California faces growing demand for water. The strongest evidence for this is the overdraft of groundwater, especially in the Central Valley. The total amount of groundwater overdraft is uncertain, but normal year overdraft estimates range from 0 .5 to 2.5 million acre feet in the Central Valley., The overdraft was undoubtedly considerably higher in 2013-2015. Even in normal years, water is in critical supply during the summer and early fall months in our California region. Conditions are worse during droughts. Whatever can be done to increase groundwater storage and stream runoff during those periods would greatly contribute to the agricultural economy, and health of our streams, fisheries and watersheds.
Various measures have been and will be taken to respond to California’s need for additional water. The Legislature passed bills regulating the overdraft of groundwater, but they will not have any real effect for a decade or more. Cities are conserving, in part due to higher water rates and in part due to drought inspired conservation, but with a very high demand for water from permanent crops like almonds, overall water demand in California is likely to either remain stable or increase.
Building more surface and underground water storage capacity is favored by water agencies and political leaders because of the proven ability of storage to provide reliable water supplies. But a new study by the Nature Conservancy, CH2MHill, and UC Davis demonstrates the limited ability of new storage to provide new water supplies. That study concludes that no more than 5-6 million acre feet of new surface and groundwater storage can be productively put to use. Even with full integration of this storage with existing water infrastructure, and with a Delta facility in place, this new storage would not produce much more than a million acre feet of new water. Further, the cost of this new infrastructure would be more than twenty billion dollars.
Even with expenditures of this magnitude, it is apparent that water storage alone will not meet all of California’s water needs.
POTENTIAL WATER BENEFITS OF YELLOW STARTHISTLE MANAGEMENT
California must seek additional ways to make better use of the precipitation it receives. One way to do so is vegetation management. For example, a recent report by UC Merced, UC Berkeley and the Environmental Defense Fund calls for returning Sierra Nevada forests to the densities found before 1800, with a resulting increase in runoff due to lower water use resulting from thinning highly dense young trees.
Another method of habitat manipulation to increase runoff could be through the removal of dense stands of weeds which use more water than other native and non-native vegetation, such as annual grasses. Yellow starthistle (YST)(Centaurea solstitialis) is such a weed.
YST is found throughout California, especially in central California and northward, typically to about 5900 feet (1800 m), and sometimes at higher elevations. It is common in the Sacramento Valley, San Joaquin Valley, Sierra Nevada foothills, Cascade Range, Klamath Ranges, eastern North Coast Ranges, and the central-western region. It thus largely overlaps the region of most water origination and use in California.
YST is also common and invasive throughout much the western United States, and is also found in almost every other states except a few states in the South
Yellow starthistle distribution throughout California (from DiTomaso et al 2006) and proposed study area.
Invasion of rangeland by Yellow Starthistle dramatically reduces the quality of rangelands for livestock and wildlife. Yellow Starthistle has increased its coverage in Northern California from one million acres in 1981 to 10 million acres in 1997 to fifteen million acres today, and according to the Western Shasta Resource Conservation district it continues to steadily invade new acreage
YST is not suitable for grazing livestock, and is toxic to horses. Bees make good honey from YST, but otherwise millions of acres of California are being lost to wildlife and ranching uses. Losses to ranchers from YST are in the tens of millions of dollars a year in lost productivity and control costs.
YST on millions of acres of BLM, Forest Service, National Parks, and various state lands often go untreated or barely treated due to the costs of control.
Coinciding with the spread of YST in the Sacramento Valley, expected flows in streams tributary to the Sacramento River have declined in the past 20 years. While cause and effect has not been established, it is not unlikely that YST is cause of this streamflow decline.
YST has been shown by UC Davis researchers to use far more water than the annual grasses it usually displaces. YST uses up to .36 acre feet of water per acre more than annual grasses. This is because it is very deep rooted (up to 2 meters or more), and is highly efficient in water uptake. UC Davis researchers have estimated that up to a million acre feet of water could be saved if YST could be widely controlled. This water would stay in the ground, maintaining groundwater levels, and eventually appearing as surface water in streams, providing benefits to fish and humans
Harvesting water where it falls will be increasingly important in California, as climate change transforms snowfall to rainfall, making it harder to retain higher winter runoff in surface reservoirs, which must be operated for flood control purposes. Water captured in soil will be increasingly valuable under these conditions. This is because runoff is most valuable when it comes in the spring and summer. Higher groundwater levels are valuable at all times. YST is not likely to affect groundwater levels or runoff in the winter, when the plants are not substantially growing. The effects of YST removal on water levels will be strongest in the spring and summer, the main YST growth and transpiration periods.
YST is easily controlled using Integrated Pest Management methods, making it susceptible to economically efficient control. The cost of controlling YST averages $25 per acre. It can be controlled by burning, mowing, grazing by goats, and through chemical means. Once it is controlled, spot control is necessary on an annual basis. That costs $5 per acre.
If YST is controlled on heavily infested lands, the cost of the water saved would be less than $20 per acre foot on a long term basis. This compares to up to $400 per acre foot for most conservation programs, more than $1000 per acre foot for new surface and groundwater storage, and up to $2000 per acre foot for desalination.
In addition to the economic benefits of greater water generation, reducing YST will greatly benefit cattle ranchers. 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 would benefit native fish species and help listed rivers meet temperature TMDLs.
To convince water managers that these savings are real, the Natural Heritage Institute has raised funds to hire Dr. Joseph DiTomaso from UC Davis, an internationally recognized expert on YST and its control; and Dr. Mike Deas, a highly respected hydrologist. They have designed and are implementing an experiment on four adjacent small test watersheds, to compare the water savings on controlled and uncontrolled watersheds. In the two controlled watersheds YST will be removed, while it will be allowed to remain on the other two watersheds.
After an initial year to measure water runoff baseline, YST will be removed in two watersheds. Then surface and groundwater quantity and quality will be measured on all watersheds for three years, and the water savings will be determined. If the savings are significant, a statewide YST control program would be designed. It would be implemented by water agencies who wish to use the water savings.
This demonstration project will cost about $580,000 and take 3 years.
Nothing humans can do has greater potential to produce affordable new water supplies in California than Yellow Star Thistle control.
 Faunt, C.C., ed. (2009), Groundwater Availability of the Central Valley Aquifer, California: U.S. Geological Survey Professional Paper 1766, 225 pp.
 INTEGRATING STORAGE IN CALIFORNIA’S CHANGING WATER SYSTEM. Nov 2014. Lund, Munevar, Taghavi, Hall and Saracino. https://watershed.ucdavis.edu/files/biblio/Storage_White_Paper_20Nov2014.pdf
 Costs and Losses Imposed on California Ranchers by Yellow Star Thistle Alison J. Eagle, Mark E. Eiswerth, Wayne S. Johnson, Steve E. Schoenig, and G. Cornelis van Kootens Rangeland Ecol Manage 60 :369-377 1 July 2007