Come Rain or Shine

Precipitation Intensity: Research, Challenges, and Opportunities

October 05, 2022 USDA Southwest Climate Hub & DOI Southwest Climate Adaptation Science Center Season 3 Episode 10
Come Rain or Shine
Precipitation Intensity: Research, Challenges, and Opportunities
Show Notes Transcript

Last month we heard from two experts on drought and aridity; this month we’re covering the other end of the spectrum and focusing on precipitation intensity and heavy downpours in the Southwestern US. Two hydrologic modelers, Dr. Dave Goodrich and Dr. Eleonora Demaria, discuss their research on intense precipitation events, as well as some of the implications of observed trends, associated challenges, and perhaps surprising opportunities. Image credit: Hans, Pixabay.

Publications mentioned during interview:

Demaria, E.M., Hazenberg, P., Scott, R.L., Meles, M.B., Nichols, M. and Goodrich, D., 2019. Intensification of the North American Monsoon rainfall as observed from a long‐term high‐density gauge network. Geophysical Research Letters, 46(12), pp.6839-6847.

Zhang, F., Biederman, J.A., Dannenberg, M.P., Yan, D., Reed, S.C. and Smith, W.K., 2021. Five decades of observed daily precipitation reveal longer and more variable drought events across much of the western United States. Geophysical Research Letters, 48(7), p.e2020GL092293.

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[00:00:00] Emile Elias: Welcome to Come Rain or Shine, podcast of the USDA Southwest Climate Hub 

[00:00:05] Sarah LeRoy: and the USGS Southwest Climate Adaptation Science Center, or Southwest CASC. I'm Sarah LeRoy, Science Applications and Communications Coordinator for the Southwest CASC. 

[00:00:17] Emile Elias: And I'm Emile Elias, Director of the Southwest Climate Hub. Here we highlight stories to share the most recent advances in climate science, weather, and climate adaptation and innovative practices to support resilient landscapes and communities.

[00:00:33] Sarah LeRoy: We believe that sharing some of the most innovative forward thinking and creative climate science and adaptation will strengthen our collective ability to respond to even the most challenging impacts of climate change in one of the hottest and driest regions of the world.

[00:00:55] The contents of this podcast are for informational purposes only, and should not be interpreted as endorsement for any of the products, technologies, or strategies discussed.

[00:01:06] Emile Elias: Last month we heard from two experts on drought and aridity. This month we're covering the other end of the spectrum and focusing on precipitation intensity and heavy downpours in the Southwest.

[00:01:17] This year, across the US and in some other parts of the world, we've seen what can happen when those often much needed rainstorms dropped too much, too fast. From soil erosion to infrastructure damage and even tragic loss of life, we've seen the power of water in the damage that flash flooding can cause. 

[00:01:39] Today we are with two hydrologic modelers, Dr. Dave Goodrich, a research hydrologic engineer with the US Department of Agriculture -  Agricultural Research Service in Tucson, Arizona. Dr. Eleonora Demaria is a hydrologist for Pima County Regional Flood Control District and has also worked with Dr. Goodrich on precipitation research at the Walnut Gulch Experimental Watershed. Dave and Eleonora, welcome.

[00:02:07] Thanks for joining us. 

[00:02:09] Eleonora Demaria: Thank you. My pleasure.

[00:02:11] Emile Elias: I'd like to dive right in and ask you about the state of the science now in 2022. What would you say is our current collective understanding of changes in extreme rainfall? And in our region, how does that relate to the North American monsoon? And let's go ahead and start with you, Eleonora. 

[00:02:31] Eleonora Demaria: Yeah. Well, I would like to say that in the last probably 10 years, the understanding of how precipitation behaves at the sub daily time scale, less than 24 hours, has increased tremendously. Part of that increase in knowledge and understanding came from an international effort, which is called Intense, which is led by the World Climate Research Program, and what they did was to collect sub daily rainfall precipitation across the whole planet. So that allowed researchers from different countries and different institutions to start analyzing sub daily rainfall. And in the US that was done by USDA-ARS, the Agriculture Research Service, through efforts like the LTAR, we will talk about that later.

[00:03:23] At the same time, the computational power has increased. So now we can, or researchers can, run models at the very fine spatial scale, and they do that with regional climate models as the models that are used for the weather forecast, for example. So these models, what they do is they allowed us to represent the convective processes that are so crucial for rainfall initiation and development.

