Establishing environmental flow targets to protect aquatic communities is a priority for numerous programs in California including in the Delta. In this first of three brown bag seminars focusing on flow targets and ecology, Dr. Yarnell discusses her recent work using a functional-flows approach to develop statewide environmental flow recommendations. The approach allows for the rapid development of flow regimes that consider ecosystem and human needs and can be applied in an adaptive management framework, allowing for changing conditions and needs.
Dr. Sarah Yarnell is a research hydrologist with the Center for Watershed Sciences at the University of California Davis. Her research focuses on integrating the traditional fields of hydrology, ecology, and geomorphology in the river environment. She is currently working with a broad coalition of academics, researchers, and resource managers to develop the California Environmental Flows Framework, which has been in process over the past year.
“It started when several of us realized we were working for different agencies and basically doing the same thing,” she said. “So we started meeting and trying to push forward a path that would be more inclusive across the state, more comprehensive across the state, and more applicable to multiple agencies at once, so what I’m going to talk about today is this approach that we’re actively working with and still developing over the next year.”
California is a huge, diverse state with a lot of different water problems, and many questions related to water withdrawal and water diversions, such as how much flow can be diverted for irrigation, how best to deal with stormwater runoff, wastewater treatment, and restrictions on groundwater withdrawals.
“All of these questions relate to water, but they can be very difficult to answer, and might have a different set of ecological and environmental flow problems that can be associated with them, so the task is not simple,” she said.
Dr. Yarnell acknowledged that there are areas in better shape than others that could be protected, but it’s hard to prioritize those areas, knowing that there’s going to be withdrawals and impacts that happen all across the state in all of the different watersheds. “In addition, because California is so huge, there’s a huge geomorphic and geologic topography and variability that happens across the state, so even if you put one set of environmental flow restrictions in place for one type of use, that exact same pattern or prescription might not work in a different geologic and geomorphic area,” she said. “So the same type of flow regime might not be suitable for different geologies, even if it is suitable for the type of water issue or water problem that you have.”
This has led to a lot of different approaches and challenges to actually creating environmental flow regimes across the state as it can be hard to balance the needs with the broad range of demands that happen. “Right now there’s really no mechanism for coordinating and information sharing among the agencies, so we have agencies that tend to have a different approach and different set of standards for developing environmental flow criteria than others do,” she said. “So a lot of times, two agencies or two groups come together to come up with a flow prescription for a particular watershed, and they can actually have competing recommendations or competing prescriptions that end up then getting negotiated.”
This points to a need for a coordinated statewide environmental flow framework to help deal with these issues as many agencies have a need to set environmental flows, such as State Water Resources Control Board, Department of Fish and Wildlife, US Forest Service, and USGS, and others, but they might have different ecological endpoints, different species, and different management needs.
Dr. Yarnell said their group is trying to come up with a more inclusive approach to developing environmental flows. Their approach was to start with a science-based approach about the hydrology of the different watersheds in California and the ecology of the different diverse species that we have, and combine those together to come up with environmental flow targets that are then based on the best known science at this point in time, she said.
“Based on that then, if you can have a cohesive approach to developing those and a single set of flow criteria to start with, then you can move into balancing the beneficial uses and really having a discussion that becomes more of policy-based decision,” she said. “If there’s a particular single approach to developing these flow targets and having a starting point to begin negotiations and discussions, and taking into account these beneficial uses, then you have a simpler path forward to coming up with actual policy and regulations. Everybody is starting at the same point before they begin their negotiations, basically.”
Dr.Yarnell said that a tiered approach to setting flows starts with a single cohesive method to set coarse-scale or broad flow targets across the entire state; then depending on the specific needs within the watershed, the geomorphology, the ecology, the particular agencies and stakeholders that are involved, the flow targets are refined using more site specific e-flow methods; and then having a portal, a clearinghouse, or a website as a place to make the data openly available and disseminated to the public and the various agencies involved.
“We feel that by taking this approach, it provides a series of steps that everybody can follow and can agree upon to get you to a specific flow prescription that can be specific for your site but starts from the same starting point,” she said.
