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Mapping Watersheds [Nature
Posted on April 4, 2018 @ 10:58:00 AM by Paul Meagher

This blog is a continuation of my recent blogs on rivers and flow patterns (see Part 1, Part 2, Part 3).

If you want to understand river flow you will need to study some watershed maps. The watershed of a stream is the collector area that feeds water to that stream. Properties of the watershed determine how that watershed responds to a rain event. The watershed response determines stream flow volume.

A digital elevation map (DEM) can be used to render the surface of the watershed landscape. Other maps can be overlayed onto this map to indicate, for example, infiltration rates and hydraulic roughness. Finally, if you overlay a grid onto these maps you can compute on a cell-by-cell basis (using slope, infiltration, runoff, roughness as parameters) the contribution of that watershed cell to the stream flow. You can add precipitation to this model to compute the response of the watershed and predict flow volumes.

This is a physics-based approach (versus the more common empirical/statistical approach) to simulating and predicting stream flow. It sounds fairly simple in theory but there are many details that need to be figured out if you ever wanted to do this in practice. A good book for learning those details is Distributed Hydrologic Modelling using GIS (3rd Ed., 2016) by Baxter E. Vieux.

A Geographic Information System or GIS is a piece of software that not alot of the general public uses. Now that I have a reason to use one, to map watersheds, I began looking around for a free opensource GIS system. From my research, the premier opensource version appears to be QGIS.

I am using a windows desktop operating system for the software so I downloaded an older stable 64-bit version (2.18.18-1-Setup-x86_64.exe) from http://download.osgeo.org/qgis/windows. This is a large download that installs alot of opensource geo software for windows so expect download and installation to take awhile. The latest version (v 3.0.x) can also be downloaded and run alongside the older version if you want to experiment with a buggier version of the software without as much tutorial support as the stable version. The QGIS software can also be run on other operating systems by finding the appropriate repository to download from.

My first impression is that QGIS is full of features that would take quite a while to master. There appear to be good learning resources out there for it. It is used in industry and by governments so it is not immature software. Powerful geomatic software is now within the reach of the average Jack and Jill.

To understand river flow we have to understand the larger context of the river. That larger context is called the watershed and it can often be defined precisely based on how the land drains.

A watershed is like the market for a product or service. It may be a small watershed that supplies a small first-order stream or it may be a large watershed that supplies a third-order stream. Properties of the marketshed determine the flow of traffic to your product or service. If you have alot of time on your hands, you might use GIS systems, overlays, and grid-based computing to model how consumer traffic might flow to your product or service.

Those who advocate for a bio-regional economy often make use of the watershed concept to delimit what is considered to be the local economy. Proactively, you should make an effort to trade with those who you share a watershed with. That seems like a crazy idea in some ways but in other ways it is reflective of how things worked historically and that history still bares an imprint on current trading practices if you look at trade through a watershed lens.

Another reason to trade with someone who you share a watershed with is because if that person lives upstream or downstream from you, then you are all interconnected by the various uses that you make of the water resource. Water quality and volume is a function of the whole watershed, not just some section of it. This is a very important connection that watershed users share and could be the basis of thoughtful trade patterns. Restricting your trade to just those within a certain distance from you (e.g., 100 mile diet) can cut across watersheds. That may not be that important to you, but it is a factor that is perhaps less arbitrary than a simple distance measure for defining what is local or not.

In conclusion, mapping watersheds is useful for understanding and predicting water flows. Techniques and technologies used for mapping watersheds might also be relevant to mapping marketsheds. Finally, the quest for sustainability dictates that the marketshed and the watershed should become more aligned.

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