[00:03:54] In particular in the Southwest, where we have monsoon processes that which are convective processes. The parameters that can be used in those regional climate models allow to as close as possible to nature, represent how the cloud formation and the rainfall processes really happen. And the scale is really, really small.

[00:04:16] Just to give an idea is 2.5 miles, four kilometers, and it can be done at an hour, one hour time resolution or less. There is still expenses. There still takes a lot of time and memory to run those models, but scientists have been able to simulate processes with models and at the same time we have been able to validate those simulations with the observations that have been collected.

[00:04:46] So that is allowing us, the scientific community, to understand what's happening and what could potentially happen with rainfall intensities at small temporal scales, scales that are shorter than one day. A colleague of ours, Dr. Chris Castro at the University of Arizona, he has done a lot of research using regional climate models to simulate the monsoon.

[00:05:15] The monsoon storms in the area and he's finding that these very high resolution models can represent quite accurately the processes that are needed for the development of these convective, summer convective storms. 

[00:05:31] Dave Goodrich: Well, Ellie, I know the models are getting better in terms of computational time steps and spatial, but don't they still have a hard time predicting when moisture from either Gulf of California or Gulf of Mexico gets injected or comes north to be triggered by the heating. You know, have they got that one sorted out? 

[00:05:55] Eleonora Demaria: Yeah. . Sometimes. Two things. One is the weather forecast, and they have a hard time with that. But when they go back, for example, what Chris Castro does at the University that he goes and simulates particular storms, they can do that much better.

[00:06:14] But yeah, no, the uncertainty is still really big. So it depends. And one of the things that those regional climate models do is they, they can represent the topography better. And you need topography for, for monsoon, for storms think that they're doing better, but they, they're not perfect. And, and another thing is what is good for a hydro meteorologist is not good for a hydrologist.

[00:06:40] Because the model can say it’s raining over the, the mountains, but for a hydrologist, if it rains in the east side of the mountain is one story, and if it rains on the west side, it's another story. It's very important where the storm is. It doesn't matter for an atmospheric scientist, that's not, it's not what they're looking at.

[00:07:03] They say, Oh, it's raining on the mountains, good enough. So when we, when hydrologists want to use that information for stream flow forecast or for warning systems, it might not be good enough, although, because the location is not right. 

[00:07:20] Emile Elias: Thank you. Great. That's a great overview of where we are now. Dave, anything you'd like to add?

[00:07:26] Dave Goodrich: No, I think Ellie covered it well. 

[00:07:29] Emile Elias: Great. Well, you both published an article a few years ago called "Intensification of the North American Monsoon Rainfall as observed from a long term high density gauge network." And your results tell a different story as compared with previous model simulations and analysis of shorter records of observation.

[00:07:52] So can you tell us about that research and what you studied and what you found? Dave we'll start with you. 

[00:08:00] Dave Goodrich: Yeah. Let me provide a little bit of background on this. ARS in the fifties actually started to build larger scale experimental watersheds, sort of in the 50 to 150 square miles of drainage. And they found that they had, they had collected a bunch of data during the drought or after the thirties drought on small field scales.

[00:08:27] And they realized they couldn't transfer those results to larger scales. So in the fifties there was a, actually a Senate bill passed to create these watershed centers. And Walnut Gulch, which is what we'll be talking about today, was selected as one of these watersheds. So this is a 60 square mile watershed.

[00:08:52] It's centered, or Tombstone is roughly in the center of the watershed, and currently we have almost a hundred rain gauges at the watershed. But for the analysis, we used a smaller number because that number had the longest length of continuous records and, you know, why do you need so many gauges?

[00:09:15] Well, when they first put the watershed in, they put in twenty rain gauges, and they ended up recording runoff, but no rainfall. So they were missing the rain entirely. So, you know, as many of you know, these monsoon thunderstorms can be very spatially compact and do not necessarily cover very large areas.

[00:09:42] So we got our data again from the Walnut Gulch Experimental Watershed, and looked at it at a very fine time scale. The ARS experimental watersheds are, to my knowledge, the only set or network of rain gauges that collected data at the sub hourly time scale from dating from the 1950s and 60. And, and that's important because, you know, the, generally most people agree that the major warmup occurred in the climate in the, the seventies and eighties.

[00:10:19] So, you know, we're recording fine scale, temporal resolution before and after the climatic warmup. We're still wrestling with data from the other locations. Unfortunately, they, they actually designed the experimental watershed network, so it was looking at rainfall and runoff. Those were the two key variables to collect. But they didn't specify common instrumentation.

