Soquel Creek, Capitola, California; Photo by Rachel Lovinger

WESTERN GROUNDWATER CONGRESS: Depletion of interconnected surface waters: Not that simple

Sierra Ryan is a water resources planner with the County of Santa Cruz.  In this presentation from the Groundwater Resources Association‘s 2019 Western Groundwater Congress, Ms. Ryan tells the story of how the Santa Cruz Mid-County Groundwater Agency balanced the various perspectives, authorities, and interpretations of the DWR regulations in writing the portion of their Groundwater Sustainability Plan that pertained to the depletion of interconnected surface water (sustainability indicator or undesirable result #6).

I promise you that it was not as simple as we thought it was to begin with, so for those of you in the medium and high priority basins that have the 2022 deadlines, I hope you’re thinking about this now,” Ms. Ryan advised.

The Santa Cruz Mid-County groundwater basin is a critically-overdrafted basin located along the Central Coast where a history of overpumping has led to areas of seawater intrusion.  Projections are that there will be less groundwater recharge due to climate change, so the concern is to maintain pumping and take other actions to ensure that the seawater intrusion doesn’t increase.


The Sustainable Groundwater Management Act requires the identification of interconnected surface waters within the basin; if there are interconnected surface waters, the groundwater sustainability agency (GSA) needs to determine the rate of depletion whether that depletion is significant and unreasonable.  If it is, the groundwater sustainability plan (GSP) must address it so it doesn’t get any worse.  (Ms. Ryan noted that the GSA must also take into consideration GDEs or groundwater dependent ecosystems which are species and their habitats that rely on groundwater, but that part is not part of this presentation.)

It actually seemed pretty simple at first, but I’m telling you now, it’s not,” she said.  “It’s deceptively simple if you put together a little flow chart like we did and we thought great, we’ll just work our way down this flow chart.  First, you start by identifying your groundwater dependent ecosystems.  For us in Santa Cruz County, there was a lot of pre-prioritized natural habitats and our endangered species are something the community values. This was always going to be something that was important to us, second to not having seawater in our drinking water, but still very important.”

The next steps were identifying the interconnected surface waters which was simple; prioritizing where the species and habitats are; calculating the amount of depletion; defining the statements of what is significant and unreasonable; and then setting minimum thresholds and measurable objectives.


In retrospect, one of the best decisions the GSA made was to form a surface water working group early on which was both a technical advisory committee and a stakeholder advisory committee.  “These were stakeholders, the people who have an interest, as well as technical experts,” said Ms. Ryan.  “They aren’t going to charge you; they are happy to be participate and be involved.  They are also potentially the people who are going to write you nasty letters during your comment period and may write nasty letters to DWR when it moves on, so you want to get them involved early on and we’re really glad that we did.”

Using the California Natural Biodiversity Database, the Nature Conservancy’s resources for identifying groundwater dependent ecosystems, and local databases that were available, they put together a number of maps (lower, left) using the groundwater model that showed where there were different habitat types and different species.

The model also identified interconnected surface water with the areas of most connection shown in darker blue (upper, right).

The regulations only requires the GSA to consider sustainability indicators in the places where there is groundwater pumping because that’s the crux of what SGMA regulates, she said.

On the east side of the basin, the surface water isn’t connected to the groundwater, there’s over 200 feet of separation between the two of them and a big thick clay layer in between, so we weren’t really worried about that area,” Ms. Ryan said.  “There is a state park, so there’s really no pumping that’s happening there.  On the west side of the basin, there is a granite substrate with limited pumping, and the area by the coast is served by the municipal water district that uses surface water, so there wasn’t really much pumping there, either.  It basically left this one creek, Soquel Creek so that’s where we were going to put our focus.”

They worked with the surface water working group to prioritize the species, deciding that steelhead and coho were the top priorities, not only because of their special status, but because they need the most water, especially during the peak dry season for breeding and rearing of their young.  The riparian forests rely on water year around as well.

The priority species were like umbrella species,” Ms. Ryan said.  “If we can meet their needs, if we could take care of them, everyone else was going to benefit and we didn’t need to spend a lot more time focusing on the habitat needs of each one of these, so it made sense to us to do it that way.”