The group Dr. Yarnell has been working with has developed the tier 1 or coarse-scale e-flow targets so there is a methodology that can be applied across the state in a broad sense framework. It’s a three-step process, the first is to develop a single stream classification across the state that is based on the hydrology of the area.
“It allows us to group the different streams within the state into a series of types that then can be used to describe what the reference hydrology would be for that area,” she said. “To describe that reference hydrology, we’ve come up with this approach of what we’re calling dimensionless reference hydrographs. Once we know what type of stream-type we’re in and what type of reference hydrology it has, we can look at a series of metrics, or quantifiable components about that hydrology that we know can relate to the ecology within that particular system.”
“Using this rough guidance or this approach, we then can come up with some broad scale criteria that are functionally based for the ecology and hydrology of that particular system that provide a set of rapid or coarse or broad-scale flow criteria that then can be refined with environmental flow methods that are more traditionally used,” she said.
Dr. Yarnell then went into greater detail about how the approach works. The functional flows approach is based on a paper that was put out in 2015 in Bioscience that said that given that it’s simply not possible to replicate a full natural flow regime with the over-allocation of the water supply and the structure that the state has, there are particular aspects of the flow regime here in California that really serve a particular purpose or function. So in restoring the flow regime, which aspects of the regime are the most important ones for driving the actual physical processes of what’s happening in the stream?
“We want to focus on those components that support the natural disturbances, such as large floods that can scour and rework sediment, that promote the physical dynamics that create habitat within the streams and really drive the resulting ecosystem functions that happen,” she said. “It’s this dynamism and these physical dynamics within the streams that actually drive the high biodiversity that we do have in this state, and so restoration of those processes can be the best thing that we can do, given the confinement of a lot of our river systems.”
Dr. Yarnell said that it’s important to take into account the actual geomorphic setting as having the space for the water to actually do the processes is equally as important as having the water to actually move sediment and create those processes.
Functional flow is defined as any component of the hydrograph that provides a distinct, geomorphic or biogeochemical function. Dr. Yarnell reminded that they’re not interested in trying to replicate the exact pattern and the exact nature of that full natural flow regime, because that’s simply not possible with the way water is managed here in California. “Instead let’s focus on those pieces that we know through science-based literature and previous studies have a real distinct function,” she said.
She presented a typical hydrograph for a mid-elevation Sierra Nevada stream. It usually has a dry season from July through October; there are typically some small floods in the fall when the rains start. There can be large storm events in the middle of the winter, especially during atmospheric rivers or large precipitation events, and then as the snow melts from the higher elevations, there’s a pronounced distinct springtime pulse.
“So we might pick out particular components – in this case, these four components that are highlighted here, because these ones serve particular functions,” Dr. Yarnell said. “Let’s work on preserving those and restoring those within our streams and allowing the rest of the flow regime to be captured into a reservoir dam or reallocated in way that it currently is.”
She noted that the functional flow regime is reflective of the natural patterns that occur both in space and time. “We don’t want to release a peak flow down in the middle of spring when species are spawning,” she said. “We want to match the timing that’s occurring within the natural system.”
Dr. Yarnell then discussed each of the four components in the example. The wet season initiation flow provides important ecological and biogeochemical functions, such as preparing the riverscape by clearing the channel bed of the organics; it’s a flushing flow that goes through and reactivates the hyporheic zone. The wet season initiation flow is also a cue for a lot of native species to start their migration upstream, such as Delta smelt, but also salmon species in the north part of the state are cued to begin their migration upstream when these first fall flows come down. “It really kickstarts a lot of those biogeochemical processes when you rewet a stream after a low flow in the summertime,” she said. “So there are both biogeochemical and ecological processes that are tied to this particular cue at this particular time, so this might be a piece of the hydrograph that we would put a lot of value on preserving or restoring within an environmental flow regime.”
