As climate warms, summer monsoons to produce less streamflow

As climate warms, summer monsoons to produce less streamflow

Photo caption: A monsoon rain event in the East River watershed of Colorado, a pristine, high elevation, snow-dominated headwater basin of the Colorado River. Credit: Xavier Fane.


New study holds implications for future water supply in the Colorado River Basin

 

Las Vegas, Nev. (Monday, Feb. 1, 2021) – In the summer of 2019, Desert Research Institute (DRI) scientist Rosemary Carroll, Ph.D., waited for the arrival of the North American Monsoon, which normally brings a needed dose of summer moisture to the area where she lives in Crested Butte, Colo. – but for the fourth year in a row, the rains never really came.

“2019 had just a horrendous monsoon,” Carroll said. “Just the weakest monsoon. And we’d had a few years of weak monsoons before that, which had really gotten me wondering, how important is the monsoon to late summer streamflow here in the Upper Colorado River basin? And how do monsoons influence the following year’s streamflow?”

Working in partnership with colleagues David Gochis, Ph.D., of the National Center for Atmospheric Research and Kenneth Williams, Ph.D., of Lawrence Berkeley National Laboratory, Carroll set out to explore the importance of monsoon rain in streamflow generation in a Colorado River headwater basin.

The team’s findings, which are published in a new paper in Geophysical Research Letters, point to both the importance of monsoon rains in maintaining the Upper Colorado River’s water supply and the diminishing ability of monsoons to replenish summer streamflow in a warmer future with less snow accumulation.

Their study focuses on the East River watershed, a pristine, high elevation, snow-dominated headwater basin of the Colorado River and part of the Watershed Function Scientific Focus Area (SFA) program that is exploring how disturbances in mountain systems – such as floods, drought, changing snowpack and earlier snowmelt – impact the downstream delivery of water, nutrients, carbon, and metals.

Using a hydrologic model and multiple decades of climate data from the East River watershed, Carroll and her colleagues found that monsoon rains normally deliver about 18 percent of the basin’s water and produce about 10 percent of the annual streamflow, with streamflow generated primarily in the higher elevations of the basin.

“The amount of streamflow produced by monsoons, while not geographically extensive, is actually somewhat substantial,” Carroll said. “It was larger than I thought it would be. That doesn’t mean all of that water gets to a reservoir – some likely is used by riparian vegetation or irrigation, but it still does go to meet some need within the basin.”

DRI scientist Rosemary Carroll stands in the East River measuring stream discharge in Colorado.

Desert Research Institute scientist Rosemary Carroll measures stream discharge in the East River, Colorado. Credit: Kenneth H. Williams.

Next, the team explored the ability of these summer rains to produce streamflow during cool years with high snow accumulation, and during warm years with less snow accumulation. During cool years with more snow, soil moisture levels were higher going into summer, and greater streamflow was generated by the monsoon rains. During warmer years with low snowpack, dry soils absorbed much of the monsoonal rains, and less runoff made it to the streams.

“You can think of the soil zone as a sponge that needs to fill up before it can allow water to move through it,” Carroll said. “So, if it’s already depleted because you had low snowpack, the monsoon then has to fill it back up, and that decreases the amount of water you actually get in the river.”

As the climate warms, snowpack in the Rocky Mountains and other mountain systems is expected to decline, leading to reduced streamflow. Rising temperatures also lead to increased soil evaporation and increased water use by plants. According to the results of Carroll’s study, these changes will reduce the ability of water from the monsoon to make it to the river as streamflow.

“Our results indicate that as we move toward a climate that is warmer and our snowpack decreases, the ability of monsoon rain to buffer these losses in streamflow is also going to go down,” Carroll said. “So, the monsoon is not some silver bullet that is going to help mitigate those changes.”

The Colorado River is a critically important resource for people living in Southern Nevada, where it accounts for about 90 percent of the water supply. Although runoff from winter snowpack provides a much larger proportion of streamflow each year than the monsoons, the monsoonal moisture is important to both ecosystems and people in part because it arrives at a different time of year. And in a system like the Colorado River, where every drop of water is allocated, if monsoon rains do not arrive, it creates a shortage somewhere downstream.

“In terms of water resources, if monsoon rains are useful and contribute to late-season streamflow, then the loss of that water obviously has implications for the ecology of these systems,” Carroll said. “This water is really important in supporting aquatic habitat there. But it’s also really important for human use. If any amount of water that we rely on isn’t there,  then something has to give. The Upper Basin will have to consider how they are going to manage their water to meet those downstream obligations.”

