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.
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).
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.
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%.
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.
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).
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.