Recharge and Replenishment – What’s the difference?
By Warren Tenney
Whether you tune into a public meeting on water in Arizona or read any of the numerous newspaper articles on how our State is preparing for future Colorado River shortages, you’re bound to come across the terms ‘recharge’ and ‘replenishment’. Both are tools used to manage our precious surface water and groundwater resources more effectively. However, the terms are often used interchangeably despite meaning different things in Arizona water management.
For instance, there are notable differences between utilizing artificial recharge for underground storage (storing for the future) and replenishment (replacing what has been pumped). Aquifers are not simply underground bathtubs; they are complicated with layers and barriers. Recharging supplies for the future is like padding your savings account when you have extra income while bringing added benefits to the aquifer. Conversely, replenishment is more like paying back a loan of groundwater by returning water to the aquifer.
Let’s take a more detailed look at each term:
Recharge is a process by which water supplies are added to the aquifer. While some natural recharge occurs due to rain or surface water seepage, other methods have long been used to artificially recharge the aquifers in central Arizona. In fact, Arizona has been a leader in underground water storage after working for years to have it recognized as a beneficial use of water.
Methods of recharge include:
- Constructed Underground Storage Facilities where water is flooded into a large basin built to allow water to percolate into the aquifer below, or where water is injected into a well to infiltrate and be stored in the aquifer.
- Managed Underground Storage Facilities where water is released into a pre-existing natural stream, wash, or river channel to percolate into the aquifer.
- Indirect Recharge, also known as in-lieu recharge, where renewable water supplies are used for irrigation instead of groundwater, thus reserving groundwater that otherwise would have been pumped in the form of a credit for future use.
Most importantly, recharging our aquifers with renewable supplies allows those supplies to be stored now for use at a later date. When water is stored underground using any of the above methods for future use, it generates Long-term Storage Credits (LTSC). LTSC represent a volume of recharged water reserved as a credit that can be recovered later. When recovered, LTSC are characterized the same as the water supply that was initially stored underground. For example, if Colorado River water is recharged, it is legally considered Colorado River water once it is pumped as a LTSC.
Replenishment refers to the intentional replacement of groundwater back into the aquifer after it has been pumped and is often achieved through the process of recharging surface water supplies. Replenishment typically takes place due to Arizona’s Assured Water Supply regulations, which prevent new growth in the Active Management Areas (AMAs) from relying on mined groundwater. One way to satisfy this requirement is to replenish groundwater after it is pumped by recharging an equal volume of renewable water to replace what was withdrawn from the aquifer.
The Central Arizona Groundwater Replenishment District (CAGRD) provides a means by which landowners and municipal water providers in three of the AMAs can replenish their groundwater pumping to comply with the regulations. Water providers and homeowners who pay for membership in the CAGRD can rely on groundwater. The CAGRD is then tasked with finding renewable supplies to replenish that groundwater within the same AMA. Importantly, not all groundwater users are subject to the Assured Water Supply requirements, and all sectors withdraw large volumes of groundwater that are not replenished. Unreplenished groundwater use is a major contributing factor to the Phoenix AMA’s struggle to reach its safe-yield goal.
Both the tool to recharge surface water supplies for storage and future use and the requirement that new groundwater uses be replenished have served to create a more resilient water management framework in Arizona. Water managers have numerous options for storing surface water supplies in preparation for future shortages or drought impacts. And our finite groundwater supplies are better protected when their use is replenished, all while still permitting growth and economic development in areas with less direct access to renewable supply options.
So, while it’s important to recognize the distinctions between recharge and replenishment, we fully expect that these water management tools will continue to be utilized and improved upon to protect and sustain our water resources into the future.
For over 50 years, the Arizona Municipal Water Users Association has helped protect our member cities’ ability to provide assured, safe, and sustainable water supplies to their communities. For more information, visit www.amwua.org.
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