Nestled in the hills of California’s San Joaquin Valley, a tranquil pond invites flocks of ruddy ducks, pintails, and shovelers to feed at its shores. The water is crawling with the larvae of brine flies and midges that these waterfowl love.
But this pond doesn’t constitute the ideal refuge. White, encrusted salts can be seen at the water’s edges, evidence that it is as saline as the ocean. The body of water is an evaporation pond, an artificial basin made for trapping drainage waters that have no other place to go in the tightly managed agricultural valley.
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With an annual rainfall of about 12 centimetres, this part of California’s Central Valley requires extensive irrigation to produce the abundant fruits, vegetables and nuts it supplies to the nation’s supermarkets.
But when routinely applied to fields, imported waters and the salts they carry can build up in the soil, threatening to impair fledgling crops and decrease crop yields. To make matters worse, a pervasive layer of clay in the region’s soils acts as a barrier, keeping water from trickling farther into the ground.
To meet these geologic challenges and reach their irrigation goals, area growers have installed subsurface tile lines and drains to carry drainage water to evaporation ponds. But the ponds can create problems of their own.
Dennis Corwin, a soil scientist with the U.S. Department of Agriculture, is familiar with the difficulties associated with evaporation ponds.
“These ponds are a considerable liability to farmers because they take land out of production and are time consuming to manage. On the west side of the San Joaquin Valley, for every nine acres of land in production, one acre is needed for evaporation ponds.”
In addition to their high salinity, some ponds may contain above normal amounts of selenium, boron and arsenic, which can be toxic to plants, wildlife and the bird populations that migrate through or overwinter in the area. Selenium shows up in the valley’s soils and waters because nearby mountain ranges are composed of marine sedimentary rocks enriched with the element.
Corwin is working with Westlake Farms to help farm owner Ceil Howe III lessen the need for evaporation ponds by reducing the volume of drainage water. Corwin has demonstrated a way to shrink the ponds and grow a profitable crop at the same time.
He plants a high quality, salt-tolerant forage crop such as bermuda grass on non-productive saline soils, and then irrigates it with the drainage water. With careful management, the grass crop will flourish and provide affordable forage for livestock.
The project, now in its sixth year, includes a team of soil, crop and animal scientists plus a resource economist. The grower now uses land that would probably never have been used.
“Cattle can graze on the bermuda grass,” Corwin said.
“With eight paddocks, we can rotate the animals so they’re kept off the fields being irrigated.”
An animal scientist is monitoring animal health, since some elements such as molybdenum can cause digestion problems in ruminants.
The researchers have met their goals of significantly reducing drainage-water volume, the need for evaporation ponds and bringing nonproductive soils back into use.
“We’ve shown a way that drainage water can be reused even in the most difficult soil conditions in California,” Corwin said.
Initially, the soil being planted with the test forage crop was poor, with high salinity, sodium and molybdenum levels. The irrigation water from the evaporation ponds was about half as high in those elements as the soil. According to Corwin, when managed properly, these poor soil and water conditions can be overcome.
“When you apply good quality water with low salinity and sodium levels to saline-sodic soil, the water just ponds on the surface. With these conditions, you need irrigation water that is high enough in salinity to enhance infiltration, but low enough to leach high levels of salt in the soil,” Corwin said.
“The soil at the Westlake site is high in clays that expand when wet. Once the soil is wet, it can be difficult to get more water into it to remove salts. To remedy this, the farmers let the soil dry out so it forms cracks that the water can run through. The key is adding enough water to fill the cracks and leach the salts without over irrigating and sealing the soil.”
The soil is improving at the test site and so is the forage crop.
“It’s looking better each year,” said Corwin. “At first the site was patchy and had weeds. It looked so bad that you couldn’t even tell we were running an experiment. Now, the bermuda grass is expanding and filling in areas that were previously bare.”
But the big question is sustainability over the long term. What will the soil quality be 30 years from now? While almost six years of data are significant, the researchers hope to continue monitoring the site.