[00:10:48] And one of the main reasons was after in the thirties, forties and fifties, NRCS or at Soil conservation Service spent lots and lots of money putting in conservation structures. And so one of the key things that the ARS watersheds were supposed to help answer because of western water rights, if you do these conservation structures, are you going to impact water rights of downstream users. And you know this, this came up in one of the Arizona court cases and they used data from Walnut Gulch to show that because of ephemeral channel losses there, once you get some distance downstream, you really aren't having a significant impact on downstream water use.

[00:11:36] What Ellie found in her analysis was that the average rainfall intensity change per decade is about 10% from everything from 30 minute storms to 24 hour storms. So, you know, think about that. In just two or three decades, you're increasing the intensity of events by 30, 20, 30%. You know, if we're looking out to 2050, 2080, 2001, if these trends continue, you're really talking about highly impactful intensification, even in the southwest here.

[00:12:18] And I'll, I'll pass it over to Ellie, and she's the one who conducted the majority of the work, and she can tell you more about the actual methods and data.

[00:12:27] Eleonora Demaria: Yes, thank you, David. Yes, David's totally right. The, the length of the records for the USDA ARS rain gauges or Experimental Watersheds is not found anywhere in the country.

[00:12:41] So, and that was, was what allowed us to do the analysis because before we did the this analysis, there was a lot of, in the research and the literature, there was a lot of contradictions between what the intensities of rainfall during the monsoon, what was happening with them? We used, so for our analysis, we used 58, 59 rain gauges in that really relatively small watershed, 60 square miles, and we were able to use 57 years of data started in 1960.

[00:13:15] So that that data was, rainfall data was available at at the five minute resolution. So every five minute there was an observation. And what we did was to also to include in the analysis rainfall observations from different networks. One from a network, from the NOAA rain ranges, and also a, a data set that is based on rain gauges but it's interpolated. So the data is on a regular grid. But those, the two data sets I just mentioned, were at the daily time scale and most of the analysis in most of the research on extreme rainfall events has been done using daily data because it's what's available. So what we did was using Walnut Gulch, sub daily rain gauge data. And the other two data sets, which are at the daily time set, we did a trend analysis. And what we learned was that if you look at the whole 57 years of data and look at the trends, it was very hard to find a conclusive direction, increases or decreases, in rainfall intensity.

[00:14:25] So what we did was she used an approach with what was called moving windows. So we computed those trends every 30 years of, of data and we moved the window and we noticed that in the Walnut Gulch sub daily rainfall data, we could see that around the mid seventies, the, the trends in rainfall intensities became positive.

[00:14:50] They were negative before and they became positive and they were statistically significantly positive. When we looked at the daily data, we don't see any trends in the rainfall intensities. So what we learn is, What data you use to be really crucial to, to understand changes in rainfall intensities. If we are looking at the monsoon storms, which during the monsoon in the Southwest, it rains for half an hour to maybe one hour, maximum two or three hours.

[00:15:24] That's it. So if you have a rain gauge that, that, that only records once a day, you, you will miss the, the distribution of that storm. So the other thing we learn is that, as Dave mentioned, with one rain gauge, you can miss the whole storm. For in the case of Walnut Gulch, we had 59 rain gauges, but it didn't mean that every single rain gauge was up capturing those increases in rainfall intensity.

[00:15:54] No. Some of them were, some of them were not. So, in order to understand a very complex atmospheric system like a monsoon, monsoonal storm, you need really fine temporal resolution of rainfall and also a very dense network of rain gauges trying to understand, past and future changes in rainfall intensities using daily observations may lead to wrong conclusions.

[00:16:24] And going back to the models, most of the time the climate models, if they are validated or, or the regional climate models, if they are validated with observations, they will use daily observations. So they are training the models with observed data that doesn't have the information We need to understand how processes work.

[00:16:45] What we learned is we also looked at the, with within those 57 years of data, we, we did was to divide that, that time period in two sub periods of 28 years each, and we looked at changes between the, the most recent period and the, the least recent period and we saw that yes, in the most recent 28 years, the events are, have become more frequent and those events that became more frequent are the ones that are more intense. The ones that have what we call the return period of 20 years, or more. So that indicates that extreme events have become more extreme in, in this part of the Southwest. 

