the rains have returned?  As explained last spring, a single wet winter after a dry period can replenish snowpack, soil moisture, and surface water reservoirs, but groundwater basins may take many years or even decades to recover.

An average or wet winter may make up for water level losses of one dry year, but often not much more.  Also, the amount and location of groundwater level recovery varies with other factors such as the local reliance on groundwater or chronic overdraft.

At the end of the most recent drought, the near average 2016 precipitation in Northern California helped stabilize groundwater levels, and some areas saw groundwater level recovery. The extremely wet winter in 2017 expanded groundwater recovery to most of California (Figure 2).

Throughout California, the wet winter of 2017 refilled groundwater storage leading to higher water levels in spring of 2017, when compared to spring 2016. Source: DWR 2017

In many areas with significant groundwater pumping, therefore, two average to wet years are not enough for groundwater to recover from several dry or drought years.  For example, the change in groundwater levels over the last 5 years (Figure 3) or the past 10 or 17 years (Figure 4) shows that groundwater aquifer conditions can have a long memory.

Figure 3: In most of California’s groundwater basins, the wet winter of 2017 did not refill groundwater storage to where it was before the 2012-2016 drought, in the spring of 2012. Source: DWR 2017

Figure 4: For most of California, 12 of the past 18 winters (and 7-8 of the past 11 winters) were below average or dry. As groundwater levels in most basins need one average or wet winter to recover from one below average or dry year, many areas are several average to wet years short of reaching water levels observed in spring 2000. Source: DWR.

The lack of groundwater level recovery is partly from persistent below-average precipitation in the past 20 years. This can be seen by comparing the long-term change in groundwater levels with the cumulative deviation from average (CDFM) statewide rainfall (Figure 5). The recent twenty-year sequence of more below-average years than average or wet years appears as a decline in the orange line in Figure 5. For comparison, DWR’s groundwater data and tools website includes groundwater level change maps of the difference in groundwater elevations over various time periods, with pie charts indicating the regional and statewide percent of wells increasing, decreasing, or staying relatively neutral (e.g., Figures 2 and 3). We can construct a groundwater level change index, for example, by subtracting the statewide percent of wells with increasing water levels from the statewide percent of wells with decreasing water levels over a period of time.  A positive number indicates more wells had increased water levels than decreased water levels, while a negative number means more wells have lower water levels than higher water levels.  For example, for Figure 2, the statewide groundwater level change index for 2016-2017 is computed as (30.7%+5.4%-1.0%-6.3%) = +28.8%.

The cumulative deviation from mean statewide precipitation (CDFM) since 1896 (blue line) shows that we reached peak surplus in 1983, 1998 and 2006. But by 2016, after a nearly steady ten-year decline, the deficit reached levels similar to the early 1990s. Note that the CDFM is, by definition, zero at the beginning and end of the averaging period.
Average and wet years can make up for groundwater decline in below-average years: The orange line indicates the difference between the total number of average to wet years to date and the number of below average to dry years to date. If an “average year” includes any year with at least 97% of average precipitation, then an equal number of years have been “average or above” years and “below average or dry” years since 1896 (difference = 0). For 1998 to 2017, five more years were “below average or dry” than “average or above”.

Figure 6 shows this groundwater level change index for 1, 3, 5, and 10 year periods preceding each year from 2012 through 2017. The long-term trends of all four indices – perhaps most so for the 10-year index – are similar to the precipitation trends– as precipitation deficit increases, the groundwater level change index becomes more negative (more and more wells with decreasing water levels). However, as the deficit decreases, fewer wells have decreasing water levels, and more wells have increasing water levels.  This very simple analysis doesn’t account for other factors that can affect long-term changes in groundwater levels, but shows the strong effect of the continued precipitation deficit, relative to 1998.

Figure 6: Comparison of the CDFM (blue) shown in the previous figure with a groundwater level change index that captures relative groundwater level change over the last 1 year, 3 years, 5 years, and 10 years prior to the year indicated at the bottom axis. The 10 year index most closely follows the precipitation CDFM.

What should well owners and operators expect for summer and fall of 2018 if it remains a below average to dry year? This would be like 2007 and 2012.  Both 2007 and 2012 followed wet years with surface reservoirs in good condition, like 2018.  Additionally, 2007 and 2012 had below average precipitation and a thin snowpack.

So, with a below average to dry 2018, groundwater levels would likely decline similarly to 2007 and 2012, but not as drastically as in 2014 or 2015 when additional groundwater pumping occurred from lack of available surface water for irrigation (Figure 7).

Figure 7: Unless April is exceptionally wet, expected water level changes between last fall and this coming fall will be of similar magnitude as between fall of 2011 (following a wet winter) and fall of 2012 (following a relatively dry winter, but with surface water storage carry-over from 2011 to support cities and agricultural irrigation). Source: DWR.

One thing is certain – California’s climate will continue to be variable.  And if the past 20 years are a guide, groundwater levels may have a difficult time recovering.  This reinforces the importance of drought contingency planning, especially for overdrafted groundwater basins and in basins with issues related to declining groundwater levels.

Thomas Harter is a Professor and Associate Director at the Center for Watershed Sciences. Bill Brewster is a Senior Engineering Geologist with the California Department of Water Resources.

 Further reading

CaliforniaWaterblog. Post-drought groundwater in California: Like the economy after a deep “recession,” recovery will be slow.

DWR Drought Page

Spring 2017 Groundwater Level Data Summary

USGS Runoff Estimates for California

DWR Groundwater Information Center Interactive Map Application

DWR Data and Tools Page