Farms serve as natural experiment site for large-scale groundwater recharge

Leigh Bernacchi
January 18, 2018

Dr. Helen Dahlke’s research contributes to restoring California’s water reserves.

Dr. Helen Dahlke is an Associate Professor of Integrated Hydrologic Sciences in the Department of Land, Air and Water Resources at the University of California, Davis.

Dahlke was interviewed by UC Water’s Program Coordinator Leigh Bernacchi for the release of the UCTV Sustainable California video and open-access publication on the alfalfa research in California Agriculture. To learn more about Dahlke's work, visit UCTV. A special thanks to UC ANR and UC Davis Communications for contributing the video.

UCW: How do you explain groundwater in basic terms?

HD: Usually I borrow a metaphor from my UC Davis colleague Thomas Harter, a groundwater specialist with the University of California Cooperative Extension. He always talks about the Central Valley as a bathtub. It is filled with sediment of various sizes and diameters, and groundwater is more or less the water that occupies the pore spaces between those sediment grains. It is the water stored in the voids between the sediments.

There are pockets in the groundwater system where we can pump the water out more easily, and then there are pockets where it is really hard to get the water out. Those are typically in areas where we have a lot of fine materials such as clays that don’t allow quick conveyance of that water to a groundwater well for example.

UCW: In the video, you say 100 million acre feet (MAF) of water has been pumped out of the Central Valley. Where did this water come from and where does it go?

HD: The Central Valley’s groundwater is a renewable resource, and it is renewed every year. When we have precipitation falling in the winter, it’s falling onto the land surface, our mountains, and some of that becomes streamflow that we can see in our rivers. But a lot of that water is also infiltrating into soils and slowly moving in the subsurface. It flows downslope with gravity and is accumulating in the Central Valley. This process has been going on for thousands and thousands of years.

Right now the problem that we have is that we are extracting water at greater rates than it is replenished or recharged naturally from precipitation. The resource is renewable. Just we exceed the use of it. During the period 1960-2016, we have extracted about 80 million acre feet beyond what is naturally replenished from rainfall. If we go further back in time, and look since the 1920s, we have extracted 150-160 MAF.

80 MAF is 700 years of the annual water use for the District of Columbia. Another way to think about it is that the entire volume of Lake Tahoe is 120 MAF, a lake that is over 500 yards deep.

UCW: As we enter the (hopefully) rainy season, what are some of the updates you have on your on-farm groundwater recharge research?

HD: We have completed our on-farm experiments on alfalfa and almonds. The results of the alfalfa study are published in California Agriculture


For alfalfa, we had two experimental sites where we tested different water amounts and different timings of winter water application for groundwater recharge. The main take-home message from the research is that if you do groundwater recharge on soils that have high infiltration rates, there seems to be little harm to the alfalfa, if you follow some of our findings:

  • Wait until the alfalfa is mostly dormant,
  • Apply 4-10 feet of water in the winter,
  • Use pulsed application (e.g. normal irrigation practice) so that air can reenter the root zone, and
  • Don’t keep the field permanently flooded.

The research was conducted in Scott Valley, and some of the follow up research that we want to do on this topic is repeating the experiment in a warmer climate. Right now we’re looking to find sites in the Central Valley where the crop does not go completely dormant in the winter. We want to see whether or not there is any negative effect of recharge on that.

Learn more about flooding alfalfa on the UCANR blog.


We also had three almond orchards where we did experiments. We’re in the third year of the study; most of the plant physiology studies have been completed when we measured stem-water potential to see how well hydrated the trees are; we also took pictures in the root zone to see how root growth was impacted by the recharge. We also did yield measurements before and after the recharge event.

Study setup

We looked at three sites that had different ratings for soil infitration capacity based on the SAGBI, Soil Agricultural Groundwater Banking Index. On the SAGBI, high infiltration rate soils are ranked excellent, good, or moderately good.

Study Design for Flooding Orchards
Site LocationSoil rating (SAGBI24" of rechargeImpact on trees
DelhiExcellent24" Benefitted. Trees stayed more hydrated during the growing season, salts were flushed, longer lifespan of almond roots.
ModestoModerately Good24"No observed difference
OrlandModerately Poor5"Had to discontinue flooding due to overland flow. 


root zone photo
Dr. Astrid Volder, Assistant Professor in the Department of Plant Sciences
at UC Davis, designed underground cameras to study how flooding 
affected almond roots.

It looked like the additional recharge in the winter had no negative effect at all. The trees seemed to be pretty well adapted to these types of conditions in general.

In terms of tree impact, from the two data years of yield we have, we did not see a significant difference between the control and the flood treatment. The trees at the sandy Delhi site looked like they were benefitting from the recharge in the winter. They stayed more hydrated during the growing season and the median lifespan of roots was about 30-70% longer than in the control. There might have been a benefit from flushing some of the residual salts and nitrates out of the root zone. And at the Modesto site, which is the site that is flood irrigated throughout the growing season, it looked like the additional recharge in the winter had no effect at all. The trees seemed to be pretty well adapted to these types of conditions in general.

Future plans

There is interest from the Almond Board of California to investigate how recharge during the growing season—when trees are leafed out and actively blooming or growing—whether that would have the same negative effect on trees.

There is definitely evidence that you can harm almond trees if you overwater them during the growing season, so we need to do more research on that. But so far, it looks like the trees that are planted on suitable soils, such as well-drained soils, don’t seem to have an issue with additional water during tree dormancy.

UCW: What resources are available for farmers?

