The San Joaquin Valley – California’s largest agricultural region – has been in a state of perpetual water stress that can only be partly attributed to the latest drought. Decades of unchecked pumping have resulted in a chronic groundwater deficit averaging nearly 2 million acre-feet per year – equivalent to about two Folsom reservoirs. The clock is ticking for overdrawn basins to comply with the state’s 2014 groundwater law and get their water supply and use into balance.
One strategy that can help make a substantial dent in this deficit is to encourage more storage in the valley’s depleted aquifers. While logistical and economic constraints will limit how much extra water can be captured from local rivers, we estimate that there is potential to reduce the deficit by up to a quarter with increased groundwater recharge. As a first step to realizing this potential, the State Water Resources Control Board will need to develop a simple, rapid way to determine when river flows exceed the amount of water required for environmental purposes and downstream users.
The very wet winter of 2017 hints at what’s possible. Even though high-river flows can only be captured for a short span of time – and despite serious shortcomings in the infrastructure required to actively store more water in aquifers – water districts managed to put an estimated 6.5 million acre-feet into local aquifers last year. An additional 4 million acre-feet may have been recharged “passively” through seepage from river channels and regular irrigation practices on farmland. The combination of active and passive recharge brought the valley’s groundwater balance sheet into the black for the first time since 2011.
Our new research shows that taking these practices to the next level will require overcoming some significant hurdles and creating new incentives for recharge. Valley water managers we surveyed listed infrastructure barriers as the top impediment to getting more water underground in 2017. As one manager told us, “We need more of everything: conveyance to areas most conducive to recharge, and surface-water agreements with neighboring districts.”
Even with significant new infrastructure investments, it is probably not feasible to capture all the extra water in high-flow years such as 2017. Top priorities include making better use of existing infrastructure (canals, surface reservoirs and recharge basins) and determining where additional investments are warranted.
Recharge potential in the valley depends on the capacity to move more water from the wetter northern half of the valley to the drier southern half, where overdraft is more pronounced and conditions are more favorable for recharge. But conveyance capacity has fallen since the latest drought because extra groundwater pumping caused lands to sink around the Friant-Kern Canal – the key infrastructure for moving San Joaquin River flows to the southern valley – which is now running at just 40 percent capacity in its southern reach. Determining how much water could be captured cost-effectively with existing or enhanced infrastructure is an essential step for evaluating new infrastructure projects and upgrades to existing systems.
One unknown is understanding how releases from the state’s large reservoirs can be timed to help improve groundwater recharge. Moving water stored in reservoirs for dry years into groundwater basins during the fall months can make more room in reservoirs for winter storms. This will require establishing agreements with parties who currently store this water in surface reservoirs, so they are confident they will be made whole when the water moves to groundwater storage.
Another limiting factor is the availability of suitable lands for spreading water, and the willingness of local farmers to recharge on those lands. Actively recharging on farmland has high potential: the valley has roughly 5 million acres of irrigated cropland – more than half of which has at least moderately good conditions for direct recharge. But farmers have concerns about how spreading water on their fields might affect crop health and yields. Pilot studies are under way to look at impacts on some crops, which could help allay fears. Farmers may also be reluctant to take on the costs of recharging if the benefits aren’t adequate. Districts in some basins are developing incentive programs that could be used as models elsewhere.
Increasing groundwater recharge will require high levels of cooperation among a wide variety of stakeholders. Local and regional partnerships – for capturing and moving water efficiently, making new investments and devising projects that bring multiple benefits – will be key to success. The stakes are high, as groundwater is the state’s most important hedge against drought. Moving quickly to resolve roadblocks can help us prepare for the next wet year’s recharge opportunities.
The views expressed in this article belong to the authors and do not necessarily reflect the editorial policy of Water Deeply.