Understanding Rain-on-Snow Events with Anne Heggli

The Sierra Nevada Mountain range, as of March 2023, contains a snowpack with more than 200% of an average year’s snowfall. Water managers across California and Nevada, states that rely on the snowpack as the region’s largest supply of fresh water, are celebrating what this means for alleviating some of the worst impacts of a widespread and ongoing drought. But with snowfall occurring at low elevations in unusual places, the possibility for warm atmospheric rivers to cause flooding increases. These storms, called rain-on-snow events, are the focus of DRI’s Anne Heggli, who is studying ways to improve our ability to forecast and prepare for these potentially hazardous storms.

Under the guidance of DRI’s Ben Hatchett, Ph.D., Heggli is working with the Nevada Department of Transportation and the National Weather Service in Reno to build better forecasting tools for rain-on-snow events, which will improve safety alerts and storm preparation across the state. DRI sat down with Heggli to learn more about her work, when rain-on-snow events are the most problematic, and why snowpacks don’t simply absorb rainfall like a sponge.

Above: Anne Heggli inside of a snowpit at the Central Sierra Snow Lab. 

DRI: Your Ph.D. work focuses on rain-on-snow events, can you tell us more about that?

Heggli: My Ph.D. work is focused on leveraging existing monitoring networks to try to find ways that we can maximize the investment that we’ve already made to learn about patterns with our snowpack to further our understanding of rain-on-snow processes, and to help inform decision makers on what exactly is happening in the mountains, hour by hour as these rain-on-snow events take place.

I got started with this because a water manager for a hydropower company in California told me that ahead of these atmospheric rivers, she felt like they were flying blind. They had no idea how the snowpack was going to respond.

DRI: And how are you doing that?

Heggli: The western U.S. has this great snow telemetry monitoring network, called the SNOTEL network, that’s run by USDA Natural Resource Conservation Service. All the stations collect hourly data for air temperature, precipitation, snow depth, snow water equivalent, soil moisture and soil temperature.

We really use the daily data, but the hourly data has not been applied. And I felt like that was a great opportunity to analyze this data to shave away at some of the uncertainty and help inform the people who are managing our water in the Sierra Nevada.

This data is especially important for the warmer atmospheric rivers that move through and put rain up over the crest of the mountains. It’s a way for us to understand what’s really happening in the deeper snowpack and what percentage of the watershed is actually contributing to runoff. The benefit of the SNOTEL network is the soil moisture sensors. In the Sierra Nevada, those have been installed since 2006, so there’s quite a lengthy record of soil moisture data there. And when a rain-on-snow event occurs and the rainfall makes its way through the snowpack, there are these really prominent signals in the soil moisture data, so it’s a way to actually verify if the snowpack is releasing rainwater or snowmelt.

The soil moisture data is key for my research because I can identify when the snowpack is releasing water, and then look at the snow density, air temperature, and precipitation. That way I can identify the patterns that are present every time the soil moisture has these really dramatic responses to find the ingredients that produce more impactful runoff rain-on-snow events.

DRI: Is soil moisture a measure of melted snow, or is it rainfall that’s passing through the snowpack to the soil?

Heggli: That’s one of the things that is kind of unknown. There’s been an assumption that the snowpack is melting. But some of the research in my first paper shows that during these rain on snow events, snow melt is not the primary driver of runoff in deeper snowpacks. Shallow snow will be obliterated, but in the deep snowpack, sometimes that snow will actually absorb part of that rainfall. But essentially, except for very exceptional events — like the 1997 flood event and February 2017 in the Sierras — snow melt typically is not part of the runoff process in the deeper snowpacks. However, in the shallower snow at lower elevations, it can begin to melt and then that increases the amount of water available to runoff into the streams.

It’s really about trying to tease out whether the runoff during rain-on-snow events comes from melting snow, or is it just rainfall and increased runoff efficiency? What exactly is producing the runoff and why is it so hazardous? What are the ingredients of a perfect storm for those major rain-on-snow flood events?

