If every year were an average water year, the Tuolumne River could provide enough water to sustain a vibrant agricultural economy as well as a healthy river ecosystem. The problem is there are good years and bad years, and when a number of dry years line up we experience water shortages, often pitting economic interests against the environment.
This year we experienced the opposite, as torrential storms dumped near-record precipitation on the Tuolumne River watershed. The reservoirs filled quickly and, beginning in January, maximum allowable releases from Don Pedro Dam were required to prevent future flooding downstream.
More water in excess of flow requirements was released into the Tuolumne River than what the three water agencies operating on the Tuolumne – the Modesto and Turlock irrigation districts and the San Francisco Public Utilities Commission (SFPUC) – use in about two years.
While it’s likely there will always be a debate over how much water should flow down the river to protect fish and wildlife and maintain water quality, few would argue that there wasn’t a considerable excess of water this year.
So, what could be done to capture and store some of the excess water in wet years for future use during dry years?
The answer lies right under our feet.
Stanislaus County is blessed with excellent soils for groundwater recharge, and sits upon two large groundwater sub-basins – Modesto and Turlock, on either side of the river – with many times the storage capacity of Don Pedro Reservoir. While neither sub-basin is classified as over-drafted, there are concerns that pumping could increase as a result of higher in-stream flows required by the State Water Resources Control Board to help revive the San Francisco Bay-Delta and rivers that feed it. Over-pumping of the aquifer could reduce its reliability and possibly lead to land subsidence, threatening important infrastructure.
It would be prudent to explore potential new recharge opportunities to ensure the continued viability of groundwater pumping without causing harm to the aquifer. Such a program would help meet the Sustainable Groundwater Management Act (passed in 2014) requirement that levels of pumping and recharge be in balance.
The Stanislaus and Tuolumne Rivers Groundwater Basin Association and Turlock Groundwater Basin Association have done a good job establishing Groundwater Sustainability Agencies, as required by SGMA. The next step is to create Groundwater Sustainability Plans. We are hopeful these plans will include active recharge programs during wet years, and look forward to engaging in the process.
The viability of recharge programs has already been demonstrated. For example, a 20-acre recharge basin managed by the Merced Irrigation District replenishes 25 acre-feet of groundwater per day. The State is eager to support similar projects, as funding for earthwork and infrastructure is available through the California Water Bond, which allocated $2.7 billion for water storage projects.
Another option is for the Irrigation Districts to partner with the SFPUC, which might be interested in establishing a groundwater bank similar to its water bank in Don Pedro Reservoir.
With further study and implementation of groundwater recharge, we could capture more water during wet years, improve in-stream river flows every year, and continue to support a prosperous agricultural economy during dry years.
Peter Drekmeier is Policy Director and Zarine Kakalia a Summer Fellow with the Tuolumne River Trust.