With the right infrastructure in place, water districts and agencies can manage surface water and groundwater as a single source, using one to balance the other when surface water or groundwater levels are low. Further implementation of conjunctive management practices across the state could yield between .5-2.5 million acre-feet.
Conjunctive management refers to the planned use and management of both surface and groundwater resources to maximize the availability and reliability of water supplies in a region. With the right infrastructure in place, water districts and agencies can manage surface water and groundwater as a single source, using one to balance the other when surface water or groundwater levels are low, thus reducing diversions and groundwater pumping while enhancing supplies.
Conjunctive management projects can be implemented to meet multiple objectives, such as improving local or regional water supply reliability, reducing groundwater overdraft, increasing flood protection, meeting environmental needs, improving groundwater quality, and countering land subsidence.
COMPONENTS OF CONJUNCTIVE WATER MANAGEMENT
Effective conjunctive management captures excess water when it’s available, and stores it in the groundwater basin for use during extended dry periods. To do this effectively requires monitoring, evaluation of the monitoring data to develop local objectives, and then use of monitoring data to establish and enforce local management policies. Each aquifer has its own unique characteristics, so scientific studies are needed to understand the geology of aquifer system, the recharge areas and process, and the flow directions and gradients of the groundwater.
The two principal components of a conjunctive water management program are recharging of the groundwater basin and recovery of water through groundwater pumping.
Natural recharge occurs when water percolates into the aquifer from surface water sources, both natural sources such as streams, rivers, lakes, as well as surface water conveyance facilities, and applied irrigation water. It is typically the slowest method of recharge and is relatively unmanaged.
Direct percolation recharge is accomplished by putting water in basins and allowing the water to percolate into the aquifer. Percolation basins constructed on highly permeable geologic materials can result in a rapid and economical way to recharge the aquifer. This form of recharge requires large, dedicated areas of land, conveyance facilities to deliver water to the basins, and annual maintenance to sustain the high percolation rates.
In-lieu recharge occurs by providing surface water to users who would normally use groundwater. The aquifer will partially be recharged by the application of surface irrigation water, and the reduced demand on groundwater leaves more water in the basin for later use.
Injection wells (or aquifer injection) is a method where water is injected into the aquifer by operating a well backwards. This method has the advantage of working in almost all geologic conditions and in relatively small scales where other recharge techniques are less suitable.
With conjunctive management, recharge of the aquifer and groundwater pumping and use are cycled over a period of time to balance them. This can and typically does lead to larger than normal declines during periods of more intensive pumping and could pose problems for other groundwater users in the basin who may not be part of the conjunctive management effort.
CONJUNCTIVE MANAGEMENT IN CALIFORNIA
Conjunctive management in California is nothing new; many agencies have implemented conjunctive management programs for decades to meet demands when surface water is cutback, to replenish declining groundwater levels, and to help control subsidence. Conjunctive management programs can range from the relatively large projects such as the Semitropic Water Storage District at 2.1 MAF to the small 2289 AF program run by the Compton Water Department.
Conjunctive management projects can be implemented locally, as well as regionally and potentially statewide. Projects implemented locally are easier to design and implement, and should be an integral part of the local agency’s water management portfolio. However, with appropriate infrastructure and responsible management, conjunctive management could potentially span multiple regions and achieve greater benefits.
An important legal issue that must be addressed as part of a conjunctive management program is how to resolve the ownership extraction rights for the artificially recharged water in groundwater basins that are managed by multiple jurisdictions or have multiple landowners.
The question is whether the water that has been artificially added to a groundwater basin is the property of the entity that added it, or once commingled with the existing groundwater, does it then become groundwater governed by the prevailing statues of the water code? The question becomes even more complex if the timing of the artificial recharge interfered with the natural recharge, which would belong to all overlying landowners.
POTENTIAL BENEFITS AND COSTS
Conjunctive management can be used to improve water supply reliability and sustainability, to reduce groundwater overdraft and land subsidence, to improve or protect groundwater quality, and to improve environmental conditions. However, to the degree these benefits are realized depends on how well the surface water and groundwater are managed as a single source to maximize the beneficial uses of water. The benefit derived from effective conjunctive management is further limited by the combined surface water and groundwater production capacity of the project area.
Costs for implementing conjunctive management include a wide range of facilities and as such are site-specific. Some projects require relatively minor upgrades to existing infrastructure, while others require extensive construction of new facilities. Therefore, costs for implementing new conjunctive management projects are highly variable. In general, urban uses can support higher project costs than agricultural uses.
HOW MUCH COULD CONJUNCTIVE MANAGEMENT POTENTIALLY YIELD?
According to the California Water Plan, the practice of conjunctive management in Southern California is estimated to provide 2.5 MAF of annual average water supply. A conservative estimate by the Department of Water Resources in 2003 estimated the potential yield for further implementation of conjunctive management projects throughout the state is 500,000 acre-feet, while more aggressive estimates say that amount could potentially be 2 MAF.
FOR MORE INFORMATION
Helpful documents and websites …
- Conjunctive water management: What is it? Why consider it? What are the challenges?, brief by Toccoy Dudley and Allan Fulton
- Conjunctive water use in the Sacramento Region, brochure by the Sacramento Groundwater Authority
- Conjunctive Water Management: A Solution to the West’s Growing Water Demand?, submitted testimony to U.S. Subcommittee on Energy and Resources by Jason Peltier
- Conjunctive surface water and groundwater management under climate change, journal article at Frontiers in Environmental Science
For more information on groundwater …
More on conjunctive management from Maven’s Notebook …[posts-by-tag tags=”conjunctive management” number = “5” excerpt=TRUE date=TRUE]