Groundwater limits in California's San Joaquin Valley

Farmers in the San Joaquin Valley began relying on groundwater around the 1920s. By 1970 the land had sunk more than a foot across about 5,200 square miles, half the valley, and by as much as 28 feet in places, and the compacted aquifer lost storage it cannot get back.¹² The droughts of 1976-77, 1986-92, 2007-09 and 2012-15 each cut surface water and pushed pumping up again.¹² The sinking has reduced how much water the Delta-Mendota Canal and the California Aqueduct can carry, and repairs have been expensive.¹² The Sustainable Groundwater Management Act of 2014 requires local agencies to bring their basins into balance, by 2040 for the critically overdrafted ones, and most of the valley's basins are in that group.^2,9,17 Since 2023 the Department of Water Resources has found the plans of seven basins inadequate and referred them to the State Water Resources Control Board.^9,10 The Board put Tulare Lake on probation in April 2024, citing subsidence, water quality and water levels, and Tule in September 2024.^3,4,5 It returned Chowchilla, Kern, Kaweah and Delta-Mendota to the Department of Water Resources' oversight after their agencies revised their plans, the last in April 2026, and is still weighing Pleasant Valley.^1,6,7,9,10 On probation, most pumpers must measure and report what they pump and pay $300 a well plus $20 an acre-foot.^2,11 Those pumping more than 500 acre-feet a year must meter their wells, though Tule lets them estimate pumping from evapotranspiration where that captures the use, and Tulare Lake will consider alternatives case by case.^3,5 In Tule every pumper within two miles of the Friant-Kern Canal inside the Eastern Tule agency, a subsidence management area, must meter, apart from small household users.⁵ On 12 August 2026 the trial court barred the Board from enforcing probation in the Tri-County Water Authority and Southwest Kings agencies' areas of Tulare Lake while the Kings County Farm Bureau's lawsuit goes on; the rest of the subbasin remains on probation.^3,4 Households keep reporting dry wells: the state's dry well site counts 73 statewide so far in 2026, 30% fewer than by this time last year.¹⁵ The state's dataset holds 6,213 distinct reports once repeated rows are removed (the file has 6,228 rows); 4,374 come from the valley's eight counties, San Joaquin south to Kern, 1,870 of them from Tulare County alone.¹⁶ PPIC's Water Policy Center estimates the valley may see 500,000 to 900,000 acres of farmland fallowed by 2040, up to a fifth of its irrigated land.¹⁷

Who needs an answer

• Groundwater sustainability agencies, which set pumping limits, minimum water levels and subsidence targets, and decide how to share the cuts among their growers.
• The State Water Resources Control Board, deciding whether probation in Tulare Lake and Tule has worked and whether to impose an interim plan and manage the groundwater directly.
• Farmers deciding which fields to fallow, which to switch to crops that need less water, and whether to buy or sell water.
• Families and small water systems on shallow wells, who lose their water first when levels fall, and the counties and agencies that pay for replacement wells and tanks.
• Whoever runs the valley's canals, since a canal that sinks carries less water.
• Counties and towns whose economy rests on farming. PPIC puts farming and related industries at 14 percent of the valley's economy and about 340,000 jobs.¹⁷

The domains it chains

Surface water arrives in some years and not others, and pumping makes up the difference. Pumping lowers water levels, the aquifer compacts, and the land and the canals on it drop for good. Shallow household wells go dry first, and water quality changes as levels fall. Crops differ in how much water they need. Fallowed fields raise dust and weeds, and farm jobs carry the towns.

What a twin should let people try

• Where to cut: the same total reduction in pumping spread evenly across a basin, or concentrated near the canals and in the areas sinking fastest, and what each does to subsidence and to shallow wells.
• Dry wells: at what water level a given cluster of household wells goes dry, and how many would need deepening or replacing under each agency's minimum thresholds.
• Fallowing against other crops: leaving land bare against planting winter grain forage, cover crops or desert crops such as agave and jojoba, given that bare fields still lose more than half their water to evaporation.¹⁷
• Drought: a repeat of 2012-15 with pumping limits in place.
• Measurement: how well estimates of pumping from evapotranspiration match metered pumping, since the Board accepts both in Tule.⁵

Where to start

The Department of Water Resources publishes InSAR measurements of ground movement for more than 200 of the state's groundwater basins on 100-metre points, from Sentinel-1 radar processed by TRE Altamira and checked against GPS stations, updated quarterly.^13,14 Its dataset page still describes the record as running to April 2022, but the April 2026 update report extends it to 1 April 2026.^13,14 The Dry Well Reporting System publishes each household report with its location and, where given, the well's depth.^15,16 Stream and groundwater observations are in USGS Water Data, and the USGS has measured aquifer compaction with extensometers on the valley floor since the 1960s.¹² The use case on land use covers what changes when farmland changes use.