Invisible waters: At the base of the river
New study looks at Colorado River groundwater in a warming world.
Good morning, and welcome to Western Water Notes.
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One of the things I like about journalism is you get to make the invisible a little more visible. In the case of my work here, that often means writing about the hidden ways water moves through our communities and ecosystems. It’s why I’m so interested in how water gets from Point A to Point B. And it’s why I focus a lot on groundwater.
There are all sorts of ways to measure, model and estimate groundwater but compared to a stream or spring, groundwater is far more invisible to us — flowing underground through complex and varying geological structures we don’t always fully understand.
I’ve started to refer to our inability to see it as groundwater blindness.
I get why we all have a little groundwater blindness: You can recreate on a river. You can listen to its peaceful sounds. You can see it right there, supporting birds, fish and plants. You can see diversions in real-time. Watching groundwater on the other hand…
But it turns out groundwater is way more visible than we might assume. It actually is part of the river. Not always. Not everywhere. Yet in a whole lot of cases, you can see groundwater because it is the river. Rather than thinking of groundwater and streams as disconnected, it’s often more accurate to think of them as intertwined — because, in a lot of cases, groundwater can arise at the surface. Depending on the geology and physics of an area, groundwater can flow into rivers, providing an important water source, especially in dry times. This portion of a given river is known as baseflow.
Things are changing though.
For one, we (humans) have punched a lot of groundwater wells into the ground. When water is pumped from a well it can intercept or “capture” water that would otherwise have flowed to a river or spring. On top of this, the climate is changing too. Warming affects the way water cycles through the environment and how groundwater moves.
Both are critical issues that Desert Research Institute hydrologist Rosemary Carroll has researched. Carroll is based in Colorado, but her research on how groundwater interacts with streams is informed by her past work in Nevada. On the Walker River. The Humboldt River. Along Cleve Creek. I caught up with Carroll last week, and we talked about a new study she led looking at how groundwater depletion could mean less water in the upper Colorado River, which sustains life across the Southwest.
The study, published in Nature Water, developed a detailed model of the East River, a tributary representative of the mountain-streams in the Colorado River’s headwaters.
What it finds is that warming poses a risk to groundwater storage and thus how water moves to streams — with potentially major implications for other Colorado River headwaters. The model also shows that groundwater contributes an important baseflow supply in the East River, and it suggests that gaining a better understanding of groundwater could help prepare water managers heading into a warmer future.
Here is how the press release from the Desert Research Institute put it:
The study showed that consistently warmer temperatures resulted in sharp declines in groundwater levels that were unable to recover to historical average levels during wet periods. Isolating the warming seasonally demonstrated the strong impact of warmer summers on water table declines, as the atmosphere increases evaporation rates, plants increase their water uptake, and soils dry out. The largest declines in water table elevations occur in the subalpine forests where conifer forests are most dense.
I talked with Carroll about how she got interested in some of these questions and the paper’s main findings. Here’s a little bit from our interview, edited and shortened:
What was your interest in doing this study in the larger context of the science that’s out there? My work in Nevada looking at groundwater and surface water interactions, primarily in agricultural settings, and understanding that those are connected. Then moving some of my research up into the headwaters of these mountain systems.
These agricultural systems tend to — in Nevada — depend on snowmelt-dominated sources and they depend on snowmelt in some regard. And so my research has taken a turn to these mountain watersheds, and there's just really very little information on groundwater in mountain basins, in particular deeper groundwater. This is an area we don’t understand very well.
Classically, [models of] mountain systems have regarded groundwater as being maybe a relatively stable, constant source. We don't really need to explicitly model it. We can just assign a baseflow and just have it be constant over time.
Broadly speaking, what do you find in this study about how important groundwater is to streamflow? What we find is that if you look historically, groundwater is a stable source of streamflow… This model says that there's about 25% of streamflow annually that is coming from groundwater. Now that percentage can go up or down based on that year’s snowpack coming into the stream.
And so that idea that ‘you can have constant baseflow and it's always the same,’ that sounds good until you start to really see compounding climatic events.
When we had an event like 2018 — a very low snowpack year, really warm conditions [and] on top of that we had really no monsoon for about five or six years — we see this big drop off in water table elevations in the model. We also see that in our data.
Over the last eight to ten years, we're seeing something shift here in real-time. What are the implications of that if we took the same precipitation [and] let's just crank up the temperatures and see what happens [in the model]. And what happens is you start to see a system that kind of starts to unravel and groundwater starts to significantly change and has an impact on streamflow.
I maybe should have started with this question to situate and better understand. One of the things I was wondering is where human activities come into this with groundwater pumping. I’m curious how that plays out when you’re thinking about the interactions that you’re talking about in the paper. Human interactions are really important. They're not in this paper, and they're not in this basin so much. This basin was in part chosen because it's relatively unimpacted by management of the water system.
So would it be accurate to say that this is a model looking at how the natural system might be changed by climate change? “Yes. There is no human-decision making in this model, and I do an exploration in this paper across mountainous watersheds. And those are defined in a very specific way. Generally speaking, they might all be flowing into a mainstem that is managed but the mountain [streams] contributing to it are less so.”
On the Colorado River and in other places in recent years, you’ve seen 90% or 80% of normal snowpack to then only see something like 30% or 40% runoff. Do you think that the baseflow issue is factoring into that? I think it’s starting to. I do. I think there’s a lot of answers to that, and I think it’s a complicated question. Papers have been out saying vegetation water use [is responsible]. So that’s a big part of the puzzle. If it’s warmer, then you’re going to have more evaporative losses out of the basin and your vegetation is going to need more. Now the timing of that warming is important…
But I’m going to say groundwater is part of the story and I suspect will become more important as we move forward throughout the century. I’m trying to say don’t ignore it.
A photo as we near summer ☀️
The Truckee River upstream of downtown Reno on Sunday.
Got to spend some peaceful and beautiful time on the river this weekend.
Until next time,
-Daniel