Peak magnitude flows are really the primary disturbance that happens in the state’s systems. There were five of these in the 2016-17 water year. “In some of the rivers that we monitor at the Watershed Center, we had five ten year floods that happened through some of the northern Sierra Nevada rivers with our wet year this year. And they completely redistributed, scoured, eroded, and redeposited new sediment,” she said. “It really reset that system and provided a lot of new habitat for the species to use. And it reset a lot of these successional processes that we know are really important in riparian communities.”
If peak flows are eliminated by a dam capturing water, which allows peak flows to go downstream in only the wettest of years, it doesn’t allow for the resetting of habitat that happens typically on a two to five year basis. “So it’s important to maintain the full magnitude of those peak flows,” she said. “And in fact, we might argue that it’s probably better to retain one large peak flow at full magnitude, and skip and hold back the rest of the flows that are coming into your reservoir then to reduce the peaks of five flood flows and allow five moderate flows to come down. Having a full magnitude peak flow in the wetter years or every 2 to 5 years can have a huge benefit, geomorphically as well as ecologically within the system.”
Dr. Yarnell noted that peak flows are most effective when they have access to a floodplain to the water can flow out onto the landscape. “We can put a flood flow down a channel or a river channel like the Sacramento River, but if it’s not accessing a floodplain and really moving that sediment around a floodplain, it might not have as much of an ecological benefit. If we have a place where we can combine that peak flow with access to the floodplain, you’re really getting the most bang for your buck in making that water work that much more for you in terms of restoring ecological processes within the system.”
Studies in the last five years have shown that spring recession flows are also important in California. There is a distinct wet season and a distinct dry season, and the spring recession is the transition from an abundant, high flowing river environment with a lot of cold water down to the low limited flows of summer. “If you are behind a dam and you shut off the spring recession and close the gates and immediately go from a high flow condition to a low flow condition with no transition between the two, you’re fundamentally changing and altering the habitat that’s down there in the channel,” she said. “Allowing a gradual recession that has some amount of duration allows for quite a bit of ecology and geomorphology to actually happen within the system.”
“As the flows gradually recede, you’re getting redistribution of the sediments that were mobilized by the high flows, you’re getting resorting of those sediment into different size patches that then create different habitat locations for different species, you’re limiting the riparian vegetation encroachment by keeping areas wet and slowly drying them out which delays any establishment of vegetation, and these annual cues in the springtime are often the cues for outmigrating salmon or outmigrating native fish off the floodplain back into the channel,” she said. “These species are looking for that gradually warming, gradually receding flow to be their cue to move to a better environment for them in the summer. And so we would argue then that maintaining this transition between the wet season and the dry season provides a lot of ecological benefit.”
Lastly is the dry season, low flow. Dr. Yarnell noted that there have been a lot of studies that talk about what the magnitude of low flows should be, but it’s also important to consider what the natural summer flows were like, such as was the stream ephemeral and it’s been changed to perennial, or was the stream perennial and is now ephemeral.
“We would argue for maintaining emphermerality or perenniality of different streams in different systems to preserve what the species have adapted to,” she said. “Some of us would argue that going down to a lower flow in September is what species are adapted to and what more natural systems would exhibit if they had higher flows in the spring, rather than maintaining a constant flow from June through August. … In reality you need a little bit more water in the spring and then you can allow for less water in the summer. Our native species were adapted to a lot of dry years where you’d go down to very low, very warm flows, but it was for a short period of time. And so if they are healthy and robust, and have this spring water available to them, they might be more adapted to being able to survive that condition of the low, warm flows in the late summer.”
Maintaining the interannual flow variability is also equally important. California naturally has wet years and dry years, so these extremes naturally occur anyway, but the native species are adapted to this.
“A really warm, dry low flow year might be a great year for macroinvertebrates and native amphibians, and not such a great year for fish,” she said. “Conversely, a wet cold year might be a really great year for some of the native fish species and not a good year for the amphibians. The biodiversity that California has which is some of the highest on the West Coast, is based on these tradeoffs and alternating these different types of niche and habitat availability from these wet to dry years. In fact in all dry years or all wet years, there are going to be less types of habitat available overall, and so supporting the full biodiversity needs to allow for this interannual flow variability.”
