Pumping impacts to Devils River

A geoscience team from the Southwest Research Institute in San Antonio, Texas, is using computer modeling in an effort to understand how surface water and groundwater interact.


A geoscience team from the Southwest Research Institute in San Antonio, Texas, is using computer modeling in an effort to understand how surface water and groundwater interact.

Here's a look.

Well, the problem is that Texas is in a semi-arid area, particularly the western part, and there are increased demands for the water.

Limited resources.

And so it's become very important to understand the relationship between surface water, groundwater, the demands, river flows, spring flows.

And we've been working on different studies in order to look at that.

In particular, we've just finished a study on the Devils River, which is in a very challenging area because of the limited amount of precipitation and the demands that are being placed upon those water resources.

This is a map of South Central Texas.

And what we are showing are the different watersheds.

Some go off into the Colorado River, off into East Texas.

The rest of those funnel down to the Rio Grande.

And what you see is, at the center of this image is the Devils River.

You see the Lower Pecos.

And if you look at all the water that is discharged out of Amistad Reservoir, about 1/3 of it is recharged out of Val Verde County.

And most of that comes out of Devils River.

So even though the Devils River watershed looks rather small relative to some of these other watersheds, it provides 15% to 20% of all the water in the lower Rio Grande.

Green provides an interesting graphic to demonstrate the region's relationship between rock and the flow paths of groundwater.

An easy way to visualize this is, think of it as pipes in a sponge.

So, the rock is the sponge.

These preferential flow paths are pipes.

So, we've placed them along some of the major tributaries in the Devils River, as shown in this graphic.

So if you happen to put in a well that's close to one of these pipes, you're going to do very well.

If you happen to put in a well away from the pipes, you're going to be lucky to get a few gallons a minute.

And so that's what this graphic shows.

It's pipes in a sponge.

But what happens when too many wells are pumping water from the ground in the area?

The concern is whether there's going to be any impact from future water-resource decisions.

And by that I mean, there are some thoughts of maybe putting in some well fields in order to intercept some of that groundwater and use it wherever the need may be.

It may be locally.

It may be at some distance, where they'd need pipelines.

The concern is, at what point does that groundwater pumping impact the river?

And that's a hard question to answer.

We've been working on it since about 2009.

And we've made some progress in the last few years, and we did that by developing a groundwater model, a computer model, and a surface-water model.

But we base that on a lot of underground work.

We've looked at the chemistry of the water.

That tells you a lot about where the groundwater goes, how it flows.

We've done sub-surface imaging using geophysics, where we try to understand what the rock looks like, particularly near the rivers.

We've looked at the hydraulics, which is the capacity of wells in the area.

Green and other scientists at Southwest Research Institute combined the data from surface water and groundwater to create a new numerical model that can analyze and predict water availability in the region.

This picture shows how the numerical model was assembled.

So, on the left, you see a sort of a dendritic pattern.

It looks like a leaf of a tree.

And you see all these conduits that have been placed in the model that have preferential flow, allowing faster groundwater flow.

Then, on the right, you see the rest of the model, and that includes the rock matrix.

The rock matrix is what we call the sponge.

These conduits are what we call the pipes.

And when we couple those together, and groundwater and surface water, we have a rather intricate coupled model that takes into account both surface water and groundwater.

Now we understand a fair bit better how these two watercourses, surface water and groundwater, interact out in this area, and we have, I think, developed a good tool in order to help manage the resources as we go forward.