HD: We have some tools out there that can really be used to do a check on whether your particular land parcel is suitable, soils that are excellent, good, or moderately good. If that’s the case, you might be a good candidate for doing groundwater recharge.

The next step is to get in touch with your water district if you have one so you can discuss whether they are planning on doing any recharge, if they do how you could get access to surface water in the winter for recharge.

It’s important to know about water rights: If you have a riparian property and have a riparian water right, you actually cannot use or divert water for groundwater recharge because it is against the law to store water for more than 30 days under a riparian water right. It would be better if you had either an appropriative water right or you are affiliated with a water district and all of the water right permitting is handled through the water district.

There are good examples for this. Tulare Irrigation District has a program where they have farmers sign up for groundwater recharge, and they get a certain credit for the water they are recharging. Pajaro Valley is another example for incentive programs. They have a pilot of net metering in places where farmers can receive credit for their summer water bill if they recharge water in the winter. All of that is pretty encouraging and could be discussed with your individual water district.

UCW: How can farmers and landowners know if their property is suitable for recharge?

HD: That’s possible. Our colleague Toby O’Geen has developed an online tool called Soil Agricultural Groundwater Banking Index. It’s a tool that has been developed purely using soil survey data: combination of different soil characteristics and ranking of different soils for their suitability for recharge.

Among well-drained soils, we’re looking for:

  • high saturated hydraulic connectivity (often occurs in soils with a sandy texture and less clay or restrictive features such as hardpan)
  • soils that are not prone to forming crusts (if you were to recharge those soils, it could be that the surface of the soil would be clogged up with fine sediment, reducing infiltration capacity),
  • flat surfaces, and
  • soils naturally low in salt content or other chemical constituents because they could be leached to the groundwater.
SAGBI is a useful suitability index for anyone interested in recharging groundwater on their land.
It incorporates five critical factors for recharge. Learn more on Soil Agricultural Groundwater Banking Index.

SAGBI is soil-based so it does not include any information like depth to the groundwater table or hydrogeology, which, of course, are also important to get the water to the groundwater table. Land IQ has taken the SAGBI map and added hydrogeology and depth to groundwater. And Sustainable Conservation have a tool together called GRAT: Groundwater Recharge Assessment Tool.

UCW: You also have some research on available surface water flows for recharge. Tiffany Kocis and you recently published on the “Availability of high-magnitude streamflow for groundwater banking in the Central Valley, California” in Environmental Research Letters. There’s also a cool interactive website that shows where the flows are available. What were some of the goals of that research?

HD: I have to admit. I haven’t worked much with groundwater. All of the experiments have used surface water and I am actually a surface water hydrologist.

UCW: I always think of Dahlke as synonymous with groundwater. That’s interesting.

HD: In order for on-farm recharge to really work out in the long-term, there are several key factors that have to come together: location, soil and crop suitability are some of the factors, but you can’t do recharge if you don’t have excess surface water nearby.

One goal of our project that was funded by UC ANR three years ago was to basically look into surface water availability for groundwater recharge. There has been a report recently by the CA Department of Water Resources, the WAFR report, or Water Available For Replenishment report, which also presents some numbers on how much surface water is available.

We took a slightly different approach. We just decided to look at historic streamflow records. And basically we were interested in big storm events that are happening in the winter. We always talk about the flows going out to the ocean. How much water is going out to the ocean from those flows, when are they occurring, how often are they occurring, and can we summarize that information in a comprehensive way for the Central Valley?

We selected 93 stream gages that had records of more than 50 years in length and did a statistical analysis that focused on flows above the 90th percentile, that is, the 10% highest flows that have ever been recorded on that stream gage.

There is a lot of runoff that is generated just in the Central Valley on our ag lands that is contributing to stream flow below the rim reservoirs. We actually underestimate how much water we have just running off of the surface during these big storm events in the winter that we could capture if were to do a better job of managing our fields in the winter instead of allowing all that surface water to flow off. For streams we should also think about using more cover crops or making sure that surface water stays on the fields and is captured there.

We actually do see quite clearly what is happening over time because we have the real-time USGS stream-flow data that you can look at. You can’t start diverting surface water without a permit unless you have a water right in place that allows you to divert surface water in the winter season for a given purpose, whatever it might be, probably not irrigation, then you really cannot make use of these excess flows as they are happening. California’s water rights situation with respect to recharge is something that we are working on now. The agencies recognize that really if we want to achieve sustainability of our groundwater aquifers, the only source of the water where that could come from are the big winter storm flows. We have to find a way to legally use that water for groundwater recharge.

UCW: Finally, you have a new grant from UC Water to study water quality issues. Why is water quality a concern for on-farm groundwater recharge? And why are you interested in that?

HD: California groundwater is already known for being contaminated with nitrate, often from crop fertilizers, so applying more water on fields in the winter could potentially increase the risk of more nitrate contamination. Any legacy nitrate that might be stored in the unsaturated zones (the zone between the soil surface and the water table) might be mobilized if you apply large amounts of water because nitrate is very soluble. So in order to really understand if we do this on-farm recharge, what does it do to the nitrate leaching, we are currently doing some laboratory soil column experiments, where we simulate recharge events as we would conduct them in the field to understand how much leaching is occurring with various application amounts and timings on different soil types. The UC Water funding basically provides funding to model processes so we can apply hopefully our understanding more broadly to other areas.

UCW: Thank you for your time and your critical research.

Learn more: 

"Flooding alfalfa fiels has high potential for groundwater recharge." UCANR blog. By Jeannette Warnert.
"Managed winter flooding of alfalfa recharges groundwater with minimal crop damage. California Agriculture.

Watch online: UCTV Sustainable California.