Where the Colorado River flows through the Palo Verde Valley in Southern California, the stream is an essential source of water for crops that are grown and reaped year round. But on a 1,000-acre-swath of land just north of Blythe, Calif., the irrigation canals flood densely planted fields that no one plans on harvesting.
Cottonwood, willow and honey mesquite trees grow there as part of a long term plan to protect endangered, threatened and other at-risk animal species along the lower Colorado. The trees, planted on converted agricultural lands that have long been hydrologically separated from the river, depend on irrigation to survive. An irrigation monitoring project is underway to make sure the effort is as efficient as possible in an already dry region where climate change could make water even more scarce.
Before flood control and irrigation infrastructure tamed the Colorado, the river would periodically breach its banks and deliver moisture to the soil in its floodplain.
“The Colorado River was really flashy and really wild before the dams were put in,” a said Matt Grabau, a restoration specialist with GeoSystems Analysis, Inc., a consulting firm working with the U.S. Bureau of Reclamation on the Lower Colorado River Multi-Species Conservation Plan. “Some of the historic accounts of the sediment loads that went into the delta are just phenomenal. The peak flows would take out huge swaths of land along the Colorado.”
As part of the conservation plan, the U.S. Bureau of Reclamation has since 2006 planted almost 1,000 acres of the Palo Verde Ecological Reserve north of Blythe with trees conducive to the recovery of endangered species like the southwestern willow flycatcher and the yellow-billed cuckoo. The reserve was once entirely agricultural land, and the irrigation infrastructure in place there waters the trees and moistens the soil instead of the historical inundation of flood waters.
The plan is having some success, and several species have been found using the restored habitat. But it’s not clear that the irrigation scheme is as efficient as it could be. That’s important, because the Multi Species Conservation Plan is a 50-year program in an area where water could become increasingly hard to come by.
One reason for uncertainty is that the irrigation canals are watering the fields of trees just like they were rows of cotton or alfalfa. While irrigators have a reasonable idea of how water will flow across row crops, it less clear how it moves through dense willow stands.
“We don’t know right now how the water is moving across the field,” Grabau said. “We think it’s going to be important in the long term to monitor that because it’s really going affect the irrigation efficiency and how much water is required.”
To that end, GeoSystems Analysis in 2012 installed a network of irrigation and soil moisture sensors across a 75-acre section of the ecological reserve that was planted with willows and other species in 2007. Dozens of custom sensors detect the presence of water to track how it moves across the field. At 26 sites, soil moisture sensors measure how long water stays within the top few inches of soil. Some stations also have soil moisture sensors installed 1-, 3-, and 6-feet deep to capture a profile of how much water is available to tree roots.
With some additional analysis, Grabau said the irrigation monitoring network will eventually serve as a predictive tool figure out how often the field needs to be irrigated to meet soil moisture objectives. This will help fine-tune the water application to balance keeping the trees and soil healthy with using only as much water is needed.
That fine-tuning is important because climate models predict the Southwest will see less rain by the end of the century, and water use in the region is already a contentious issue.
“Based on climate projections and potential future scenarios, I don’t think it’s going to be going away,” Grabau said. “If anything, it’s going to be more contentious. I think coming up with those kind of quantitative tools will hopefully help in the long-term management. That’s really what the goal is.”