[00:17:29] Sarah LeRoy: Thank you both for that very good description of your research and your findings and, and especially, you know, why you were able to observe these changes in trends when, when others weren't and, and the reason behind that with the sub daily data. Dave, I'd like to ask you this next question since you're a part of the Long Term Agro-Ecosystem Research Network, and so I'm wondering if you know of any plans to expand your research on precipitation intensity to other parts of the country?

[00:17:57] Dave Goodrich: Yes, we are. The LTAR or the Long Term Agro-Ecosystems Network really captured many of the larger ARS experimental watersheds, so they are now LTAR sites. So there's also experimental ranges, there's some biological research stations. So yes, our intent is to go back in time just like we did at Walnut Gulch to the LTAR experimental watershed locations, and do the same sort of analysis that Ellie described, and we're, we're in the middle of that right now.

[00:18:36] So it, it is important. And again, all of the gauges are recording at hourly or sub hourly levels, and again, typically the watersheds have a much denser spatial network than you would find from the average weather service or NOAA Rain gauge. 

[00:18:55] Sarah LeRoy: Great, thanks. Eleonora. So in addition to this long-term research that we've been discussing, you're also a hydrologist working for the Pima County Regional Flood Control District, as we mentioned earlier.

[00:19:08] Could you tell us a little bit about what you do there and whether this research on rainfall intensity informs your work at all with Pima County? 

[00:19:17] Eleonora Demaria: Yes. Well, I was hired three years ago to work for the county, and part of the reason I was hired was because of my, or my experience with climate change and rainfall intensities because Pima County Regional Flood Control District is quite ahead of many, many other counties in the country regarding climate change.

[00:19:40] We are getting ready. We know that rainfall intensities have been getting stronger. And that models, climate models, predict, predict or project increases in intensity. So we are, the county is, is getting ready to, to be prepared. And how, how are we doing that? Well, we are doing studies, analysis because we want, the county wants to revisit, review the, what's called design storms is what the magnitude of the rainfall storm that is used to size infrastructure. So we are in the process right now of doing that. Designing new design storm or generating new design storms. And also the other thing that is important is if, if it rains more, if it rains more intense, that potentially means that more people could be flooded.

[00:20:36] So how are we going to prepare for the future? Because one of my jobs, or the job in the district is to delineate the inundation. To create inundation maps. So if the, if the storms are more intense and it rains more, or, or, or is more flashy, that means that the, those inundation maps might in the future become wider and that means that more people could be potentially be impacted from that.

[00:21:07] We, the county is preparing for that and thinking, once we have new design storms, how are we going to regulate? How are we going to change our regulations? How are we going to change the design parameters we use for anything culverts, bridges, any type of infrastructure that the, the county designs. So going back to the question, the research I did with Dave, it was very. I would say crucial for me to, to the job I'm doing now.

[00:21:43] Although I, I'm not doing the job myself. We are hiring a consultant team to do the analysis, but I oversee the project and, and we are in the process of changing, adapting, and being ready for the future. 

[00:21:56] Emile Elias: Thanks for mentioning that it's easier to see those signals in precipitation intensity in the northeast and other parts of the country and that that the value of the work that you did is partially because and reliant on that dense rain gauge network so that you could really see that sub daily precipitation intensity is increasing and has been increasing since the seventies or so.

[00:22:19] So yeah, thanks for mentioning that and, so you looked at a long, long record, but now I wanna bring it to, to now and it seems to the most recent year, it seems like there have been a lot of very intense rainfall events this year causing flooding in some areas of the Southwest and other parts of the world.

[00:22:39] So I'm wondering, do you think this is an unusual summer or is this type of rainfall intensity something that we can and should expect to see more frequently in the future? And we'll start with Dave on this question. 

[00:22:54] Dave Goodrich: That's a really difficult question. Again, because of the things we've mentioned about the spatial variability of storms, I've, I've got a little graph up of one of our rain gauges just north of Walnut Gulch, and this was from oh, August 20th. We had a little over four inches and just over a period of two hours. So one, one of our folks down in Tombstone who services the gauges said he had never seen erosion like this before, and he's been working at our location for 40 plus years.

[00:23:32] So, you know, we've had two good monsoons, but you know, in the last 20, 22 years since the mega drought started, you know, we've, we've really had some bad years with the occasional average year. I won't say there's too many good years in there or not. Actually I think there, there's been a greater impact on winter rainfall than summer rainfall.

