Commentary: Surface water, groundwater storage work together
By Danny Merkley
During El Niño rains this winter, University of California researchers flooded this Modesto-area almond orchard as part of a groundwater-recharge experiment.
Photo/Joe Proudman, UC Davis
"Flexibility translates to resilience."
With that simple, four-word sentence, a new academic study emphasizes the important links between surface water and groundwater storage in the West, and the need for the two forms of storage to complement each other in tackling chronic water shortages.
The study was published last month in the journal Environmental Research Letters and was led by geologists from the University of Texas, with participation from U.S. Geological Survey researchers from California and Arizona. It looked specifically at how to enhance drought resilience in those two states.
In the Central Valley of California, as we know, farmers and other water users have had to rely on groundwater to a much greater degree the past few years, as drought and endangered-species protections have reduced surface-water supplies. The researchers see that as opening opportunity to refill depleted aquifers, noting that storing excess surface water in aquifers when it's available "can greatly enhance the reliability of water supplies."
The study looks at groundwater storage using two methods, which the researchers describe as conjunctive use or managed aquifer recharge. The researchers define conjunctive use as substituting surface water for groundwater, thereby reducing groundwater pumping and retaining groundwater in aquifers. They define managed aquifer recharge as using surface water to recharge groundwater, for example through the use of "spreading basins" where water is directed during wet years to percolate underground. In California, this method is considered "groundwater banking" and requires permits from the state water board.
In the Central Valley, the study says, importation of surface water via federal and state water projects supports groundwater storage. Recharge from surface-water irrigation increases groundwater storage. The report notes experiments with flood irrigation on fallow lands and perennial crops to enhance recharge, and says studies in Arizona indicated flood irrigation "should be very effective in recharging aquifers."
"Irrigators in the Central Valley maintain surface water and groundwater-based irrigation systems, which is essential" for groundwater storage, the report says. Additional recharge occurs naturally during extremely wet periods.
For optimal management of water resources, the researchers say, "it is important to understand the similarities and differences between groundwater and surface water storage systems." Surface reservoirs can accommodate rapid storage of large water volumes, whereas recharge systems generally feature slow infiltration rates, which the study says limits their ability to respond rapidly to climate extremes.
Here's how I describe the difference in talking about the need for all types of storage, as California decides how to spend the $2.7 billion voters approved as part of the 2014 water bond: Put a stopper in your sink, fill your coffee pot with water and then pour that water as quickly as you can into the sink. The sink holds the water as a surface reservoir would. Now, fill the pot again and pour it rapidly into a coffee filter filled with grounds—and you'll have a mess. Much of that water will waste away onto your counter and kitchen floor, because the water can't percolate through the filter fast enough, just as water can't quickly reach aquifers.
It's an important distinction because some organizations want to focus all of the available bond money on groundwater storage—and groundwater storage alone simply can't handle the flashier storms that come with the tropical weather systems that often help to break California droughts. If we are to recharge groundwater basins, we need strategically placed, new surface storage to capture precipitation the way California receives it today.
The study notes that recent increases in climate extremes related to both wet and dry periods in California and Arizona require additional water storage "to buffer water supply variability related to these extremes."
Importantly, given the recent concerns in California about management of increased flows during the El Niño storms of the past winter, the study says that flood flows to the Pacific Ocean from California that exceed environmental flow requirements "could provide additional water" to support expansion of groundwater-recharge programs.
"It is important to understand that it is not surface versus subsurface reservoir storage but integrated management of both," the study says, concluding that "integrated management of surface and subsurface reservoirs should be used to optimally manage water supply variability, with surface reservoirs capturing flood flows and transferring to aquifers for longer-term storage."
In some ways, it's akin to managing a checking account and savings account: Surface reservoirs act as a "checking account" for deposits and withdrawals; groundwater storage is the savings account that we build whenever possible so it can be tapped when the checking account runs low.
As Farm Bureau has long maintained, California must continue to pursue an all-of-the-above approach for making our state's water supplies more secure, including storage both aboveground and underground, plus continued investments in water recycling, coastal desalination and enhanced water efficiency.
As the study says, flexibility leads to resiliency, and California must remain open to pursuing all strategies to provide water for its people and environment.
(Danny Merkley is director of water resources for the California Farm Bureau Federation. He may be contacted at firstname.lastname@example.org.)
Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.