They accomplished the first three steps rather easily; they had only had two working group meetings at this point.


However, at the next step, calculating the surface water depletion, it became more complicated.  The SGMA regulations require the GSA to identify the rate or volume of surface water depletion caused by groundwater use that has an adverse impact on beneficial users and might lead to undesirable results.

It actually still kind of made sense if you have geology like this beautiful diagram where you have the surface water system that you’re worried about,” said Ms. Ryan.  “If the flow in the creek had been 10 cubic feet per second, and then you turn on those two wells and all of a sudden, the flow in the creek is 5 cubic feet per second, that means you have a depletion of 5 cubic feet per second.  That makes a lot of sense if your geology looks like that.”

Our geology looks like this,” she said.  “This is a cross-section of our basin and we have a lot of different wells, and it starts to get really complicated.  If there’s a little creek, is this well depleting this creek?  Or this this well depleting this creek way over here?  Or is it depleting both, and to what degree?  It’s not a simple as just turning off a well and seeing what happens because there’s a lag time … it could be a 20 year timeline between using a well and when you really see the full impact in your surface water, so to turn off all these wells for 20 years is not practical from a management standpoint.”

So they looked at what data was available.  First, it’s important to note that stream flow is influenced by a lot of different factors, she said, presenting a list of most of them, roughly in order of importance.  Rainfall and runoff from the watershed is a direct input to a surface water body; surface water extractions are the water taken out of a surface water body.  The vegetation uses water through evapotranspiration, which peaks during the hot times of the day, especially during the summer, and it can have a really big impact on flow.

There is the interflow, which she explained was much like a sponge effect where if there’s been a number of rainy years, there is still flow even in dry years; if there is a number of dry years, the flow might stay low in the next wet year.  Then there is the subsurface geology which can play a factor, and then there are the groundwater extractions.

All of these things affect surface water flow,” said Ms. Ryan.  “This is the only one that SGMA gives us authority over which is one of the least influential, so already trying to just look at flow becomes really hard.”


Trying to figure out surface water depletion impact based on flow alone is really tricky, even in this area where they already have good data, such as pumping data, deep aquifer groundwater elevation, precipitation, and surface flow.

It’s all really complicated and messy,” she said.  “We have looked at this data every different way you can, and there really isn’t a significant link that you can draw between the pumping down here and the flow up here.  There are so many other variables happening that it becomes really messy, and we don’t want messy if you’re making laws about this.  If you’re going to be telling people that you’re restricting their pumping based on some sort of messy figure over there, it becomes really challenging.”

So with help of the surface water working group and their consultants, they concluded that they were going to be unable to quantify the link between pumping and surface water depletion so they decided to use groundwater elevation as a proxy.

What this means is if the groundwater elevation next to the creek is staying at a certain level or going up, you’re not causing any more damage to the creek,” Ms. Ryan said.  “If you bring those levels down right next to the creek, then you might be causing basically what we call a losing water surface body where the water starts leaving the creek to feed the groundwater basin.  So it made sense conceptually.  Then we would develop all of our plans to basically maintain or increase those groundwater elevations adjacent to the creek.  So we thought we had it all figured out.”

By this time, they had already spent months, digging through the data, trying to find the connections, talking to DWR and others, and they had finally come to this conclusion.

Then we were reminded by DWR and the agencies that the SGMA regulations require that you be able to make a significant correlation between the groundwater elevation proxy and whatever you’re using it for before you can move forward and manage it that way,” she said.


We were having trouble doing that.  We couldn’t really find that link, so we ended up having to use our groundwater model to calculate what the average depletion was based on the groundwater elevations and it seemed to work pretty successfully.  We came up with a number of 1.4 cubic feet per second, so what that’s saying is the groundwater pumping adjacent to this creek or in the basin in general has caused a depletion of 1.4 cubic feet per second, and so if we keep our groundwater elevations at or above where they are now, we do not expect that number to be any bigger than it is.”

By now, they had spent all of their money and most of their time getting the background work, and they hadn’t gotten to writing up the content for the plan and there was less than a year to go until the deadline, so they were getting a bit panicked.