[00:24:03] And, you know, that of course means we don't have a greenup in the spring. Vegetation becomes drier, soil moisture is lower, and you know, that may be a contributing factor to greater wildfire situation. So, hard to say. And, and it's also difficult when you hear in the news they're reporting the monsoon totals from one rain gauge at the Tucson airport.

[00:24:32] And so, Pima County, actually the flood control district has a number of rain gauges around Tucson that you can go online and, you know, see the storm, see the totals, and you know, there are parts of town with easily three times as much rainfall than was recorded at the airport. So, you know, hard to give a definite answer on that.

[00:24:59] I don't know. Ellie, what do you think?

[00:25:01] Eleonora Demaria: Yeah, I agree with you. It depends on what rain gauge you use. The, this particular 2022 monsoon season in Tucson has been normal or above normal, so yeah, it is a tough question to answer, but the only thing I wanted to add is that the climate models predict that, or we have equations.

[00:25:27] Scientists have physical equations that relate the temperature in atmosphere with the amount of water vapor that the atmosphere can hold. It's like a sponge. So these more intense rainfall events are potentially going to, happen more frequently in the, in the future. So that's, that's what models show and we, we are seeing now doesn't mean that it will be every year.

[00:25:54] It will be every other year, it will be three times in one season, we don't know that, but there is a very high likelihood that more intense rainfall events will happen more frequently in the future. 

[00:26:08] Emile Elias: As we prepared for this conversation, we started talking about the impacts of increasing rainfall intensity on the landscape, and you mentioned some of those just a minute ago with increased erosion.

[00:26:20] We were thinking about those impacts on the economy and on people in the future in the Southwest. And so is there anything we can do to minimize the negative impacts and possibly increase some potential positive impacts? And so Eleonora, we'll start with you on this question, given your position at Pima County.

[00:26:42] Eleonora Demaria: Yes. Well, in the Southwest and in Tucson, we want water. We, we are desperate for water. So one thing that we can do is to capture the water. It's just keep in mind that in a natural system when it rains, roughly 60% of the rainfall stays in that system. When we built houses and roads, that 60% decreases to 10%.

[00:27:10] So that's what generates an increase in runoff and creates problems downstream. So one of the things that can be done is to create basins, little, little catch basins, detention basins, retention basins where you can store water that can be used to grow trees for vegetation. And that helps in, in cities in the urban core.

[00:27:35] But it only works for storms that are not large. So for relatively small rainfall events, you can do that. You can get the water from the street into the, into these basins. You, of course, you can capture the water in your house and use it for, for irrigation later. So those are positive impacts. Things we can do and things that the county's doing, thinking about it and doing at a different scale.

[00:28:07] Sometimes we have big systems when, when we have space because that's the problem. When you have a city that's already built with buildings and houses and roads and everything, there is no available land. And if there is available land, nobody wants to sell it because it's valuable. So the, the, the possibility of building some type of basin to capture the water to be used later on for any type of use it is, is, is really limited. 

[00:28:40] So the regional flood control district does that when there is a problem area, a vacant lot that gets flooded. We acquire it from the owners and we built what we call a detention basin so the water gets stored in that basin and evaporates and infiltrates. Naturally, it gives that opportunity to create a park because it rains so few times during the year.

[00:29:07] So most of the time it's an empty basin. So what we do is we design a park around it so people can enjoy it, and when it rains and it floods, the water gets stored. Now we, what we need to keep in mind as well is that as the watershed gets, gets bigger, you need a bigger rainfall event to flood that place.

[00:29:31] So a three hour event, isolated three hour monsoon storm will flood downtown Tucson, downtown Phoenix, part of a city. But we could have a very wet monsoon where everyday it rains, every single day. And we had some of those examples, this, this summer. So what happens is what Dave was saying, the basin gets saturated, the soil moisture gets saturated.

[00:29:58] So what happens, the big rivers, the big tributaries in our case is the Santa Cruz River. Gets flooded and that is there is no retention or detention basin you can build that will accommodate that amount of water. So in that case, what we do is to remove people, if possible, from the flood plains or we create protections, levies, or, or structural protections.

[00:30:28] So there are two scales here. There is the short, intense, short duration, intense scale that will affect a very localized area, and there is, there are winter storms that can last for days or tropical depressions during the fall that, those are the ones that really flood our big main tributaries, and those are the ones that are more dangerous for, for, for humans and for also economic losses.

