By Scott Hamilton, President, Hamilton Resource Economics
It was an impressive year for groundwater recharge in Kern County. However, far more is needed if Kern is to achieve groundwater sustainability without significant loss of farmland. The current water shortage is in the vicinity of 2 million acre-feet per year, of which the San Joaquin Valley portion of Kern County accounts for around 300,000 acre-feet. Between 15% and 18% of the irrigated farmland in Kern County could go out of production if nothing is done — a revenue loss that exceeds $1 billion dollars per year.
In their groundwater sustainability plans (GSPs), water district managers in Kern County have identified more than 30 projects that could increase groundwater storage and improve water supply reliability. Those projects will cost around $225 million and contribute over 100,000 acre-feet per year to fixing the water shortage. The average capital cost of those projects is around $130 per acre-foot. Additional projects were identified in GSPs but did not specify yield or cost information and so could not be included in the total. Regardless, likely half of the projects needed to achieve sustainability have been identified.
If there were more places to recharge water, more water could have been stored this year. San Luis Reservoir was full toward the end of March, and State Water Project pumps were operating at reduced capacity for an additional three months. Given Kern County’s ability to divert water from the California Aqueduct, around half a million acre-feet were not brought into the county because there was no place to put it. The recharge facilities were full of Kern River Water. How then can groundwater recharge capacity be increased?
Dedicated multi-benefit recharge basins (such as those providing groundwater recharge and wetland habitat) are one option. They have appeal as an effective way to utilize ag land that may be fallowed, but with land acquisition, earthwork, and new plumbing, they can be expensive.
Traditional on-farm recharge — the flooding of dormant orchards or fallow fields — has been gaining acceptance as an economical method for recharging groundwater. But it is not without its problems. These include the unpredictability of water supply and having to manage recharge around farm operations. There has also been concern about the potential impacts of mobilizing fertilizer and pesticides from farm fields into the groundwater table. The science underlying these fears is far from extensive, and the work that has been done generally shows improvements in groundwater quality from recharge operations, not degradation. Those concerns aside, the overlap of an orchard’s dormant season with the availability of flood water can be fairly short.
Another alternative is reverse tile drains. These are like conventional tile drains but the water flows in the opposite direction — into the field instead of away from it. The reverse tile drain systems are installed prior to the development of fields for orchards. They have also been installed on row crop land. The reverse tile drain system consists of perforated pipes installed in gravel in trenches 9 to 12 feet below the soil surface. That depth is necessary to separate the recharge from the root zone. The trenching, installation, and refill occurs in a single pass. When the system is not being used for irrigation, the recharge water delivered by the water district passes through the farm filtration system and into standpipes that connect to the buried perforated pipes. The system has a number of advantages over surface flooding of fields: the recharge does not interfere with farm operations; there is little risk of mobilizing recently applied fertilizer or pesticides; and recharge can occur through the growing season without the risk of compounding disease problems, greatly extending the duration of recharge.
The systems are not inexpensive. Lidco Inc. installed more than 25 systems in the Valley over the last five years. Glenn Drown, Lidco’s Central Valley manager, indicated installation costs run between $100 and $160 gallons per minute (multiply that by the capacity of the farm filter system to get an estimate for a field). That cost is substantially cheaper than building dedicated recharge ponds. But whether or not that is a good farm investment depends on a number of factors. Primary among these is how much credit the farmer is likely to receive for the water recharged on his or her farm. Water districts throughout the Valley have vastly different policies for what credit farmers get for recharging flood water on their own property. In some districts, farmers get a credit to their groundwater account of 90% of the recharged water. As the need for more on-farm recharge becomes apparent, incentives are likely to increase in many districts. As SGMA implementation incrementally reduces the capacity to pump groundwater and limits farm operations, more on-farm recharge may make sense for farmers in water-short districts. A second consideration is the frequency at which flood water will be available. That also varies from district to district. And some farms are better suited to a reverse tile drain recharge. Farms with sandy soil at a depth that does not overlie the Corcoran Clay will be able to recharge more water for longer periods. Recharge in less-than-perfect conditions can still be effective and economical.
When California is wet, it tends to be wet everywhere — meaning the eastside rivers are running high while 100,000 cubic feet per second might be passing through the Delta to the Pacific Ocean. During very wet years when some fields are being flooded, it is difficult to think about capturing more water. But doing so will reduce flooding and is essential to achieving groundwater sustainability in Kern County.
