By Sarah Sarfaty Epstein

Groundwater has long been the unseen lifeblood of irrigators across the state, and some are now taking an active role in replenishing it. When and where surface water has been insufficient, Californians have drilled down, steadily depleting the aquifer, as shown in Figure 1.

California’s cumulative groundwater overdraft is over 100 km³, equivalent to almost 18 Shasta Reservoirs at full capacity (Dahlke et al, 2018; NorCal Water Assoc). This has led to land subsidence, surface water depletions, seawater intrusion, increased groundwater pumping costs, and reduced groundwater availability (Dahlke et al, 2018). Ending overdraft is a requirement of regional Groundwater Sustainability Agencies under the Sustainable Groundwater Management Act (SGMA).

Aquifers naturally recharge when water percolates into the soil. Intentionally replenishing the aquifer to keep pace with pumping, through a practice called Managed Aquifer Recharge (MAR), is an important strategy, and the state’s agricultural lands have emerged as a field of opportunity for implementation (Harter, 2015). California’s aquifers can store at least three times as much as surface reservoirs (Dahlke et al, 2018). As such, not only does recharging the aquifer stabilize the supply available for current groundwater pumping, but it could play a compelling role in California’s climatically altered future where storing water is of utmost importance, due to diminished snowpack and increased rainfall as models suggest. 

There are a variety of approaches to implement MAR. Infiltration basins date back to at least 1917 in California. Shown in Figure 2, they are natural or manmade depressions filled with diverted surface water, storm water, or treated wastewater, with the intention of percolating into an underlying aquifer (Dahlke et al, 2018). A more emergent practice, called flood-MAR (Figure 3), is described by DWR as “an integrated and voluntary resource management strategy that uses flood water … for groundwater recharge on agricultural lands and working landscapes” (Marr et al, 2018). While these practices are not exclusive to agriculture, when they occur on private agricultural lands, they have come to be known as Ag-MAR. 

Some literature has taken a narrower approach to defining Ag-MAR exclusively as spreading flood flows on crop-land as a secondary use for land, compared to basins which require land set-aside for MAR (Levintal et al, 2023). However, the distinction between these two practices, and the exclusion of infiltration basins on agricultural land from the term Ag-MAR, may cause confusion. Additionally, using Flood-MAR interchangeably with Ag-MAR may confuse the practice with other non-agricultural uses of flooding to recharge the aquifer. California’s water and land managers may benefit from clear and consistent terminology to assess what is appropriate for their region. 

Issues related to groundwater and recharge permeate most of California.  In particular, Pajaro Valley receives attention for its water agencies innovative groundwater recharge net metering, or ReNEM program that aims to address what has been estimated to be 12,100 acre feet of annual overdraft (Miller et al, 2020). Pajaro Valley, which includes parts of Santa Cruz, Monterey and San Benito Counties relies more on groundwater to meet its freshwater needs than most other parts of the state. Water demands are both agricultural and municipal (Schmidt et al, 2012).

Like many things in the world of California water, regional variation is key. In the Q&A below, Dan Hermstad of Santa Cruz County Resources Conservation District discusses ReNEM, comparing the practices of infiltration basins and Flood-MAR and their regional applicability. 

Q&A

How would you explain the ReNEM program to someone unfamiliar with it?

Dan Hermstad: ReNEM is essentially a mechanism to incentivize private landowners to improve stormwater recharge primarily on their lands. There are many program partners, starting with the private landowners who volunteer their land to participate. There are no acquisition payments or fees paid to use the land, but they receive a rebate payment for water infiltrated. Then a third party certifier, in this case it is us at the Santa Cruz County RCD alongside Dr. Fisher’s lab at UCSC, quantifies the resulting recharge. Finally, the Pajaro Valley Water Management Agency, who receives our estimated amount of infiltration, pays the rebate based on our agreed on a standard formula. 

Our area is unique in that the wells are metered and people pay for pumping groundwater. So, we used the fees that PV Water charges for pumping to get at the rebate cost – currently about 50% of that charge. It is not meant to be a payment for the water but rather to support maintenance and operation of infiltration basins. We don’t want this program to be an additional cost for landowners. 

How many active agricultural MAR sites are currently working within the Pajaro Valley? Are they all using recharge basins or do any use flooding techniques?

DH: We have three active sites and each of them are infiltration basins. We often refer to them as MAR basins. The infiltration basin approach was chosen partially because we are unique from other parts of the state.