Additional information:

The full text of the study, Efficiency of the Summer Monsoon in Generating Streamflow Within a Snow‐Dominated Headwater Basin of the Colorado River, is available from Geophysical Research Letters: https://doi.org/10.1029/2020GL090856

For more information on Rosemary Carroll, please visit: https://www.dri.edu/directory/rosemary-carroll/

For more information on the Watershed Function Scientific Focus Area (SFA) program, please visit: http://watershed.lbl.gov/ 

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The Desert Research Institute (DRI) is a recognized world leader in basic and applied interdisciplinary research. Committed to scientific excellence and integrity, DRI faculty, students, and staff have developed scientific knowledge and innovative technologies in research projects around the globe. Since 1959, DRI’s research has advanced scientific knowledge, supported Nevada’s diversifying economy, provided science-based educational opportunities, and informed policy makers, business leaders, and community members. With campuses in Reno and Las Vegas, DRI serves as the non-profit research arm of the Nevada System of Higher Education. For more information, please visit  www.dri.edu.

Meet Rosemary Carroll, Ph.D.

Meet Rosemary Carroll, Ph.D.

Rosemary Carroll, Ph.D., is an associate research professor in DRI’s Division of Hydrologic Sciences. She has been a member of the DRI community since 2000 when she was hired as a research hydrologist. Rosemary works remotely from Crested Butte, Colorado, where she studies mountain hydrology. She recently published a paper in Geophysical Research Letters titled, “The Importance of Interflow to Groundwater Recharge in a Snowmelt-Dominated Headwater Basin,” so we sat down to talk to her about the project and her other work at DRI.

What is your background, and what do you do at DRI?
I pursued both my Master’s and Ph.D. in hydrology at the University of Nevada, Reno. I joined DRI upon the completion of my Master’s degree in 2000 working primarily on groundwater modeling projects. In 2006, my family and I moved to Colorado while I was finishing my Ph.D. on mercury transport in the Carson River. I’ve been able to maintain projects at DRI and build new science programs because of the wonderful support of my division director as well as from DRI faculty and staff who still look out for me despite not being on site.

My current research is focused on groundwater and surface water interactions. Specifically, I create numeric models, or computer simulations, of watersheds that begin high in the mountains and are fed primarily by snowmelt, like those in the East River where I live. I am trying to understand how snow dynamics influence the amount of groundwater that feeds into mountain streams. In 2014, I began working with Lawrence Berkeley National Laboratory using the East River as their experimental watershed to quantify how mountain systems store and release water and solutes and the relationship of that process to climate. Through these efforts, I am interacting with a wide range of scientists from universities, national labs, and federal agencies as well as with water managers in the state of Colorado.

What are the challenges in studying hydrology in mountainous landscapes like the East River?
The challenges are largely associated with either lack of data or the difficulty in collecting data. Mountainous watersheds contain steep terrain and extreme weather to make access, safety and maintaining deployed sensor networks difficult. I am in charge of the East River stream network. Avalanches are a very real problem here, and some of our stream field sites require skiing 20 miles round-trip to sample in the winter. In the spring, streams are fast and cold and not safe to wade. Spring runoff can also wash away equipment, erode banks and make rivers very turbulent. All of this puts traditional techniques of observation to the test and can mean lost data. We also spend quite a bit of effort protecting equipment from animals. Beavers, moose, elk and cattle are an inevitable part of planning a sensor network in the Colorado Rocky Mountains.

It sounds like mountain hydrology involves a lot of time outdoors. How often do you go out in the field?
I am part of a larger team of field scientists and technicians, so I go out about once a week but I now largely oversee several others. The fieldwork is rigorous, and the conditions are not easy. There’s a lot of hiking and backcountry time, including skiing and snowmobiling, and there’s intense spring runoff to contend with. My next big field push will be in September and October to make sure all our equipment is winterized before snow begins to accumulate.

Rosemary Carroll

Carroll checking weather station monitoring equipment in the East River, CO.

So taking measurements directly from streams is one thing, but modeling a watershed seems an entirely different challenge. How exactly do you build a model, and what goes into it?
Essentially what you’re doing with a hydrologic model is combining data on climate—precipitation, temperature—and watershed characteristics—elevation, vegetation, soils, geology—into a single framework to solve mathematical equations that describe how water moves through the system. The model is tested against data we can collect in the field, like streamflow, solar radiation and snow accumulation.

