In 2023, California’s flood-recharge push helped drive an 8.7 million acre-foot net increase in Central Valley groundwater storage

California’s Central Valley can look dry in summer, but winter storms can quickly change the landscape. For many years, the endless fields of nut trees and grapevines, along with other crops, have been baking in the hot sun and relying on deep groundwater wells. However, when heavy winter rains arrive, the dry dirt roads can turn into pools of standing water.That change became especially clear in 2023. Following years of severe drought that left regional water tables depleted, state officials and local growers took a major step. They directed enormous volumes of floodwater straight onto active agricultural lands. In total, California recorded an 8.7 million acre-feet net increase in subterranean groundwater storage in Water Year 2023, driven by 4.1 million acre-feet of deliberate managed recharge across farm fields and basins alongside a sharp drop in agricultural pumping, according to the California Department of Water Resources.This approach follows a simple rule of thumb. During the harsh winter period, rivers and lakes burst out, forming dangerous floods for all the downstream cities. Instead of allowing the freshwater to flow to the Pacific Ocean or burst the local levees, water planners use it for irrigation purposes on farms. It accumulates in the topsoil layer of almond plantations, grape plantations, and idle farms where it eventually sinks into the aquifer layers below.According to the California Department of Water Resources, this concept is illustrated by the state government’s Flood-Managed Aquifer Recharge, or Flood-MAR, project. The project combines flood management with groundwater recharge. By using private farmland as temporary recharge basins during winter, officials can reduce flood risk and store water underground for future droughts.During field trials conducted by UC ANR researchers, 24 inches of stormwater were applied monthly to the Modesto orchard treatment rows during winter recharge periods, enabling more than 80 percent of the applied water to percolate past the root zone into the underlying aquifer with minimal impact on tree health and yield. They also note that earlier offseason flooding tests on grapes, pistachios and alfalfa did not hurt the crops under certain conditions and did increase groundwater levels.A practical way of balancing soil and cropsA major shift came when water conservation was tested in the field. For a long time, growers believed that standing water around tree roots for long periods could damage the trees and ruin the harvest. The above-mentioned field trials by UC Davis researchers found that applying captured winter floodwaters to almond orchards on moderately to well-drained soils resulted in no measured negative impacts on canopy development, stem water status, or crop yield, while extending fine-root lifespan.When severe atmospheric river storms battered the region in early 2023, state executive orders temporarily streamlined permitting restrictions. This allowed local water districts and land managers to swiftly redirect high-stage flood flows onto active farm fields and dedicated recharge basins, capturing over 400,000 acre-feet of emergency recharge under the orders. Farmers directed several feet of muddy storm runoff onto dormant fields. The soil can absorb large volumes of water each day and filter some impurities as the water seeps into the ground.The benefits of this approach go beyond storing water for the next drought. Chronic groundwater over-pumping across the San Joaquin Valley has caused land subsidence in many areas, as the ground surface collapses. When aquifers are depleted, clay layers beneath the surface compress under the weight of the soil above, permanently reducing underground storage space and cracking canals, roads, and bridges.Recharging these aquifers increases pore pressure, which can slow subsidence and help protect infrastructure. It can also dilute salts and nitrate buildup from agriculture in rural drinking wells.Economic survival and the way forwardFlood-MAR is growing quickly because of legal requirements. Under California’s Sustainable Groundwater Management Act, groundwater sustainability agencies must ensure that overdrafted basins are brought into balance over the coming decades. If groundwater use does not match replenishment, regulators will impose significant pumping cuts.According to land-use projections by the Public Policy Institute of California, growers across the San Joaquin Valley may need to fallow or permanently retire between 500,000 and 900,000 acres of irrigated farmland by 2040 to comply with SGMA groundwater balance mandates. That makes managed recharge an important way to reduce land retirement. By using farm fields as temporary recharge zones during high-flow winter periods, agricultural districts can secure long-term water supplies without taking large areas of productive land out of cultivation.The 2023 floods became a crucial trial for this method. Diversion gates were enhanced, ditches were cleaned, and temporary pumps were used to direct water from the irrigation canal on private fields. Water quality, soil absorptivity, and plant condition were studied by university researchers and state hydrologists at hundreds of involved sites in order to plan future floods.Since there is an alternation of prolonged droughts and severe winters with storms, surface water management is no longer sufficient. One response is to capture floodwater in wet years and store it underground.