[00:30:56] So the, what the county does is, and the city of Tucson as well, we build as much as when we have the opportunity, we have a space in the right of way, we will build basins, or we've built curve cuts that will guide the water into a basin and that will allow to, to increase the vegetation and, but those attenuating only the small events.

[00:31:24] He also mentioned negative impacts, and I think Dave made reference to that, which is erosion. In areas where there is no, not rural, no urban, rural areas, the problems with erosion due to more intense precipitation is, it is a big concern. And I was going to say that these short intense, short duration rainfall events are very important for sediment transport because I think the, the 15 minute duration rainfall intensity is what is used to model or to simulate when the erosion process starts in a watershed. 

[00:32:07] So if these really short duration storms get more intense, there is certainly is going to be more erosion in in rural areas and in areas that are not as controlled with cement, with soil cement, with culverts, with bridges, as are the cities.

[00:32:26] Dave Goodrich: Yeah. I'd like to add a little bit to that, I guess first on this, this notion of runoff and recharge.

[00:32:32] There's been, I think, very convincing evidence that in hot areas that have a thick vadose zone, so that's, you know, basically the pore space of the sediments that is not saturated with water. And basically the findings are that rainfall over an area does not reach deep recharge or reach the aquifer.

[00:33:05] The plants have adapted to taking it up and the, they're called suction terms. And, and you see this when you look at a glass of water and this, this might be getting a little too deep in the weeds. But water clings to things because it's a polar molecule. And basically the water, unless you get a lot of water going into the unsaturated zone, the water will cling to these particles. 

[00:33:37] So the system before development then is that, you know, you don't get a lot of recharge. So when Ellie talks about urbanization, you now effectively have new water, if you wanna call it that, or new manageable water that's created by the fact that you're putting in impervious area and you're compacting the soils.

[00:34:01] So what happens in that case or the findings about recharge is you only get recharge where you're concentrating a lot of water in a small area like ephemeral channels or putting a lot of water into something like a dry well. So, we've shown, again with Walnut Gulch data that you know, we, we can see and have validated with observations. Deep aquifer recharge from flows in ephemeral channels. 

[00:34:33] And just one other fact that you might find interesting, we did a study with the USGS on a small urbanizing watershed in Sierra Vista. So the upper part of the watershed was draining off of Fort Huachuca. And so that land has not been disturbed for 90 to a hundred years.

[00:34:58] So it's a very natural environment. Good natural vegetation, hasn't been grazed, any of that. So the boundary of the fort goes right through this area. And on the other side there's a small development. We were able to gauge the runoff coming off the natural watershed and off of the urbanized watershed in this subdivision, and there was a 26 fold increase in the amount of runoff, and this isn't models, this is based strictly on observations of rainfall and runoff. So 26 fold increase over 112 events I think we had in about two and a half years. So again, that's water, new water we can manage. And as Ellie said, if it's a really big flow, it takes a lot of infrastructure, concrete, whatever to contain that water. So, but the smaller and medium flows can be reduced for green infrastructure growing trees or offsetting groundwater pumping. 

[00:36:09] Typically in design for, let's say a detention pond or a development, there's a requirement usually by the flood control districts to say, you have to hold a storm that is characterized by its return period. In other words, how often statistically do you expect that storm to happen and how big a storm it is.

[00:36:36] Now we've heard a lot about, oh, there's, we're having these thousand year events. In Kentucky, in Missouri, and they are, they're very, very large events, but the whole theory of design storms in this return period doesn't account, or it assumes that there's no change in the average, long term average. And again, what we're seeing is that climate change is changing the overall statistics.

[00:37:11] And the average of these, what, what would categorize with prior records. So when you hear this idea about a design storm being a thousand year return period, you know, that's probably looking at data from 2010 or 2000 backward and looking at that data versus looking at the new data or the more recent data that really is being affected by climate change. 

[00:37:40] The county will say, you have to reduce the peak, and I'm making these numbers up of a five year, two hour storm that drops two inches of rainfall. So those are the parameters then that a developer can take and say, all right, I need to cut the peak off of this and here's how I'm gonna do it.

[00:38:04] And that's often with green infrastructure features. They're getting a lot more popular or putting in some type of detention structure that will hold that storm water and let it out slowly.

[00:38:16] Eleonora Demaria: One more thing that, yeah, is related to what Dave said. One thing that the county is considering is how to inject the, the water we are storing in these basins, how to inject it back into the aquifer.

