An excellent post from the California Water Blog.
The 2012-2016 drought has made many of us keenly aware of how “empty” our groundwater “reservoirs” have become. As the recent series of atmospheric rivers have left us with a massive snowpack, full surface water reservoirs (with some exceptions in southern California), and soggy soils, some questions are frequently asked:
Is the drought over, even for groundwater – if not, when will well owners see full recovery of their water table? And could the massive amounts of runoff be captured to accelerate replenishment of our depleted groundwater aquifers?
The short answers: while the surface water drought is over, the groundwater drought is not. How much longer may it last? As a rule of thumb, in many areas it will take as many above average to wet years to recover our groundwater storage, as it has taken to draw it down. And while excess runoff can be used for recharge, California currently lacks the infrastructure and capacity to divert and hold flows like those released over the Oroville spillways for infiltration and groundwater storage.
Why does groundwater storage recovery take so much time? Groundwater is by far our largest of the four water reservoir systems in California, where agriculture and urban users consume about 40 million acre-feet (MAF) each year, mostly from spring to fall:
Mountain snowpack, in an average winter and spring, holds about 15 MAF
Surface water storage reservoirs have a total capacity of 40 MAF
Soils store many 10s of MAF of our winter precipitation for use by natural vegetation, crops, and urban landscaping
Groundwater reservoirs are endowed with well over 1,000 MAF of freshwater
Dynamics of groundwater storage and water level change – a history lesson
How much and when groundwater levels rise and fall varies greatly around the state. But there are some common patterns. Seasonal variations occur due to California winters being wet and cold while summers are dry and hot. Water levels rise during winter and spring due to recharge from precipitation and recharge from streams that carry winter runoff (plenty of bank deposits), while groundwater pumping is limited (small account withdrawals).
On the other hand, groundwater levels decline during the summer and fall, when pumping exceeds local recharge.
In some regions, such as the Borrego Valley basin, the depletion has been a steady decline: each summer, water levels are drawn down more than they recover in the following winter, regardless of how wet the winter may be. In other places, the decline in groundwater levels may be less obvious: year-over-year water levels fall during drought, but recover during wet years. But the recovery during a series of wet years doesn’t make up for the depletion during dry years, resulting in long-term overdraft.
Over the past 100 years, overdraft has drained groundwater resources by 150-200 MAF, with most of that depletion occurring in the middle and southern Central Valley, and in southern California. The overall decline in groundwater storage, time and again, has led to costly replacement of wells that have become too shallow to dip into a falling water table, land subsidence, seawater intrusion in coastal basins, water quality degradation in other basins, and depletion of streams that depend on groundwater for base flow during California’s long dry season.
Groundwater levels (and storage) also change over the longer term, in response to drought or wet years. In dry years, it is common to see water levels recover less during the (dry) winter. With the early onset of irrigation in the spring and lack of surface water leading to replacement with groundwater pumping, water levels drop quickly in the summer following a dry winter. In wet years, the opposite occurs: water levels recover more strongly after a wet winter and groundwater levels are not drawn down as much in the summer, resulting in a net year-over-year rise in water levels.
Groundwater levels are the indicators that show how this bank account is performing. Rising groundwater levels mean increasing storage – more savings. Falling groundwater levels mean decreasing storage – running a deficit.
With this endowment, groundwater storage works like a large bank account. We run deficits in dry times, taking out more than we deposit (more pumping than recharge); and we run savings in wet times, depositing more into the account than what we withdraw (more recharge than pumping). Ideally, over the longer term, the savings match the withdrawals – groundwater recharge matches groundwater pumping.