Heggli’s snowpit at the Central Sierra Snow Lab during the December 2022 storm.

Credit: Anne Heggli/DRI

DRI: Why can rain-on-snow events be a problem?

Heggli: Well, it’s highly uncertain at times. There are times, like in 1997, where we knew that this very warm storm was coming in with a lot of moisture and precipitation and very high-elevation freezing levels. Sometimes when the atmospheric rivers make landfall, they’ll push against the Sierra Nevada and they’ll start to lift and at some elevation that rain is going to transition to snow. Understanding and forecasting the elevation that rain turns to snow is extremely difficult. And it can really change the amount of water that is being produced as runoff. In some storms, maybe 50% of the entire basin is contributing to runoff because of where the snow level is. But in other times, like in the 1997 event, you now have 100% of the basin actually contributing to runoff, and the more problematic floods have happened when we get a warmer atmospheric river just after a cold and low elevation snow event — just like something we just had — where there’s snow down to 2,000 or 3000 feet. When you take that shallow snow, and then you have rain come over it, it melts really quickly. Even if you only have three inches of snow at that lower elevation, the rain plus that three inches integrated over an entire area really increases the amount of runoff that’s available. My work is about trying to understand those vulnerabilities and when the situational awareness should be increased.

Another example was in 2017, when the Sierras had a rain-on-snow event in January that primed the snowpack and the soils, and then in February we had another atmospheric river rain-on-snow event, and that caused quite a bit of flooding. So, I’m trying to understand the evolution of how the first rain-on-snow event might impact the soils and the snowpack to kind of prime the system.

We can monitor these systems to understand if we have the capacity to take on some of that rain-on-snow, or if there are things like low elevation snow or prior rain-on-snow events that should really be alerting water managers. That way they can prepare by routing water to give it the most beneficial use up in the mountains or make sure they’re releasing some from reservoirs well ahead of the event so that there is the capacity to take on floods. In the worst-case scenarios, identifying the vulnerabilities early on can help inform emergency managers to decide if, or when, to start sending out sandbags and prepping for potentially failed levee systems well in advance of the impact.

Flooding in downtown Reno after a 1997 rain-on-snow event. 

Credit: Nevadafloods.org

DRI: So, one thing that water managers can do, if they have enough warning that a rain-on-snow event might be imminent, is to release water from the reservoirs to make room for the flooding event?

Heggli: Yeah, a lot of that’s controlled by the Army Corps of Engineers, and there are strict rules for operating reservoir levels during flood season. But for hydropower companies that operate very complex networks up in the mountains, they can move water between their reservoirs. Depending on the capacity of one reservoir adjacent to another, they might be able to move water between them to keep it up in the mountains. That way, we have access to it in the summertime when it’s most needed. That’s why giving them information to prepare for storms can hopefully help us save water for the most beneficial use, so they don’t have to rely solely on releasing water downstream. Of course, that’s if there is the capacity in those reservoirs to actually take on a little bit more.

DRI: How common are rain-on-snow events in California and Nevada?

Heggli: They’re relatively common. One rain-on-snow event a year is pretty common, and we have years where we don’t get any. But we’ve also had years where we get as many as five rain-on-snow events. Everything in the West is highly variable but it’s definitely not uncommon and this is by no means something new – there are photos from 1955 of downtown Reno being flooded very similarly to 1997. So, it’s something that has always been a problem in this region.

What is new is that we are now confronting a changing climate where snow levels are rising, and it’s projected that more precipitation is going to be falling as rain than snow. This means we’re kind of approaching this period of peak rain-on-snow events while the atmosphere is warming, because more rain than snow is falling but we are still getting snow for the rain to fall on. So, it’s something that we very much need to be paying attention to and it’s going to continue to be — I don’t want to say a problem, it can cause problems — but it is definitely something that we need to make sure we’re adapting to and informing our emergency managers and water managers about so they can make the best decisions with our resources and infrastructure available.