She said that the idea is by maintaining some of the components of the hydrograph, such as peak magnitude flows and these transition flows, within a single year or a season, that allows for different processes to happen, such as algal growth during the low flows, or channel erosion during some of the higher flows; also some years that have higher peak magnitude or a lower peak magnitude from a wet year to a dry year might get different processes happening. “Because we also want to maintain the full suite of natural processes that occur in our streams, we need to have this interannual variability in addition to seasonal predictable variability that happens annually,” she said.
Dr. Yarnell presented an example graph of what a functional flow regime might look like with the interannual variability. “We might still maintain all four components of that flow regime because we know that those are important, but we might change slightly their magnitude or slightly shift their timing, depending on whether we’re in a wet year or a dry year, and so the peak flows might be larger and of longer duration in a wet year because there is more water available versus in a dry year when you might have a much lower peak discharge that goes through in the wintertime,” she said. “The spring recession in a wet year might be larger and later and have a longer duration than in a dry year, it might be shifted to more like late April and it might have a smaller magnitude and shorter duration, and this sort of reflects the natural variability that we see happening anyway within an unregulated system.”
She noted that none of these components have been shifted out of their natural window of timing. “We would not suggest putting down a peak flow in the middle of April or middle of May when species are spawning. We want that really large peak flow that moves sediment to still happen in the wintertime.”
So having given the conceptual basis for the environmental flows framework, Dr. Yarnell then discussed how to use those concepts to then set the coarse environmental flows.
“The methodology for taking these concepts and putting them into practice is to focus on a particular stream classification, knowing that we have a lot of different variety of streams in California, and developing what we’re calling a non-dimensional hydrograph or a reference hydrograph for what the magnitude, timing, and pattern of flows look like in that particular stream class,” she said. “Once we have that, we can look at the statistical variability within reference gauges within that particular stream class and think about how the magnitude of the flows and the duration, and how that variability changes within an unimpaired system.”
“So for example, if I’m interested in the fall flows that are going to cue migration upstream as shown in box 1, I might look at a reference gauge or a reference hydrograph,” she said. “The bounds of that box – the upper and lower bounds would be set by the variability of magnitude that we see across years within a particular gauge, and the size of that box would be set within the natural range of timing that those types of flows occur. By looking at the natural variability within reference gauges for a particular stream class, I can then define these boxes for these particular flow targets or flow components that I want to maintain, and this gives me a range of targets then that I can start looking at to negotiate within that box.”
Dr. Yarnell said that timing can also be determined by aquatic species needs, such as the knowledge that Delta smelt have a migration cue that needs to happen between the first of October and mid November; flow targets can also be based on a particular water year type.
She then presented the stream classification that the group has developed, noting that in the beginning, there a group from UC Davis and a group from SCCRP were working independently of each other to develop a stream classification system with both efforts being funded by the State Water Board. So the two groups came together at the end of last fall and worked to reconcile the two classifications. The UC Davis group’s approach started from the hydrology and hydrologic methods and came up with a stream classification; the SCCRP group started from biology, and came up with how the biology impairment then coordinated with the hydrology, and when they were reconciled, there was 80% overlap between the two classifications.
“This gave us a lot of confidence that this classification is fairly accurate and fairly representative of the different types of streams that we have out in the state,” she said. “There was a decision framework that got put into place about how to deal with the 20% that had either one or two classes, and what you’re seeing here is the reconciled classification between these two efforts that has 9 stream classes for the state.”
The classification is reflective of the catchment properties, such as the geology, elevation, and topography of the basins, and the rainfall patterns. This provides a way to assign all the streams in the state to a particular stream class that will have a particular type of pattern and range of variability of flow. For example, the yellow represents the snowmelt class which is in the highest elevations of the Sierra Nevada where the entire hydrology is based entirely on the snowmelt; they don’t get winter storms. The bulk of the water comes down those rivers in the springtime when the snow is melting. The orange on the North Coast is the winter storm class, which has no snow but is entirely driven by the precipitation events that hit the North Coast when the winter storms come through.
