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?

More than four years after a WPX Energy Appalachia wastewater impoundment leaked and contaminated the drinking water of five Westmoreland County families, the shale gas company has agreed to pay the state a $1.2 million penalty.
But the families still don’t have sufficient, permanent water supplies, said Melissa Marshall, community advocate for the Mountain Watershed Association, an environmental organization.
It is next to Donegal Lake, which drains into Loyalhanna Creek, a tributary of the Allegheny River.
“We quickly worked to rectify the situation from its onset, immediately emptying and ultimately closing a containment pond for recycled water as soon as we became aware of potential issues,” Mr. Swan said.
“Importantly, no individual’s health was ever at risk even by the most stringent state and EPA standards,” he said.
The company also installed single-faucet water filtration systems in the five homes, but according to Ms. Marshall, those systems don’t provide enough water.
The families, who live near Stahlstown, must supplement the filtered water with bottled water.
“The filtration systems are completely insufficient in providing for the water needs of the families, and that’s been true for years,” Ms. Marshall said.
This settlement doesn’t help the residents, who are still in dire need.” She said DEP consent orders issued over the past three years requiring WPX to replace the permanent residential water supplies don’t specify the amounts of water the company needs to provide, and the filtration systems the company installed in the five residences don’t come close to meeting the families’ needs.
“My interview notes say that on the high end, one family estimated the reverse osmosis system made three gallons every six hours.