As part of our modeling approach, we integrate LiDAR (light detection and ranging) radar imaging of snow through the NASA Jet Propulsion Laboratory Airborne Snow Observatory (ASO). ASO essentially produces a 3-D map of snow depth. We use these detailed snow maps to show how snow redistributes through forces like avalanches or wind. We see that the majority of East River snow resides in the upper subalpine, or the zone between the tree-less alpine environment and the forested subalpine. The upper subalpine is a mix of barren and low-density conifer forests.

Rosemary Carroll

Carroll measuring water content in snow of the East River, CO.

What does your hydrologic model help us understand?
What our model shows is that the upper subalpine is a very important location in the watershed for replenishing groundwater supplies, which is called recharge. Snow is redistributed to the upper subalpine, where it lasts late into the spring and summer—it then melts quickly and this generates recharge. In addition, snowmelt from steep, alpine regions in the watershed is transported via shallow soil or weathered rock to the upper subalpine where it recharges into the deeper groundwater system.

Over the last several decades, the model suggests that groundwater replenished by snowmelt in this zone has remained stable, even in low snowpack years. This could mean that the water supply coming from a watershed with a large upper subalpine area may be more resilient to climate variability than watershed with little of this zone.

At least that is what our model is suggesting. The next steps are to observe this recharge process in the field, and to see if something similar is happening in other mountain watersheds with different geology. Ultimately, we want to explore how this kind of information can be used by water managers in long-term watershed management planning in the Colorado River and other snow-dominated systems around the world.

Rosemary Carroll

Carroll’s model suggests that the upper subalpine zone—where forest gives way to the alpine zone—could be a particularly important place for replenishing groundwater supplies in mountain watersheds like East River, Colorado.

DRI faculty teach water, sanitation, hygiene, and environmental issues courses in eSwatini (Swaziland)

DRI faculty teach water, sanitation, hygiene, and environmental issues courses in eSwatini (Swaziland)

Students from DRI’s WASH Capacity Building Program learn about dry sanitation during a field trip to the University of eSwatini (Swaziland) project site at the community of Buka, eSwatini. September 2018. Credit: Braimah Apambire/DRI.


 

In August and early September 2018, several faculty members from the Desert Research Institute (DRI) found themselves far from home – teaching courses in water, sanitation, and hygiene (WASH) and environmental issues in the Kingdom of eSwatini, formerly known as Swaziland, a small country nestled along South Africa’s eastern border with Mozambique.

The courses, all focused on a set of interconnected environmental issues and public health challenges referred to by the acronym “WASH” (short for water, sanitation, and hygiene) are part of an ongoing WASH Capacity Building Program, operated by DRI’s Center for International Water and Sustainability (CIWAS). This program received a five-year funding award from humanitarian non-governmental organization World Vision earlier in 2018 and provides technical capacity training to field staff who work in the WASH sector in developing countries.

Students from DRI’s WASH Capacity Building Program on a field trip to a World Vision and eSwatini Water Services Corporation Program site in Matsanjeni, southeastern eSwatini. Students learned about management of piped water supply systems, sanitation technologies and transboundary water issues. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program on a field trip to a World Vision and eSwatini Water Services Corporation Program site in Matsanjeni, southeastern eSwatini. Students learned about management of piped water supply systems, sanitation technologies and transboundary water issues. September 2018. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program on a University of eSwatini University-led field trip to the Mbabane Wastewater Treatment site. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program on a University of eSwatini University-led field trip to the Mbabane Wastewater Treatment site. Credit: Braimah Apambire/DRI. September 2018.

“The WASH Capacity Building Program is a partnership between DRI, the University of Nevada, Reno, Drexel University, and World Vision,” explained Braimah Apambire, Director of CIWAS. “We’ve developed six courses which we teach partly online and partly face-to-face, and the students take four of those courses to complete our post-graduate certificate program. In April, we taught two courses in Ghana, and the two courses that we just taught in eSwatini were the next in the series.”

The current cohort — the third since the program’s pilot season in 2016 — consists of 30 students from 18 African countries. In eSwatini, their coursework focused on water supplies and environmental management in developing countries, and on cross-cutting issues in WASH. The classes were taught by Apambire, DRI’s Rosemary Carroll, Ph.D., and Alan Heyvaert, Ph.D., and Emmanuel Opong, Ph.D., of World Vision.

Participants in DRI’s WASH Capacity Program gathered in eSwatini during August and early September 2018 to complete courses in cross-cutting issues in water, sanitation, hygiene and environmental issues. The 2018 cohort includes 30 students from 18 countries. Credit: World Vision eSwatini Communications.