The stream classifications have a variety of hydrologic attributes, such as the timing of flows, the variability of the range of flow, and the relative magnitude. The graph on the lower left of the slide shows how different each of the stream classes may be, from the groundwater-driven stream shown in red, which is fairly constant all year round, versus the flashy, ephemeral stream shown in the bottom line, which is representative of desert regions, which have flashy winter storm events of short durations with high magnitudes and dry summers.
Combining the hydrology of reference gauges, the catchment properties, the geology, and the soil characteristics, they developed a predictive model using a GIS framework to assign the classification for the rest of the streams in the state.
The dimensionless reference hydrograph is a way to combine all the reference gauges in a particular class and group those together to come up with a characteristic hydrograph for that stream class, Dr. Yarnell explained. On the bottom left is the dimensionless reference hydrograph for the North Coast streams; the Y axis is determined by dividing stream flow by mean annual flow; the X axis are the Julian dates. This provides a way to non-dimensionalize all the different gauges that might be in that particular stream class and allows for a comparison of the patterns, relative magnitude, and timing of flows for small watersheds and large watersheds.
“For example for that winter storm class, you can see that the bulk of the rain is falling in winter,” she said. “It’s very flashy; the relative range of magnitude goes all the way from zero up to 25, so 25 times the low flows in the summertime you can actually see during the wintertime. You can see that there’s not much of the snowmelt signature, that rain is really coming down in wintertime in flashy storms, and then we have a long, extended dry season low flow.”
Conversely, the snowmelt stream class shown in yellow are the high elevation Sierra streams that are driven by snow. “There is little to no rain that falls in the wintertime because it’s all falling as snow, but then once it warms up and that snow melts, there’s a really pronounced strong spring snowmelt pulse that’s quite predictable,” she said. “The range of variability there is only going roughly from one to five, so the peak magnitude flow is only maybe five times what some of the lower flow is. Those relative magnitudes aren’t quite as high, but there’s a strong seasonal signal that’s all completely tied into the timing of the snowmelt there.”
Dr. Yarnell then presented a slide showing reference hydrographs for some of the different stream classes. The low volume, snowmelt and rain class in the upper left hand corner is similar to the example hydrograph presented earlier when during the discussion of the functional flows approach.
“This is the mid-elevation Sierra stream where you’re getting influence of snowmelt in the spring,” she said. “There’s a quite distinct snowmelt recession that’s happening there in springtime, but there’s also these large peak magnitude flows that can happen in the winter that really move the sediment around in the streams. So then based on each of these stream classes, I might pick out different functional flow components that are appropriate to the stream class. For example the low volume snowmelt and rain might have all four of the components that I talked about in the functional flows approach, where the snowmelt, we might just be focusing on the peak magnitude of the snowmelt, the snowmelt recession, and the low base flows, because that’s what’s applicable to that particular stream class.”
The next step is to quantify the metrics using the non-dimensional reference hydrographs and relating those to a particular ecological function. Dr. Yarnell returned to the low volume, snowmelt and rain class example. “For example, I’m interested in maintaining those four components that I talked about, so I can use those reference hydrographs to quantify then the bounds on those boxes,” she said. “So I do want a fall flush flow, I want it to cue the fish migration, and I’m interested in what the peak magnitude or the range of magnitude for that fall flush flow is. Maybe I want to quantify that as a percent of a base flow. Then the timing of when it needs to happen; it should happen every year and maybe it should have a duration of two weeks, or maybe it’s two days, depending on what the hydrology of that particular class showed me. And so in this way, we’re taking the information out of that reference hydrograph that we feel is appropriate to the ecology of the particular function that we want to maintain, and we’re quantifying the .. non-dimensionalized hydrographs.”