For each sustainability indicator (or undesirable result), they had to decide what was significant and unreasonable, and then set minimum thresholds and measurable objectives.

This is where the science meets the policy and where politics gets involved,” said Ms. Ryan.  “People’s opinions and thoughts become less based in the science that we’ve presenting, and they are based more on their own experiences and their own values, and this is the reality of writing this kind of document.”

So they had to decide what is an undesirable situation related to surface water depletion, so the working group decided that an undesirable result would be if they were no longer able to provide the habitat necessary to keep the priority species existing in their watersheds.

We determined with the working group that we are not currently experiencing these undesirable results,” she said.  “We do still have fish, they are still breeding here, although some years are really tough and we want to improve them, but right now we wouldn’t say that we have reached this undesirable result, so that was kind of the policy and political decision that comes from SGMA.”

Next, they had to define the statement of what is significant and unreasonable.  “Our initial statement used to be super-catchy,” Ms. Ryan said.  “When we started, it was all about groundwater elevations not going below a threshold and it sounded like a call to action.  But honestly, the more DWR got involved, the more it turned into ‘the significant and unreasonable depletion of surface water due to groundwater extraction and interconnected streams supporting priority species would be undesirable if there is more depletion than experienced since the start of shallow groundwater level monitoring through 2015.’  That’s what we could agree on because it brings in the surface water depletion and our priorities, and it’s something we can then quantify for our minimum thresholds and our measurable objectives.  But there needs to be link there, and that was how we did it.”

The measurable objective was a little more aspirational.  They decided what they want to see if the groundwater elevation above the base of the creek bed so that all the creeks become gaining creeks and are benefiting from the groundwater.

In some cases that is happening now and in some cases, what we’re representing with that is basically a recovery, so it’s definitely above and beyond what’s required from SGMA but it’s what we hope to see,” she said.  “We think it is actually realistic based on the projects that we’re planning.”

They set the minimum threshold as the highest seasonal low elevation from the start of monitoring through 2015.  The groundwater elevations are low in the summer and fall, and then rise during the rainy season.   The solid line is the draft minimum threshold; the large dashed line is the measurable objective, which is above the creek bed elevation, which is shown in the small dashed line.  2015 was towards the end of the epic drought, but in their basin, customers of the water agencies cut back so significantly on their water use that they had some groundwater elevation recovery during the drought.  So 2015 ended up being the highest year of these, even though it was the peak drought time.

Ms. Ryan said that they could have chosen a lower minimum threshold, but they didn’t think the stakeholders would like that.

We wanted to do something more positive; the GSA is wanting to see some improvement and that’s why we’re putting all this work into it, so that’s where we ended up for our minimum threshold.”


The strategy the Mid-County Groundwater Agency used was to use their priority species to determine whether the current situation significant and unreasonable or not, which it was not.  Then they managed for groundwater elevations and used the model to calculate depletion, so after all of that, it breaks down to defining what is significant and unreasonable, setting minimum thresholds and defining a measurable objective, she said.

Monitoring will be key.  “We will hopefully be able to get this passed through DWR and then through all of the stakeholders because we’re going to be monitoring things,” she said.  “We didn’t have the data to show the links that we really needed, but we’re going to continue monitoring and refining things.  We have 20 years to really dial this in and it’s absolutely our intention to improve our monitoring system to really be looking for this.  Remember, our old monitoring wasn’t for SGMA, it was for other purposes, so now our new monitoring network is really for doing this, and the ultimate question is will this work?  I don’t know because it’s not over yet.”

She noted that the comments they’ve received so far have been generally supportive, although there’s some concern about using the groundwater elevation proxy.  “I know some of the wildlife agencies want to see us eventually be able to move away from that and move towards just monitoring flow but we’re not there yet,” she said.

Ms. Ryan closed by saying that they would be submitting their GSP in January 2020.  “If you’re in one of those 2022 basins, I would play close attention to what’s happening because it’s a lot of work, and so you don’t have to recreate the wheel like we did where we were making this up as we go along, so hopefully you can learn from the lessons that we have learned and take those lessons and apply them.”

Featured image credit: Soquel Creek, Capitola, California.  Photo by Rachel Lovinger via Flickr.



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