Restoring Forest Cover to Your Watershed

A 4-H Forestry Project

How is GIS used in the natural resources?

The following is an example of a problem that can be solved using a geographical information system (GIS). The Potomac-Shenandoah watershed is largely deforested and 4-H has obtained a large quantity of seedlings from the Department of Forestry to change that. The questions we are trying to answer in this project are:

1)      What parts of the watershed are most deforested?

2)      Where should our tree planting efforts be concentrated?

To answer both questions we begin with a base map that divides the watershed into smaller areas that can be more easily analyzed. We used two different base maps of the Potomac-Shenandoah watershed; one divided into counties and the other divided into small, sub-watersheds.

To answer the first question we added a layer that shows the land use coverage in the entire watershed. This layer is a map made by the United States Environmental Protection Agency and Geological Survey from satellite images taken around 1992 and is available to the general public.

The GIS program ArcView was used to summarize the land use in each county and sub- watershed. Ranking the counties and sub-watershed by % forest cover effectively answers question #1 and tells us which parts of the watershed are most deforested.

See county rank table || See sub-watershed rank table

The second question, “Where should our tree planting efforts be concentrated?” is more complicated. To simplify things, we decided that planting by students would most easily be accomplished in residential areas and that it is most effective near streams or riparian corridors. And again, we aim to plant in areas that are largely deforested.

We added another layer that shows all of the streams in the watershed.

We use ArcView to outline all areas within 60 m of a stream,

and then calculate the amount of land use types within those regions that lie within 60 m of a stream and rank by county and sub-watershed.
See county rank table || See sub-watershed rank table

Counties with riparian corridors that are less than 70% forested and have a residential land use of more than 2% are highlighted as “targeted counties.” Sub-watersheds with riparian corridors that are less than 50% forested and have a residential land use of more than 1% are highlighted as “targeted watersheds.”

These counties and watersheds are where we believe planting efforts should be concentrated.

Definitions:

Base Map- a simple map usually showing one feature that will be used in all subsequent maps and analysis. Commonly used base maps include outlines of countries, states, counties, cities, USGS quadrangles, etc.

Land Use- all land was classified into one of 8 categories, following US Government standards: water, residential, commercial, barren, transitional, forest, agriculture, and wetland. In most analysis, land use was summarized by percentage. For more information on land use classification see: http://landcover.usgs.gov/classes.html

Layer- more detailed maps showing one or more specific features that may be used in one or more subsequent maps and analysis. For examples, a map with streams was one layer we used. Other commonly used layers include: mountains, roads, population, land use, school districts, elevation, etc.

Riparian corridor- Also called a buffer, this consists of trees near streams and rivers that protect water quality by slowing down erosion and filtering out pollutants such as excess nutrients. We defined the riparian corridor to be a width of 60m on both sides of a stream. See Planting Trees in Your Backyard and Community to learn more about the benefits of corridors and how to create them.