Above: Two photos contrasting the Carson River’s flow before and after a December 2022 rain-on-snow event.

DRI: There’s been some chatter amongst meteorologists right now about the possibility for rain-on-snow events later this week (around March 10, 2023). What are your thoughts about the likelihood of this event right now and where do you see it having an impact and at what scale?

Heggli: We have seen signals for a potential warm atmospheric river (AR) and over the last couple of days the models have been converging in agreement that a lower magnitude AR is approaching. The exact location of landfall and the freezing level in the atmosphere is still uncertain at this point. However, even a weak yet warm atmospheric river, combined with all the low elevation snow, could still be very impactful. The low elevation snow, high soil moisture content, and higher river levels, which we currently have, tick the ingredients boxes for increased potential impacts from a warmer atmospheric river. It’s something to keep an eye on, but the predictions don’t yet show something huge like 1997 — there is something coming but it’s still quite uncertain how it will evolve.

DRI: Is there anything else you think is important?

Heggli: I think it’s important to communicate that the snowpack isn’t a sponge — I think there’s a really common misconception that, “Oh, rainwater moves in this uniform wetting front and just slowly makes its way down.” That’s not at all what happens during rain-on-snow events, especially higher intensity ones. The rain will hit the surface and then it looks for the path of least resistance. It uses capillary attraction to find ways to work through the snowpack, and it’ll form what they call flow fingers, or preferential flow paths. It’s like the way that you see icicles line up, the water drips in specific places. It’s something similar to that where water will find the path of least resistance and warm the snow just enough there to make its way through, which makes it easier for all the other rainwater to follow. So, it doesn’t actually need to warm the snowpack evenly to be able to progress. It’ll find these little paths, and it just basically punches its way through the snowpack.

Part of the concern with rain-on-snow events is that we have higher runoff efficiencies because the rain can punch through the snowpack and make its way to the soil and then run off. And if there’s so much rain that’s coming through that the soils can’t take it on, then that rainwater actually starts to move through the base of the snowpack. I posted a photo from December 30 when I was up at the Central Sierra snow lab during the rain-on-snow event, digging a snow pit in the rain. When I got there, there was nine centimeters of standing water at the base of the snowpack. By the time I left there was 13 centimeters of standing water at the base. So, it just really shows that the water is not able to move through the soil anymore and enter the streams that way — it’s now making its way over the surface. And that is something that can really cause a lot of flooding, because it just moves so much quicker.

Unfortunately, a lot of the work on this seems to have been forgotten and isn’t well integrated into our forecasting models. A lot of the existing models cannot handle preferential flow paths or lateral flow through the snowpack. I think this is because people aren’t out in the field making observations as much anymore, they’re relying heavily on computer simulations. These are helpful, but they also tend to remove outlier events, and in the Sierra Nevada those outliers are the events that impact us the most. You know, none of the work that I do matters until it actually matters, and then it matters a lot. We can have years where what I do is of no use to anybody. But years like this is when we really need additional information because there’s nowhere else to get information — we can’t get satellite data because of cloud cover. So, all that we have to understand what’s going on in the mountains are observational networks. That’s part of the reason that I thought, “we’ve got to use this data.”

Heggli’s experiment using instant coffee to track the flow of water through the snowpack. 

Credit: Anne Heggli/DRI.

DRI: So, you’re going out in these rain-on-snow events and digging down to the bottom of the snowpack to see what’s happening?

Heggli: Yeah. You can see from the snow surface the development of the preferential flow paths. To try to better understand these flow paths, sometimes I take instant coffee and put it in a spray bottle and spray the snowpack, and then go and dig it out. I can do different quantities of spray and then let it sit overnight and see how far those preferential flow paths progress — that way I can see the contrast of the coffee against the snow. I do that to try to better understand and observe and document what is really happening with the rain-on-snow and hope that some of these visuals help get the idea across that snow isn’t a sponge, and this is why rain on snow events are so difficult, but also interesting.