[00:38:31] It's not that simple. You might be injecting water that it doesn't have the, the quality because has been running off on the street and it's not clean. And you might be injecting it into the aquifer and maybe that's not acceptable. But so we are evaluating, yes, what are the options? How can we get that water that was supposed to infiltrate and naturally it was supposed to infiltrate, but now it's running off the watersheds, how we can get it into the aquifer so it recharges our highly depleted aquifers, at least in Tucson.

[00:39:08] There are options, but then they're not simple. That's the thing. 

[00:39:12] Sarah LeRoy: Well, I'd be curious to hear how that moves forward, because that seems like a very logical solution if the county can kind of figure out the nuances of how that would work. Thinking about mitigation efforts a little bit more, and we sometimes talk on this show about climate spatial analogs.

[00:39:28] Or you know, looking to another place where the conditions might already exist that we here would face in the future. So are there any areas in the world or even here in the US and I know Eleanora, you mentioned, you know, the Northeast is doing a lot because they've had to deal with flooding for a while. So are there other places where it might be instructive for us to look to and learn from?

[00:39:51] And so, Dave, why don't we start with you on this question.

[00:39:54] Dave Goodrich: Well, the, the influence of the Southwest monsoon is somewhat limited to the Southwest. So again, as far as big cities potentially being impacted again, you're talking Las Cruces, Albuquerque, Phoenix, Tucson, and so, again, we've, I think we're slightly ahead of the curve again because of the extensive data collection that we've done as well as the focus by Pima County on these issues.

[00:40:25] So, you know, I would think potentially Las Cruces, for example, is pretty heavily affected by the monsoon, so it would be interesting to see, you know, how this extra water that's available from urbanization might be handled there. And, you know, to see, I don't know if the records exist to see whether, you know, we could look at intensification trends there.

[00:40:51] And I, I think it's a bit of a stretch to, you know, compare our monsoon to the Indian Ocean monsoon. They're, they're so much different in magnitude that you know, drawing conclusions about all of the issues that are going on in Delta regions, the Pakistan and Bangladesh is often has horrible flooding too.

[00:41:14] That, that would be tough to draw that conclusion. However, south of us in Mexico, typically the monsoon is stronger the further south you get and the closer you get to moisture sources. And again, some of the, the bigger cities there would also be worth looking at in terms of how they're handling these issues.

[00:41:36] Sarah LeRoy: Eleonora, do you have anything to add?

[00:41:38] Eleonora Demaria: No, not really. I cannot think of anything else. 

[00:41:43] Emile Elias: Yeah. One thing I'd like to note or just ask you about is related to this concept of urbanization leading to extra water and, and in my hydrology 101 class, I remember learning water is finite and when we're thinking about the water balance of an area and Eleonora, I think you really talked about this well in terms of smaller watersheds versus larger watersheds and how that intensity matters.

[00:42:08] But I think it's really important to think about the whole hydrologic system. So we have this potentially water available as runoff, but that water is not infiltrating into the ground, and so there are some tradeoffs that may happen because of that. And those could be for ecological flows or return flows or, or water available in the future from our aquifers.

[00:42:37] And I know that's something that you at Pima County are, are thinking about in terms of managed aquifer recharge or some type of other system. So I just, for our listeners, I just wanna note that it's important to think of the system as a whole system, and I don't know if that's something that, that you're thinking about.

[00:42:56] I'm sure it is, as you do your work.

[00:42:59] Eleonora Demaria: Oh, yes. Yes, we think about it all the time because our main goal is to keep people safe. That's the goal. But at the same time, we want our rivers to be healthy because that is, is wildlife is important for, for, for wellbeing. We want our cities to be green because of the heat island effect.

[00:43:21] So yes, we constantly think about the whole system, the whole hydrologic system. What, what's the input? How that input is changing or it will, will change in the future; how we prepare for that, if we can contain the water in basins, or if we need to start building larger culverts, larger bridges, do we need to protect people?

[00:43:46] When do we need to try to relocate people who is willing to sell their property? So it, it is a quite holistic approach. And as I said, the end goal is people's safety. 

[00:44:01] Sarah LeRoy: Thank you for that. I appreciate you both being here today and it does give me a lot of hope, thinking about all of the things you've mentioned that, you know, Pima County is doing and how they're thinking innovatively for the future and how to mitigate these impacts that we're looking at.

[00:44:19] Well, it sounds like now and in the future as well. So I just wanna ask if either of you have any final thoughts before we, before we close this episode? Dave?