Our geology isn’t suited to flood-MAR, as we have a lot of fine clay sediments close to the surface. This makes for very complex geology that doesn’t allow infiltration. It creates wetlands, such as Watsonville Wetlands, which collect rain and stormwater that does not infiltrate. These are great for habitat but not recharge. Also, we have only one primary river through the valley and a small geography to flood. Not nearly the rivers you have in the Central Valley. And lastly, the crops we have out here don’t like wet feet. We are primarily specialty crops, berries and fresh produce. You do flood-MAR in perennial tree crops. 

How were the two newer ReNEM sites selected?

DH: Several factors have to come together for a site to be good. It starts with a landowner volunteering their land. Both were identified by landowners where farmland was kind of marginal because there was an existing depression in the land that kind of pooled water. Now we know they are both on paleo channels of the Pajaro River. From there we need to confirm the geology is conducive and assess how you will get water into the basin. Some sites require a fair amount of work constructing ditches for water to flow into the basin. We look at the size of the drainage into the basin as well as the size of the basin. 

To date we only take stormwater flows. One reason stormwater works here that is not always fully appreciated is that we have exacerbated stormwater flows because of the crop type. Berries often have plastic mulch and several use plastic hoop houses, all of which shed water and totally prevent infiltration.  More and more, I am interested in including an evaluation of the type of agriculture done in that upland area. These basins are critical to maintain a hydrologic balance in agricultural landscapes using a lot of plastic. 

What is the (estimated) cumulative recharge capacity of these sites, and how does it compare to the basin’s water deficit?

DH: We shoot for 100 acre feet a year from each basin. One of the three sites was built before ReNEM and does much less, something like 3-7 acre feet a year. The others are between 100 and 120 acre feet on average a year. 

The pie in the sky goal is that we would have enough basins to recharge 1,000 acre feet a year. But that is going to make a small dent in the overall overdraft. PV Water is our Groundwater Sustainability Agency, and they have various projects to address overdraft. 

These activities are taking place on private property, are there additional benefits to the landowner beyond the rebate?

DH: Not in a direct sense. Everyone is using the groundwater, so there is an incentive to be a good community steward and an incentive to replenish the resource to be able to continue farming. Landowners don’t want their groundwater to disappear or be so impaired that it is not usable. 

This program predates a lot of focus from the state on recharge. Now people are talking about Multi-Benefit Land Repurposing and the grant funds available. That may change things.  

How does the recharge interact with crops on these agricultural sites?

DH: The two main sites are farms, so they had to give up land from production. The other main interaction is that in this setting where you have plastic mulching that prevents infiltration and generates runoff, these basins become a critical part of the agricultural system. To function on the landscape in a more holistic way that is balanced, the basins are almost a requirement. If I had a magic wand, and I didn’t want to outlaw plastic in agriculture, my alternative would be putting these basins and directing water to them as an alternative to putting more stormwater on the landscape. In a way it is a novel and interesting response to that type of agricultural practice.

What makes the Pajaro Valley unique when it comes to agricultural MAR opportunities and/or challenges?

DH: We are entirely groundwater dependent. There are no surface water rights that anyone is exercising. Water use strategies in areas with Flood MAR often rely on surface water. The idea is that they are infiltrating their allotted water, or water they have a right to, so they can bank it and use it later. We don’t have surface water out here at all so there is nobody with a water right that is looking to store that water for a later date. Everyone is pulling from the same underground reservoir making it a very shared resource. It has been a cool project out here, because by necessity it is for the benefit of all. This is unique to our area, as well as the clay in the soils and heavy use of plastic because of crop type. We share some of these opportunities and challenges with neighboring areas like the Salinas Valley, and other berry growing areas like Santa Maria, Oxnard and Ventura. 

How can farmers – in PV and beyond – get involved in recharge projects?

DH: Reach out! We are in the middle of a planning effort to expand the program to neighboring areas where context is most similar. Now is a good time. 

We have heard of interest in the Central Valley too, but what they want to pursue is Flood-MAR. For people who want to get involved in recharge, it is important to understand the type of recharge you want to do, and what type you can do. No reason that what we are doing can’t be done in the Central Valley as well – but it does get more complex if people want to recover the water because they have a right to it, so it may be further down the road. But with appropriate tweaks, our net metering formula could be applied elsewhere.

Thank you, Dan! 