For more information on Anne and her research, watch this video from her presentation at DRI’s public science seminar series, Science Distilled. 

Mary Cablk: Celebrating a Career in Canine Detection, Biology, and Remote Sensing

Mary Cablk, Ph.D., recently retired from DRI after 23 years. Her journey into science began with remote sensing, and she later pioneered new fields of scientific research by integrating her experience as a canine search and rescue handler and trainer. In addition to her role as an Associate Research Professor in DEES, she served as Graduate and Adjunct Faculty at the University of Nevada, Reno, where she was instrumental to the creation of a Ph.D. program in forensic anthropology.

Among her many career accomplishments, she was the first to use detection dogs to track and locate threatened desert tortoises, as well as the first to establish that dogs can locate human teeth for recovery and identification purposes. She serves on the American Academy of Forensic Sciences Consensus Body and Standards Board, is a court recognized expert on the science of detection dogs, and is an auxiliary deputy with several County Sheriff’s offices in Nevada.

Cablk shared some of her career highlights, her plans for a busy retirement, and her perspective on how the scientific landscape has changed over the years.

Cablk, who recently retired from DRI after 23 years.

Credit: Mary Cablk/DRI.

DRI: What first brought you to DRI?

Cablk: I met a now retired faculty member, Dr. David Moat, while doing my Ph.D. at Oregon State. He was on loan from DRI and was stationed at the EPA lab in Corvallis, Oregon, at the National Health and Environmental Effects Research Laboratory. He invited me to work on a D.O.D funded project in the California Mojave Desert, so I competed for, and was awarded, a National Research Council Postdoctoral Fellow position, two years in a row. When Moat returned to DRI in Reno during the project I followed to finish out that postdoc, and that’s how I ended up here.

DRI: How did your interest in scientific research begin?

Cablk: I was exposed to satellite imagery and image processing when I was in graduate school at Duke University. I took a course in remote sensing – this was back when times were very different than they are now. We didn’t have smartphones, and we certainly didn’t have imagery on anything handheld. I thought satellite imagery was beautiful.

Art is in my genes — my grandmother was a biological illustrator. If I didn’t go into science, I was going to go into art. I thought the imagery of earth was beautiful, and then it turned out to also be data, so I got sucked into it. Everything about it appealed to me – what you could see from afar – there’s a lot of art in science, if you know how to look for it.

DRI: How did you transition into doing a lot of work with dogs?

Cablk: That started early in my career, around 2001. Right around the time when I was finishing my postdoc here, and I was a new faculty member. A Government Accounting Office report came out examining how much money had been spent on desert tortoise research, which was a lot, and what they had received in return for all that money. It wasn’t much – we weren’t getting any closer to delisting the species or reversing the downward trend. 

At that time, I had started doing search and rescue myself with my own dog. I started to see what dogs could do searching for missing people, and I thought, “Wow, this is incredible. I wonder if dogs can find tortoises.” That was really the launchpad for what became a career studying canine detection. It didn’t come easy – I was told initially by a lot of people, “that’s the dumbest thing I’ve ever heard.” Now, of course, wildlife conservation detection is huge. But back then I was one of the first to pioneer interfacing dogs with actual animals, and not just scat. We had some success, and then things snowballed and progressed. Before I knew it, I was 10 years in and a few million dollars into the research. 

I would draw from the search and rescue community to hire dogs and handlers for my Desert Tortoise K9 program, because at that time there weren’t many professional handlers like there are now. Conservation canine work is commonplace now, but back then, we were pioneering everything. It was fun – a lot of time spent in the desert, and I spent months and months living outside of military installations. That was a big part of my career.

A tortoise detection dog-in-training performs his trained alert, the ‘sit,’ near a tortoise.

