The long-term viability of irrigated agriculture in California's highly productive San Joaquin Valley is threatened by the accumulation of salt in soils and groundwater, reports a team of researchers at the University of California, Davis.

The researchers found that irrigated agriculture on the west side of the San Joaquin Valley is at risk due to the lack of fresh water, inadequate natural drainage and high water tables. The study focused on 1,400 square kilometers (about 540 square miles) in western Fresno County on the west side of the San Joaquin Valley. The findings are published online in the Journal of the Proceedings of the National Academy of Science.

"Few studies have been able to model the complex, three-dimensional hydrology and salt chemistry of an irrigated region as we did in this study," said Jan Hopmans, a UC Davis soil hydrologist and co-investigator on the study. "Our analysis shows the impacts of droughts and changes in water management on water levels and salinity, and provides insight into the long-term behavior of this irrigated agricultural system and its sustainability."

Salt build-up in soils and groundwater is a global problem that affects 20 to 30 percent of the world's 260 million hectares (about 642 million acres) of irrigated land, thus limiting world global food production. Salt is problematic for crop production because it upsets a plant's ability to take in water by its roots. If salt concentration in the soil is very high, the flow of water into the plant is actually reversed and the plant dehydrates and eventually dies.

In order to fully evaluate the effects of salinization in the San Joaquin Valley, the researchers developed a computer model that takes into consideration the hydrology and the salt chemistry of both the soils and the groundwater system. The model enabled them to reconstruct historical changes in soil and groundwater salinization in general, and specifically for the western San Joaquin Valley, starting in 1940.

The model indicated that soil salinity in the area was high in 1940, but decreased until 1975 because low-salinity snowmelt water was brought in by state and federal water projects, flushing salts out of the surface soils and down into deeper water sources or aquifers. This pattern was reversed during the 1970s as increased irrigation in the valley raised the water table, drawing up some of those salts that previously had been leached downward. As the groundwater levels rose toward the surface, farmers applied less irrigation water to prevent water logging -- and consequently increased the soil salinity. This problem was compounded by the use of more saline surface water for irrigation during occasional droughts.

The model also revealed that the dissolved mineral content of the soil and the type of water source -- whether groundwater or snowmelt -- are, in the long term, critically important in the salinization process.

The researchers forecast that, although it may take decades, salt accumulation will continue in this region, decreasing the quality of deeper groundwater sources and jeopardizing water used both for irrigation and drinking.

This research was supported by the U.S. Department of Agriculture, the U.S. Bureau of Reclamation, the UC Salinity Drainage Program and the Netherlands Organization for Scientific Research.