Looking Ahead. Implementing infiltration basins and flood-MAR on agricultural lands are important opportunities for groundwater users to take an active role in replenishing this often-invisible resource. As Dan walked us through, where they are most appropriate will likely vary across the state. Considering soil type as well as crop type is the first key step to understanding if percolation basins or flood-MAR might be best suited. Once a type of MAR is identified, it is also essential to understand available water – is there a right to surface water that can be diverted or is stormwater catchment the best option (Alam et al. 2020)? Scaling up projects such as the successful ReNEM project can help water managers around the state learn from one another and ultimately work together to address the state’s groundwater challenges.   

About the Author and Interviewee

Sarah Sarfaty Epstein is a Masters of Science student in the Energy and Resources Group at UC Berkeley. Sarah’s research is focused on climate resilience, adaptation and mitigation opportunities within California agriculture. This blog post was written as part of the UC Water Academy course. 

Dan Hermstad is the Ag Technical Program Specialist for the Santa Cruz Resources Conservation District. Dan has worked for the RCD since 2021. 

Further Reading

Alam, Sarfaraz, Mekonnen Gebremichael, Ruopu Li, Jeff Dozier, and Dennis P. Lettenmaier. “Can Managed Aquifer Recharge Mitigate the Groundwater Overdraft in California’s Central Valley?” Water Resources Research 56, no. 8 (2020): e2020WR027244. https://doi.org/10.1029/2020WR027244.

Dahlke, Helen E., Gabriel T. LaHue, Marina R. L. Mautner, Nicholas P. Murphy, Noelle K. Patterson, Hannah Waterhouse, Feifan Yang, and Laura Foglia. “Chapter Eight – Managed Aquifer Recharge as a Tool to Enhance Sustainable Groundwater Management in California: Examples From Field and Modeling Studies.” In Advances in Chemical Pollution, Environmental Management and Protection, edited by Jan Friesen and Leonor Rodríguez-Sinobas, 3:215–75. Advanced Tools for Integrated Water Resources Management. Elsevier, 2018. https://doi.org/10.1016/bs.apmp.2018.07.003.

Harter, Thomas. “California’s Agricultural Regions Gear up to Actively Manage Groundwater Use and Protection,” 2015.

Kocis, Tiffany N, and Helen E Dahlke. “Availability of High-Magnitude Streamflow for Groundwater Banking in the Central Valley, California.” Environmental Research Letters 12, no. 8 (July 2017): 084009. https://doi.org/10.1088/1748-9326/aa7b1b.

Kourakos, George, Helen E. Dahlke, and Thomas Harter. “Increasing Groundwater Availability and Seasonal Base Flow Through Agricultural Managed Aquifer Recharge in an Irrigated Basin.” Water Resources Research 55, no. 9 (2019): 7464–92. https://doi.org/10.1029/2018WR024019.

Levintal, Elad, Maribeth L. Kniffin, Yonatan Ganot, Nisha Marwaha, Nicholas P. Murphy, and Helen E. Dahlke. “Agricultural Managed Aquifer Recharge (Ag-MAR)—a Method for Sustainable Groundwater Management: A Review.” Critical Reviews in Environmental Science and Technology 53, no. 3 (February 1, 2023): 291–314. https://doi.org/10.1080/10643389.2022.2050160.

Marr, Jenny, David Arrate, Romain Maendly, Devinder Dhillon, and Shem Stygar. “Flood-MAR.” California Department of Water Resources, June 2018. https://water.ca.gov/-/media/DWR-Website/Web-Pages/Programs/Flood-Management/Flood-MAR/DWR_FloodMAR-White-Paper_a_y20.pdf.

Miller, Kathleen, Andrew Fisher, and Michael Kiparsky. “Incentivizing Groundwater Recharge in the Pajaro Valley Through Recharge Net Metering (ReNEM).” Water Management, Science and Technology. 2020. https://www.law.berkeley.edu/wp-content/uploads/2021/06/Incentivizing-Groundwater-Recharge-in-the-Pajaro-Valley-through-Recharge-Net-Metering.pdf 

Schmidt, C. M., A. T. Fisher, A. Racz, C. G. Wheat, M. Los Huertos, and B. Lockwood. “Rapid Nutrient Load Reduction during Infiltration of Managed Aquifer Recharge in an Agricultural Groundwater Basin: Pajaro Valley, California.” Hydrological Processes 26, no. 15 (2012): 2235–47. https://doi.org/10.1002/hyp.8320.