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Home » News/Events » The Journal » Vol. 13 Issue 1 » Flooding Loss and Groundwater Gain

Flooding Loss and Groundwater Gain

  • By Irv Schiffman, Board Chair

The flooding expected as a result of El Nino threatens to cause both human and property losses. But such flooding also creates the opportunity to recharge the groundwater seriously depleted during California’s long-term drought.

According to the California Department of Water Resources (DWR), during an average year, California's 515 alluvial groundwater basins and sub-basins contribute approximately 38 percent toward the State's total water supply. During dry years, groundwater contributes up to 46 percent (or more) of the statewide annual supply, and serves as a critical buffer against the impacts of drought and climate change.

DWR further reports that due to excessive groundwater pumping sinking land - known as subsidence - is happening faster, putting infrastructure on the surface at growing risk.

Thus, the replenishment of groundwater throughout the state is crucial to California’s economic and environmental sustainability.

In an important new study, University of California scientists identified 3.6 million acres of California cropland suitable for replenishing the state’s groundwater reserves.

According to the study, using the existing irrigation network, it is possible to capture flood flows from California rivers onto suitable dormant or fallow agricultural fields, allowing the surplus water to infiltrate aquifers. During storms and flood-control releases, excess river water could be routed through irrigation canals onto farms, where the surplus would seep underground to replenish groundwater. This action could also mitigate downstream flood risks.

In line with the study, UC Davis has developed a Soil Agricultural Groundwater Banking Index based on five major factors deemed critical to successful agricultural groundwater banking: deep percolation, root zone residence time, topography, chemical limitations, and soil surface condition. Researchers are presently test-flooding a number of vineyards, almond orchards and other cropland looking at infiltration rates, plant physiology, groundwater quality and costs.

There have been no comparative studies on River Partners restoration sites, although similar replenishment results may be likely: restoring riparian areas to a naturally vegetated condition is, in general, an effective way to ensure maximum infiltration of precipitation, stormwater runoff and flood water into the soil and to aquifers and other groundwater resources. And unlike agricultural lands, there is no risk of the leaching of residual pesticides or fertilizer in the soil.

Of course, our restoration sites also contribute to ground water sustainability in another way: by retiring crop or orchard lands we are retiring agricultural irrigation on that property. For example, on the 2,100 acres of property River Partners owns in the San Joaquin Valley, we will retire 10,500 acre-feet of water per year. This is a net water savings of 3,421,435,500 gallons per year. We irrigate our own plantings only in the first three years and then they are on their own.

Following years of over pumping, achieving groundwater sustainability will take decades and can only be done through a collaborative approach by all agricultural, urban and environmental sectors. Through its restoration programs, River Partners is doing its part.