Watershed Analysis

As humans, we typically think of the land in terms of our defined boundaries, like countries, states, and towns. Global Mapper can be used to load data representing these boundaries, but the program can also explore the physical topography of the land and its boundaries in the form of ridges, watersheds, and ecosystems. Understanding local topography and the watersheds of an area can help provide an insight into possible risks and contaminates to the local water system.

Watersheds, or catchment areas, are defined by the streams to which water flows. Water for all areas within a particular watershed will flow to the stream, river, or waterbody defined in the watershed. This water then flows into a larger waterway, constituting a larger watershed and ultimately ending in an outlet to a major body of water.

From a publicly available national dataset, large watershed areas for the continental United States are shown. Each watershed covers portions of multiple states with the watershed boundary not conforming to state lines.

Watersheds of Major and Minor Waterways

Looking at a singular watershed from the national dataset, the Tennessee Region watershed covers a portion of nine different states within which water flows to the Tennessee River. As this watershed area intersects with the states, it closely follows a state boundary in a couple of areas. Along the Kentucky-Virginia border, and a portion of the Tennessee-North Carolina border the watershed follows the state boundaries. This illustrates how our human-defined boundaries are also influenced by the terrain. Prominent ridgelines in an area naturally break the land into watersheds, and these physical features were often a basis for country, state, or county boundaries.

Copying a feature from one to a new layer, the Tennessee Region watershed is detached from the national watershed data layer.

In order to isolate the regions within each state that fall within the Tennessee Region watershed, the Spatial Operations tool in Global Mapper creates a new set of polygons. The Intersection operation is applied to the two layers to generate the new layer isolating sections of the nine states covered by the Tennessee Region watershed.

A simple Spatial Operation is set up to isolate the areas where specific states and the Tennessee Region watershed intersect.

Colored uniquely, areas from each state in the watershed are seen.

Selecting the new layer of features and using a digitizer option to measure the area of the polygons yields attributes for each feature allowing for a quantitative breakdown of the Tennessee Region watershed area by state. Since some states, most prominently Georgia, have several polygons representing the watershed overlap, the features are grouped into multipart polygons for each state allowing for a single group area measurement to be computed.

With options to generate measurement attributes in Global Mapper, the area of each state within a watershed is quantified.

Finding Watersheds in Global Mapper

These large watersheds describe the boundaries and water catchment areas for the region at a macro-scale, but within each area there are multiple sub-watersheds or micro-watersheds defined by smaller local waterways. The Watershed Generation tool in Global Mapper uses a terrain layer to analyze flow and identify some of these smaller watershed areas.

Based on a section of a national terrain dataset, the Watershed Analysis tool takes into account parameters for streamflow, length, and depression fill depth in the terrain. Increasing the minimum stream size will identify larger streams and waterways, creating larger watershed areas. Identifying smaller streams or tributaries in this analysis creates smaller watershed areas. The outflow from the smaller streams and areas would eventually converge to create the larger watersheds.

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Altering the stream size requirements in the Watershed Generation tool determines the watershed area sizes.

Analyzing Watershed Hazard and Risks

The ability to analyze the terrain and divide a region into sections based on naturally occurring watershed areas offers a different perspective on the physical environment and how environmental and manmade factors can impact an area extending beyond the human-defined boundaries.

One type of threat to water supplies and ecosystems is contamination from waste management facilities. Landfill runoff threatens surrounding environments and these facilities need to manage any runoff to mitigate impact. While landfill management is usually reliable, being able to analyze the area of impact for a leak of contaminated water is important for maintaining watershed health.

Using a national dataset of landfill locations, attribute calculations and queries are executed to identify solid waste management facilities that are currently open. Searching for open landfills within the Tennessee Region watershed, these points can be selected and outflow from the points is determined using another application of the Watershed Generation tool in Global Mapper.

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Two attributes are used to determine which points represent open solid waste landfills.

Mapping Flow from Landfills

To analyze the potential impact of a leak, Global Mapper’s Watershed Generation tool can be used to map the outflow from a point. Using an advanced selection option to first identify all points within the Tennessee Region watershed, and a search of the selected points to narrow the results to landfills in this region that are currently open, the selected points are then used in a water drop analysis.

The Search Vector Data tool is used to search within selected features to determine the operation landfills within the Tennessee Region watershed.

Using theTrace Flow from Selected Point(s) component of the Watershed Generation tool, a line symbolized by a series of arrows is created to show the flow of water from the point over the terrain.

With the water drop analysis, only streams are generated showing the flow from the selected landfill points over the terrain.

As water travels along the identified stream path, it may seep into groundwater and contaminate clean water resources needed for a healthy watershed. Defining the impacted area can help manage the watershed and the health of the ecosystem and water resources. Using Global Mapper’s Spatial Operations tool, the intersection of the generated water drop analysis streams with the previously determined watershed areas, or municipal boundaries provides a clear indication of the impacted area.

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WORK MADE EASY
WITH GLOBAL MAPPER

Based on a digital terrain model, water flow over an area can be mapped in Global Mapper with a variety of applications. From identifying local streams and their watershed areas to mapping potential contamination of resources, Global Mapper is used to comprehensively analyze the physical terrain and inform users of possible ecosystem risks. Want to try Global Mapper? Sign up for a 14-day free trial. You can also request a demo from one of our experts to see this workflow or other Global Mapper processing abilities.

 

Learn More

References:

“Principles of Watershed Management.” Watershed Academy Web, Environmental Protection Agency, https://cfpub.epa.gov/watertrain/moduleFrame.cfm?parent_object_id=490.

Wilson, Reid. “Map: The United States of Watersheds.” The Washington Post, WP Company, 23 Apr. 2013, https://www.washingtonpost.com/blogs/govbeat/wp/2013/11/19/map-the-united-states-of-watersheds/.

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