Participants in DRI’s WASH Capacity Program gathered in eSwatini during August and early September 2018 to complete courses in cross-cutting issues in water, sanitation, hygiene and environmental issues. The 2018 cohort includes 30 students from 18 countries. Sept. 2018. Credit: World Vision eSwatini Communications.

From left to right: Courses were taught by instructors Braimah Apambire, Ph.D. (DRI), Emmanuel Opong, Ph.D. (World Vision), Rosemary Carroll, Ph.D. (DRI), and Alan Heyvaert, Ph.D. (DRI). Credit: World Vision eSwatini Communications.

From left to right: Courses were taught by instructors Braimah Apambire, Ph.D. (DRI), Emmanuel Opong, Ph.D. (World Vision), Rosemary Carroll, Ph.D. (DRI), and Alan Heyvaert, Ph.D. (DRI). Sept 2018. Credit: World Vision eSwatini Communications.

The classroom time was interspersed with field trips to rural areas, dams, water and sanitation facilities, wastewater treatment plants, and more. Students got a firsthand look at some of the WASH challenges that are common in eSwatini and a chance to experience some of the region’s unique culture and countryside. CIWAS collaborators from the University of eSwatini gave guest lectures and organized field trips for the students during face-to-face teaching in the country.

“ESwatini is a mountainous country and very, very beautiful,” Apambire said. “It is a kingdom with a king who is the ruler of the country, and a traditional culture that is almost completely intact. Their government and NGOs, including World Vision, take interest in developing social programs that help people, especially the poor. But they still have rural areas that do not have water and sanitation facilities.”

Sibebe Rock, north of Mbabane, Capital of eSwatini, one of southern Africa’s most impressive geological features. Credit: Braimah Apambire/DRI.

Sibebe Rock, north of Mbabane, Capital of eSwatini, one of southern Africa’s most impressive geological features. Sept 2018. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program take a field trip to eSwatini's Buka Community. September 2018. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program take a field trip to eSwatini’s Buka Community. September 2018. Credit: Braimah Apambire/DRI.

Most notably, says Apambire, people of eSwatini are currently experiencing WASH challenges related to an ongoing drought, which neighboring South Africa is experiencing as well. DRI has had discussions with the University of eSwatini and some governmental departments about how the institute can help address their challenges.

“Because of the impact of climate change and reductions in rainfall, they are having some existing wells dry up,” Apambire said. “There needs to be more research to find out what some of the causes are and how to mitigate that. Artificial recharge is one option, and they probably also need to look for alternative sources of drinking water for those communities. That’s their biggest challenge right now.”

Students from DRI’s WASH Capacity Building Program on a University of eSwatini University-led field trip to a house in the Buka community where wastewater is used to grow vegetables. Credit: Braimah Apambire/DRI.

Students from DRI’s WASH Capacity Building Program on a University of eSwatini University-led field trip to a house in the Buka community where wastewater is used to grow vegetables. Credit: Braimah Apambire/DRI.

Five women are enrolled in the 2018 cohort of the WASH Capacity Building Program, receiving training that will help them become leaders in the WASH sector. Credit: Braimah Apambire/DRI.

Five women are enrolled in the 2018 cohort of the WASH Capacity Building Program, receiving training that will help them become leaders in the WASH sector. Sept 2018. Credit: Braimah Apambire/DRI.

For women and girls in many African nations, challenges related to WASH impact everything from their ability to go to school each day to the survival and well-being of their children and families. For this reason, Apambire is pleased to report that, for the first time, five of the students in this year’s cohort are female.

“DRI is helping to build women leaders in this sector,” Apambire said. “Women in Africa are the ones that the burden of fetching water falls on. When you are a girl and there is no water in your village, you spend a lot of time going to fetch water, sometimes a mile or two away. Then you are not able to go to school, so it affects education. Having women become trained as WASH professionals and go back to the villages really empowers them to become a part of the implementation and management of these projects.”

This fall, students in the 2018 cohort of the WASH Capacity Building Program will finish their coursework online, with instruction from Apambire, Seshadri (Shey) Rajagopal, Ph.D. of DRI, Emmanuel Opong, Ph.D., and John Akudago, Ph.D., WASH Sector Expert. The program is now accepting applications for their 2019 cohort.


Learn more:

Follow CIWAS on Twitter at @driciwas – https://twitter.com/driciwas

For more information on CIWAS, please visit: https://www.dri.edu/ciwas/

For more information on the WASH Capacity Building Program, please visit: https://www.dri.edu/ciwas/wash-capacity-building-program/