Remote Sensing & Lagoons: How to measure inundation with aerial images

Hey geo-loves! Hope everyone was able to enjoy the weekend despite the political events that recently transpired. Going to the Women’s March in Boston last week was a really inspiring way to see people come together.

One of my photos from Instagram of the protest last weekend.

So, I’d like to talk a little bit more about my lagoons (they feel like my lagoons because I look at them so much!) and what I’m doing with them. Right now, I’m developing remote sensing for my lagoons in Salar de Atacama in Chile. What is remote sensing? It’s when you take satellite data (which in my case are images) and manipulate that data to get information.

In my case, I’ve been taking images of my lagoons and outlining the extent of their surface area in ArcGIS. By outlining the lagoons, I’m able to measure how the surface water expands and retreats through time. I am specifically focusing on a time period after a large precipitation event that occurred in March of 2015 to see how the lagoons responded to precipitation-driven recharge. Here’s a “before and after” shot of the lagoons on March 17, 2015 versus May 20, 2015.

Can you see the difference?

There’s a pretty noticeable difference, isn’t there? To measure the surface area of the lagoons, I draw polygons around the surface water extent and then use ArcGIS to measure their surface area.

I’m really excited for some new maps with even better resolution that will come out on the Landsat Imagery website later in February. I’m also excited to take this remote sensing a step further and measure the coloration of the pixels in each image to get even more accurate surface area measurements.

So what do we see in these lagoons’ responses to the March 2015 storm? First, we see that not all lagoons react uniformly. Those differences in response may indicate variations in topography, in discharge, or in recharge based on each locale’s stratigraphy. Second, we notice that the most recent lagoon changes may be a part of an ongoing decline in the over all extent of lagoons throughout the salar.

Why are these variations in lagoon surface area occurring? That’s for me to explore further in my thesis, and I’ll make sure to keep you guys updated every step of the way. In the mean time, feel free to comment on this post or e-mail me with questions or thoughts! Have a good weekend!

¿Qué pasó con las Lagunas?

Hope you all had a lovely holiday season so far! I’m in Kentucky visiting family, and I’ve been really enjoying just relaxing. Anyway, since I’ve finished a section of my prospectus, I figured I would share a bit more on my research focus for all of you who are interested.

Photo taken by my advisor of a transitional pool looking southeast towards the Andes.

Lately, I’ve been really fascinated with the lagoons that are located in Salar de Atacama (SdA). Here’s a quick refresher: SdA is a basin in the Atacama Desert in northern Chile, which is the driest nonpolar desert in the world and is therefore a great place to study groundwater dynamics in arid regions. SdA is also the home of the densest naturally occurring brine, which is water that has a lot of dissolved halite (i.e. salt) and other compounds that make it denser than fresh water. My general interest is defining the factors (like evaporation, dissolution, and changes in the hydraulic gradient) that drive groundwater flow in brine-rich and arid environments which, as I’ve mentioned in a previous post, are unique from mechanisms seen more temperate climates. 

Eastern view of Lagunas Miscanti and Miniques, looking towards the east at the Andes. Photo courtesy of my advisor!

I think that the lagoons are the key to studying those factors further. Why? Because the lagoons are located along the boundaries of SdA’s surrounding mountains and the basin’s halite nucleus, which is basically a giant chunk of salt that has accumulated in the valley floor of the basin from tens of thousands of years of evaporation. The lagoons are also located along the transition zone between the relatively fresh groundwater and the brine. These lagoons are only slightly briny, whereas the groundwater under the halite nucleus is incredibly briny (in fact, it’s likely the heaviest brine naturally found anywhere in the world). This means that the lagoons are likely being recharged from relatively fresh water coming from the uphill Altiplano region in the Andes. So, these lagoons and the area around them are a great place to study the processes by which freshwater turns into such heavy brine.

Flamingos depend on the algae and the crustaceans that live in the lagoons. Photo courtesy of my advisor.

So how can I study the lagoons and the areas around the lagoons to figure out how this brine develops? One good way is to delineate the extent of the brine and to figure out where the groundwater becomes so concentrated with dissolved sodium and lithium. A lot of work has already defined the lateral extent of brine, but the vertical extent of the brine is still poorly defined. There are also a lot of insightful techniques for tracing groundwater flow by studying changes in temperature, isotope ratios, and dissolved lithium and sodium. I’ll make sure to explain each tracer in more detail later on.

So, based on what we know, it looks like the lagoons are responsible for generating some of the densest brines on earth. Why? The extremely high evaporation rates extract water out of the lagoons and leave behind the dissolved compounds like sodium and lithium to create the denser brine, which eventually sinks down and into the rest of the brine that underlies the halite nucleus. The lagoons are likely the only place for this process to occur because the surface of the halite nucleus acts like a barrier against evaporation with almost no porosity and a very high albedo.

Here’s a little peak into my progress! Let me know if you have any questions, and Happy New Year!

Not all groundwater flows downhill

Happy Sunday! To start off the new week, I have some delicious tidbits on my research for you. This past Monday, I went with my advisor and another grad student to Worcester Polytechnic Institute for the NSF-sponsored Water Workshop. I presented a poster on my work, so I figured I’d talk about the details of my research focus a bit more.

Standing next to my poster for the Water Workshop at WPI!

Everyone agrees that water flows downhill. But does groundwater always flow downhill? That is, does groundwater flow always follow the topography? Not necessarily. Many people before me have proved that the groundwater table does not reflect the topography for a lot of aquifers. This depends on a lot of things, including recharge, depth of the groundwater table, the height of the aquifer, and the extent of the watershed. Strangely enough, not a single one of these factors dominate whether groundwater in an unconfined aquifer flows contrary to topography. They rather work together at different intervals to create this counter-topography behavior. And someone (i.e. me?) could spend a whole career investigating how all those factors affect one another to produce this affect.

It turns out that this behavior, which we call recharge-controlled flow, happens in a lot of places around the world, including parts of Massachusetts. More commonly, you see recharge-controlled flow in arid regions like the southwestern United States and my current study area, the Atacama!

Corenthal et al. (2016)

What’s going on in the Atacama, the world’s driest nonpolar desert, is really fascinating. Figure A is a conceptual illustration that shows how the groundwater table flows under all these high peaks to reach the salar, which is a salt flat. Based on what my research team and I know, the factors controlling groundwater flow in the Atacama include recharge (or rather, lack thereof) and the depth of the groundwater table from the surface.

The lack of recharge in Salar de Atacama as the world’s driest nonpolar desert means that its groundwater needs to come from somewhere else. That somewhere else is the relatively wetter, higher elevation peaks that we call the Altiplano (i.e. “high plains” in Spanish). This difference in recharge over time creates a difference in hydrologic head that causes the groundwater to defy all the topographical peaks in the Altiplano to flow towards Salar de Atacama.

Since Atacama is so dry, this groundwater flow creates a negative water balance equation as it continues to flow from areas with little precipitation to areas with almost no precipitation at all. In other words, more water is leaving the system than coming in. Because of this imbalance, the groundwater table probably continues to lower. As a groundwater table lowers, it becomes less dependent on the topographic variations.

This behavior has a lot of interesting and concerning implications. Atacama’s groundwater, which is the area’s only source of water, is nowhere near sustainable. This point is really important for the people and businesses that depend on this water. Plus, since groundwater takes a long time to travel, the distance that the Atacama’s water has travelled means that it is incredibly old. It’s probably on the order of thousands to tens of thousands of years old!

Well, here’s a quick taste of what I’ve been focusing on this semester. I promise I’ll talk about it more soon!