Credit: Photo from Cablk et al., 2008, “Olfaction-based Detection Distance: A Quantitative Analysis of How Far Away Dogs Recognize Tortoise Odor and Follow It to Source.”

DRI: I’d love to hear more about your search and rescue work and how you got started with that.

Cablk: I got into it very early on when I was a postdoc. I had someone close to me who needed rescue in Zion National Park, and search and rescue in Zion saved his life. There is some percentage of people who get into Search and Rescue because they have a first-hand experience, or someone close to them needs rescue or recovery. I’m one of them, and it just dovetailed with my wanting to work with dogs. I’d always had dogs, my degree was in biology, and I have a lot of background in animal behavior. I was never a laboratory person.

Search and rescue really opened my eyes to possibilities for research because back then this was all new. Nowadays, we’re in a super exciting time with research into canines, canine behavior and cognition. But back then, it was literally a desert of knowledge and science. So, I just integrated what I was learning from my research into how I was training dogs in search and rescue, and then taking things that we saw on deployments and in training, and turning that around and asking questions to see if we could address those scientifically. So, I’m a little bit unusual – maybe not unusual for DRI, but certainly for a lot of people’s careers – where there’s this integration between what I do professionally and what I do in my free time. It’s been a really fun way to have a career, looking back on it.

DRI: You’re very involved in the local search and rescue groups, right?

Cablk: Yes, very much. When you run dogs for search and rescue, you either do it for a little bit, and then you get out of it quickly, or you’re in it for life – I fall into the latter category.

I’m an auxiliary deputy with the sheriff’s office here in Washoe County, the Carson City Sheriff’s Office, Douglas County Sheriff’s Office, Lyon County Sheriff’s Office, and the Humboldt County Sheriff’s Office. Over in the state of California I’m integrated with their Office of Emergency Services with the Governor’s office there.

Search and Rescue requires a huge amount of time – very few people have the time and the means to be able to do it. I feel very fortunate that I had the wherewithal and the ability to land here at DRI where I could pursue whatever research interests I wanted as long as I could secure funding. We have complete flexibility to be able to integrate something like search and rescue with science. It’s really unique here.

Cablk with her dog, Dax, at a search and rescue training course.

Credit: Mary Cablk/DRI.

DRI: Can you talk about some of your research projects?

Cablk: Well, after I learned about how difficult it was for forensic anthropologists to find teeth (which is important for body identification) I thought “You know, if dogs can find desert tortoises the size of a half dollar in hundreds of acres of desert, I bet they could find teeth.” And I saw a call for proposals that I think the Department of Justice had put out to develop more sophisticated methods to locate teeth. So, I called the program manager to get a little more information and said, “Hey, here’s my idea. I think we should look at running dogs to find teeth.” He said that was the most ridiculous idea he had ever heard. So, I hung up the phone and said to myself, “That’s fine. I’ll find another source of funding and publish the results anyway.” And that’s exactly what I did.

I published the study in the Journal of Forensic Sciences. And I was told that one year the findings were included in the American Academy of Forensic Sciences diplomate exam, which is a big deal. It was groundbreaking research at the time.

DRI: How have things changed since you first started your career?

Cablk: They have changed so much. Probably the biggest part is the development of technology. When I first started working with satellite imagery, we didn’t have the spatial resolution that we have now. I was computer line coding to do my analysis, and now people do analyses on their phones. Cellphone technology had just become smaller than a handheld brick when I finished my PhD in 1997. When we would go out in the field, we didn’t have communications with anybody. And you know, you just did what you had to do to get your research done. We were very creative. And it was fun – it was really fun.

I think for my generation of field scientists who would go out, we would dive in headfirst and get our hands dirty – that’s the fun part. Now, there’s a lot more oversight. And then of course, now we’re in constant communication.