[00:44:30] Dave Goodrich: Yeah, talk about the, the last question a little bit. We, as Ellie said, we're, we're trying to look at this added runoff water as a resource and not a nuisance.

[00:44:41] So, you know, typically in a subdivision, the law says that you are not to increase the flood risk for downstream people because you've built a subdivision or a big parking lot. So you know a lot of early design and structures were a lot of concrete and asphalt, and it's like detention ponds, keep the peak down, but get the water out of the system.

[00:45:04] So you know, again, we have this manageable resource, but I also wanna stress that the undeveloped situation, that water, that infiltrates is used by plants and evaporated. Provides essential ecosystem services for forage and wildlife. And again, as far as from, from an economic standpoint, a lot of grazing and, and a lot of ranching depends on good grasslands.

[00:45:32] And the, the issue about intensification, as Ellie said, you know, why do we, why do we care about sub daily? This goes back to question three a little bit. A lot of design storms for conservation practices and some for built environments require design storms that are smaller than 24 hours. And again, like Ellie said, you know, our storms, the big part of the storms are often over in an hour or two. Intensification as, as Ellie said, both the maximum 15 minute intensity in, let's say inches per hour, or the maximum 30 minute intensity in inches per hour is really the main criteria defining erosivity. So that's a measure of how much energy the rainfall has to dislodge soil by either splash, by the impact of the raindrops, and then subsequent carrying that sediment downstream.

[00:46:37] And our colleague Joel Biederman, who is also at the Tucson ARS Lab, he's looked at 337 long-term weather stations over the Western US with records from ‘76 to 2019. So he's finding, again, due to warming, there's a trend for reduced annual precipitation, but an increase in the variability of precipitation and that the mean and longest dry intervals are getting larger.

[00:47:14] So, you know, fewer events, but bigger events and Biederman has also written a paper that looks at the intervals of time between storms. So the intervals are getting longer between storms and, and this is contributing, I think, to why we're getting these downpours. Now, I'm talking a little bit outta my area here, but a lot of this moisture can be carried by the atmosphere, as Ellie said.

[00:47:45] And you know, it seems like when it does begin to rain, it's just got more to rain and that's one of the aspects in terms of intensification and its subsequent impacts on erosion. 

[00:47:59] Sarah LeRoy: Thanks. Eleonora, do you have anything you'd like to add? Final thoughts? 

[00:48:03] Eleonora Demaria: Well, I think that, as Dave was saying, there is a very high likelihood that we will see more intense rainfall events in the future.

[00:48:13] And it's really important, at least for an urban setting, a city like Tucson or Phoenix it's, really important to have rain gauges to be able to monitor the rainfall, the runoff to be able to inform people. So, and there is a lot of work going on in that area. People should be able to know if a road is going to be closed or not.

[00:48:40] And just to avoid danger, we need to start thinking about, and we are, we are thinking about how big we will have to build infrastructure in the future. And that will imply that some design parameters might. At least at the county level, and also regulations on where people could build or where they could live safely.

[00:49:04] And I was thinking about, about society as as a whole. And I know that there is a lot of division between it. Is this for real, is it happening or not? But I think also there is a lot of conversation about climate change and, and people, I, I see that people are open to use the rainfall as a resource, not as something that we need to get underground.

[00:49:29] Pipe it under the street and get it out of the way and never see it again. No, there is, people want to see trees. People want to see parks. People want to enjoy or be closer to nature. And, and I think this excess water that we are generating because of the impervious area is, is, is an opportunity to make our cities greener and, and cooler and, and more amenable for, for everybody.

[00:49:56] So, yeah, so I think it, it could be challenging. The future could be challenging, but also exciting. And I, I'm excited, so thank you. 

[00:50:05] Sarah LeRoy: Well, thank you. I think that's a perfect thought to end the episode on, thinking about opportunities for the future. So thank you both again for joining us to talk about some extreme precipitation and yeah, thank you.

[00:50:18] Dave Goodrich: Thank you. 

[00:50:19] Eleonora Demaria: Thank you. My pleasure.

[00:50:26] Emile Elias: Thanks for listening to Come Rain or Shine podcast of the USDA Southwest Climate Hub

[00:50:32] Sarah LeRoy: and the USGS Southwest CASC. If you liked this podcast, don't forget to rate or review it and subscribe for more great episodes. A special thanks to our production crew, Skye Aney and Reanna Burnett. If you want more information, have any questions for the speakers or would like to offer feedback, please reach out to us via our websites.