She then presented a graphic showing how the flows might be calculated. “I have a range and magnitude here that I can calculate in Excel off of these dimensionalized hydrographs, and I can look at the date that those first fall flows occur, the end date, and the range and magnitude that I see within a particular class,” she said. “The same with the winter floods; I can even look at what the 100-year reccurrence interval flood looks like, what’s the range in magnitude, what’s the range in timing, what’s the rates of recession that I typically see. We’re able to pick out quantifiable metrics that we can calculate off those reference hydrographs that I can then immediately put into an environmental flow framework.”
They have developed a tool called the functional flow calculator, which can take all of the reference gauge data in the state, apply algorithms and actually calculate the numbers to actually define a functional flow target or an environmental flow target within a particular stream.
“Just to illustrate the point, if I’m interested in having a 10 year recurrence interval flood or winter high flow in a particular stream, I can see from my non-dimensionalized graph that the flow for a particular stream might need to be between 700 and 2700 cfs, it should have a date of timing that happens sometime before November 1st and sometime in mid to late April and its duration should be between ten and twenty-nine days and it should occur once every 4-9 or 5-8 years. These numbers are being calculated off of that reference hydrology and we take that then and use that to create the bounds of what we want an environmental flow target to look like.”
This can be done for the different stream classes in the state. Each stream class has different ecological end points that can range from salmonid timing and microinvertebrate diversity in the low volume snow-melt and rain classes in the Sierra to riparian habitat concerns for amphibians in Southern California stream classes in which the timing of when the spring flows that come down can be used to limit riparian growth.
“So I could potentially have different quantifiable metrics in different classes and for different ecological end points, but I have the ability to calculate any of those metrics from these reference hydrographs, so it in essence becomes one methodology that can be applied in a variety of different ways across the state,” said Dr. Yarnell.
In conclusion …
“So just to sum up, the idea with these course tier 1 environmental flow targets is that we’re using a functional flows approach as a concept to drive the methodology,” Dr. Yarnell said. “We’re hoping to quantify and restore those components that have the greatest link to process. We know we can’t restore the whole flow regime, it’s just not possible, so let’s pick the pieces that we think based on the science at this point in time that have the greatest bang for their buck basically.”
“We’re going to use these reference hydrographs from each hydrologic stream class to quantify metrics which can then rapidly provide a range of environmental flow targets to choose from and this then becomes the first cut at coming up with some environmental flows,” she said. “Once you have the box defined or those quantifiable metrics defined for a particular region, we can further refine that using traditional environmental flow methods, if necessary or if needed. So for example, if the flow range is too big or some of the stakeholders are not interested in having such a large range of flows, then we can do on-site studies which typically happen or use additional methods like the ELOHA approach or some of the other regional IFM approaches to further refine those flows if needed.”
“One of the key pieces we all know that doesn’t often happen is that monitoring is essential,” she said. “A lot of these flows, we’re doing our best scientific guess, we’re doing studies based on it, but a lot of times you don’t actually know if it’s going to work until you go and do it for a while and actually see what happens. And so that’s a big component and piece that needs to be included.”
“And so with that … “
QUESTIONS AND ANSWERS
Question: My question is about implementation. Say I was a reservoir manager and I’m trying to decide how best to time my flows to support the downstream ecosystem, how do I know what kind of water year I’m dealing with? Especially at the beginning of the season when we have those peak flows, a lot of those happened in January or December when we may not know yet what kind of water year we’re dealing with, that as a water manager, I would be wondering if I allow this large peak flow, how do I know that I’m going to get it back later in the year? With a lot of these presumptions tied up in terms of the water year, how does that work in practice, when we can have a really wet December or January, and then things just shut off later in the year?
“This is part of the discussion that needs to happen,” said Dr. Yarnell. “It might end up being really specific for different locations. So for example, a lot of my work has been in the FERC process up in the Sierra Nevada, and you might be more willing to let a December flow pass if you’re in the upper reservoirs where they don’t have as much storage capacity; it’s more common to spill in the upper reservoirs and you might hold it in the lower reservoir until you know the water year type. Typically the water year type is declared March, 1st of April, and if you know you’re going to be looking towards a really wet year, then you might have to delay that flow that passes through the Central Valley, but historically a lot of the flood flows in the Central Valley didn’t happen until April and May anyway, so that type of question, I think if there’s not a universal answer that can happen across the whole thing and the management and implementation of that then has to go to the different sections that you’re managing.”