But we also didn’t have the education-communication side of it, to tell the world about what we were doing. That wasn’t really a thing, for lack of a better term. We would communicate within our own discipline, peer to peer and colleague to colleague, but it was difficult to explain to the public what we were doing. I have a million stories about the personal interest side of science and fieldwork, but in my generation, we were never taught how to share those stories. It was not something that was appreciated. I’m proud of the work that I did, and I’d love to share the human side of it. Like the first time the dogs found tortoise hatchlings, which are the size of silver dollars. That ability wasn’t on our radar screen, and we just sat there and watched it happen. It was like watching Neil Armstrong step on the moon – we had no idea that what the dogs were doing was even possible. I wish that we’d had an opportunity and the means to communicate that pivotal finding. Now, I see that shift in DRI and in the scientific community as a whole, towards communicating our science to the public, but back then, it was a whole different environment.

DRI: How has working at DRI impacted your scientific research and network?

Cablk: Well, I think it’s the other way around. I mean, we’re the ones that are doing the research. And we can do it anywhere. I don’t see that DRI has necessarily impacted my work, but I think that DRI has created a tremendous opportunity, and the right framework to allow professional development and growth.

Cablk with her dog, Dax.

Credit: Mary Cablk/DRI.

DRI: What advice do you have for young scientists?

Cablk: The world is so different now. Nowadays, we don’t have the hard lines between disciplines that we did before. I see the world now as an endless sea of opportunity. The one piece of advice that I’ve always given, is when you’re dealing with data analysis software, you need to learn the math behind it, and not just which buttons to push.

Go for it, have fun with it. Life at DRI is incredibly stressful. Now, on the other side looking back, I can’t imagine doing anything else. But it’s a double-edged sword. You have to have the stomach for it, especially as a woman. I do believe that challenges still exist for women, even though we’re in a different society than we were even a decade ago. I don’t know that there’s anything anybody can do externally to help women scientists find their voice and their confidence. I wish I could, because I wish I’d had a mentor like that when I was first starting out. When I showed up here, it was a sink or swim environment. But if you have the brains, and you have the passion, and the drive, and the dedication and motivation – young scientists can do anything nowadays. And they should.

DRI: What are your plans for retirement?

Cablk: Oh, I love retirement! I’m still working.  Every day is different and interesting. I am in a teaching role for the state of California Governor’s Office of Emergency Services. I teach search and rescue,  having almost 25 years of experience and training under my belt. We do week-long courses for what’s called “Winter Search Management.” We go down to Mammoth Lakes or Mount Shasta or Sequoia Kings Canyon, and teach law enforcement everything about winter searching: avalanche conditions, medical, equipment, you name it. We spend five days and at the end, they end up sleeping in a snow cave that they dug themselves.

I’m also working with Chico State forensic anthropologists and the state of California Office of Emergency Services to develop the canine portion of a new class called “search methods and identification in a burned environment.” So, when we have these massive, fatal fires that are tragic and have become an annual occurrence, we use the dogs to help locate missing people.

And of course, I’m still deploying dogs. I have the freedom and flexibility to deploy on searches and I’m still very active with the American Academy of Forensic Sciences. I still sit on their standards board and we’re working on developing national level standards. I am often invited to speak at professional conferences and meetings, for example I’ll be talking about water recovery canines with the International Water Rescue Professionals Association, MENSA, things like that. I’m still active and engaged with the canine community, and there’s certainly a scientific aspect to my involvement. Someday maybe I’ll end up on a beach, like some of my colleagues who are also retired, but I’m still pretty young and have more professional interests to pursue.

Cablk doing recovery work with her dog, Dax, at a burn site in California.

Credit: Mary Cablk/DRI.

DRI: Will you continue doing some work at DRI? 

Cablk: I’ll seek emeritus status, and then become an hourly to be able to take advantage of opportunities that might come through DRI. We have phenomenal scientists here. And I really loved working at DRI. I’m not saying it wasn’t stressful, and I’m not saying it wasn’t hard — but what a great career.