Yellow starthistle (Centaurea solstitialis) (YST) is the most pervasive invasive and noxious weed in California. Previous work showed that YST uses substantially more water than forage annual grasses it typically displaces. Soil moisture was 20% higher in annual grass test sites compared to YST test sites. Because yellow starthistle is found (sometimes in very extensive stands) on millions of acres of California, it is possible that removal of the thistle could substantially increase groundwater recharge and subsequent surface runoff. This could greatly improve range conditions, wildlife habitat and water supply, especially in the Sacramento Valley where groundwater levels are generally still fairly high and connected to surface water streams. Thus, improvements in groundwater levels in the Sacramento Valley could be expected to translate to improved surface runoff. In addition to the economic benefits of greater water generation, reducing YST will also benefit cattle ranchers. It has been estimated that statewide benefits to ranchers could exceed $20 million per year. There would also be substantial benefits to biodiversity, since native plants will do better when YST is removed. Increased streamflow due to greater groundwater recharge could also benefit native fish species and help listed rivers meet temperature TMDLs.

To test this concept, a three-year experiment is underway to evaluate the hypothesis that YST removal can improve groundwater levels and surface runoff. In particular, the ultimate objective is to estimate the potential saving of full-scale application on groundwater, runoff, soil moisture, and their interaction. The experiment is a collaboration among several researchers, including Joe DiTomaso and Leslie Roche in the Department of Plant Sciences at UC Davis, Jay Lund and Sarah Yarnell at the UC Davis Watershed Center, Mike Deas with Watercouse Engineering, and Geral Meral with the California Water Program Director of the Natural Heritage Institute. The project will be performed on four similar small watersheds covering about 30 acres on a ranch in Yolo County (in cooperation with John Anderson and Bruce Rominger). If the experiment demonstrates that water supplies can be improved, local water districts and those agencies that export water from the Sacramento Delta may find it financially attractive to pay for extensive yellow starthistle removal.