“The one benefit is that if you parse out by water year type and our water year types are five water year types, at least you know the type of target that you’re looking for and as you get into February and into March and you start to have more confidence in what type of water year you’re going to have, you might be more willing to talk about what flows to pass,” she continued. “It looks like we’re going into an above normal year, we know we’re going to be looking for some kind of flow to pass that could be of this magnitude and you start to close in on that, but it’s a real question and it does have to end up being decided within that water year basis. And it’s something we grapple with routinely, not even trying to take these functional flows into account, how much water do I store and how much flood storage do I want to have. Maybe we’re adding just one more question into that, but it’s a really good question.”
Question: So for part of your approach, you’re using actual gauge data, but yet you’re trying to create functional flows, but I’m wondering what your period of record is. A lot of your gauge data, it has been regulated for years and it may not be regulated in a way that is supportive of the ecosystem, so how do you take that into account?
“I should have clarified that when we were talking about the stream classification,” said Dr. Yarnell. “With the stream classification, there are two things. First one, the idea is that this is providing information on the reference hydrology for those, not what currently is, but what do we assume and know it should have been like or used to be like, so for that we had to go look for reference gauges. There’s only 8 to 10 in the state that have more than about a 30 year record, but there are a lot of places where you might have a 20 year record, from 1930 – 1950 before they built the dam, or there’s a 20 year record where we have naturalized flows, where they’ve gone through for various relicensing projects and recalculated what the flows are minus that regulation. There was a process that both groups went through, defined their criteria for what would serve as a reference gauge, and made sure that they were taking into account as many gauges as possible but those that really only reflected a reference hydrology, and so then using only that information. I think the final set of reference gauges is about 90 gauges across the state, and that information is available in the two papers that are associated with this. A lot of the information is hopefully going to be up on the web within another couple months. We’re working with the data.ca.gov folks to get some of this information up, but that then defines the reference hydrology or type of that stream type. What you could do then is use the current data, the actual gauge data, to also do an analysis of impaired versus unimpaired, so how is impaired is my particular stream?.”
Question: When you’re talking about those reference gauges that’s different than what a lot of people call unimpaired flow. Right? Can you explain the difference?
“There are definitely a handful of gauges that truly reflect unimpaired flow,” said Dr. Yarnell. “North Fork American, North Fork Yuba, there is a handful of them that are out there. They do not span the entire state and they do not provide enough information to do a classification for the state, so a lot of the data then that we would consider reference is historical data from the pre-dam gauge, or a naturalized flow, which is a modeled flow. I was not one of the original authors on some of these papers so I can’t give you the laundry list off the top of my head of exactly the criteria they followed, but if it was a modeled naturalized flow, then there was a set of criteria that it had to pass in order to be included as a reference gauge, and I know not all of them are gauges that come all the way into the Delta, it’s too complicated to really know an unimpaired or naturalized flow directly into the Delta, so a lot of these are the tributaries and other streams in the state, not actually what’s hitting into the Delta, so we purposely didn’t go there, it’s too complicated.”
“There are two papers that have been published off of the two original stream classifications, and in those papers, they talk about their criteria for selecting those reference gauges and that was quite a big process to figure that out, how do we have confidence in which of these reflect the reference hydrology. But then the hope is that once you’ve coalesced that into a non-dimensionalized hydrograph, it gives you variability and timings and a pattern for a particular stream type. Now I can go forward, and there is an assumption that that particular stream might match that, but if I’m not confident in that assumption, I can go out and do a site specific study for that stream, but in general, we’re going to accept that that is the rough pattern and the approximately coarse resolution so that the unimpaired hydrology should look like for that stream type, and the use that data to come up with some bounds on the flow criteria and as a way to compare it to unimpaired; how impaired is my stream, I can use that reference hydrology to compare to my impaired gauge data that I have.”
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