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 With this endowment, groundwater storage works like a large bank account.
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).
In dry years, it is common to see water levels recover less during the (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.
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.
The decline has also created groundwater storage space to replenish with extra water in wet years.
Recharge as the driver for groundwater recovery after drought.
Figure 3 shows some good examples from the Sacramento Valley (Yolo County) and the southern Central Valley (Tulare County): If neither of these resources are at hand, consider the rate at which water levels have fallen over the past five years: recovery may likely happen at about the same rate as water levels have fallen.
Irrigating suitable agricultural land with surplus winter water may allow recharge of one-half to two feet of water between December and March – allowing for additional intentional recharge in wet years of perhaps 2-6 MAF across the Central Valley, if and where water rights, infrastructure, and agricultural chemicals could also be managed appropriately (Water Foundation, 2015).
Groundwater: Where does our water come from?