Lesson 2

Interpreting Land Use Changes and Impacts on Water Quality

Lesson Introduction

What is a watershed?

The land surrounding a body of water is called its watershed. Precipitation in a watershed will either infiltrate in the ground or runoff into local streams and rivers that feed the lake. If you think of the watershed as a bowl and the lake at the center or bottom of that bowl, you can envision how any contaminants such as road salt, pet wastes, and excessive soil loss could impact and "fill-in" the bowl.  There are a number of factors that influence how much rainfall will runoff and how much will infiltrate into the ground.  These factors include the topography and elevation of the watershed.  The steeper the watershed land area, the faster the runoff will reach the lake.  The type of land cover will also affect runoff.  Watershed covered in vegetation and forests provide a chance for rainfall to be absorbed by plants and filtered into the ground.  Watersheds covered in hard or impervious surfaces cause the rainfall to runoff more rapidly because there is no plant material to stop and absorb the flow of water.

Land use Impacts on Lakes

The land uses in a watershed can have a major impact on the amount and types of pollution that ends up in a lake. The types of pollution that may enter a lake from land uses are called non-point source pollutants. They are pollutants in the watershed that may enter the lake from runoff that occurs after a rainfall.

Forested watersheds have the least impact on pollution to a lake. Rainfall that reaches a forested watershed will be captured by the trees and filtered through the soils before it reaches a stream and then finds its way to a lake. If the forest land is logged improperly however, exposed soils may runoff with the rainfall and contribute sediments to the lake. Sediments contain organic matter such as phosphorous which in turn "feeds" the lake with added nutrients. Foresters that institute Best Management Practices can reduce the impacts of logging on a watershed.

Land that is urbanized or residential may also contribute pollutants. Urban areas have hard surfaces such as parking lots and driveways which are impervious to rainfall. Rainfall that hits impervious surfaces will pick-up pollutants and carry them to the lake via local storm drains. Dog wastes, car oil, and road salts all make there way into the lake because of the runoff from land uses that are urban or residential. Another source of pollution to lakes from urban and residential land use is construction. Construction activities may cause huge losses of soil from the construction site to a local waterway. This can lead to increased turbidity of the lake and also contribute to nutrient loading from the phosphorous contained in the soil. In New York State, the Department of Environmental Conservation regulates construction sites to protect lakes from this type of non-point source pollution.

Finally, agricultural land uses may also contribute pollutants to a lake. Agricultural practices include exposing soil, the application of fertilizers and pesticides, and for farms that have livestock, the potential for animal wastes to enter local streams. Many farms in New York State are trying to reduce the impact of agricultural practices on lakes and streams by including Best Management Practices in their management of the farm.

Lesson Outcomes:

Students be able to:

• Use aerial photos to determine land uses
• Understand what non-point source pollution is and how it impacts water quality
• Determine the percentages of land use changes over time in a given place

Standards:

• Science 4: The Living Environment: Key Idea 7 – Human decisions and activities have had a profound impact on the physical and living environment. 

Lesson Objectives

In this lesson students will learn how to interpret land use changes in a given watershed area by examining aerial photos. The students will then be asked to use overlay grids to determine how much land in a given area has changed from one type of land use to another type of land use (i.e. farming to residential). After learning about non-point source pollution and its impacts on water quality students will be asked to make predictions as to what impacts the change in land use would have on water quality in the lake. To assist students with understanding how to interpret aerial photos there is a sample page provided with this lesson.

Activity

References:

Where to get aerial photos for your watershed.

Webs resources:

http://www.css.cornell.edu/iris/index.htm
Institute for Resource Information Sciences (IRIS) homepage

http://aerial-ny.library.cornell.edu/ny/
Cornell's On-line repository of historic aerial photos for Cayuga, Tompkins, and Cortland Counties

http://www.usgs.gov
U.S. Geological Survey homepage

Commercial sites:
http://earth.google.com
http://local.live.com
http://www.terraserver.com

Also check with your local Soil and Water Conservation District, Farm Services Bureau and County Historian for historic aerial photos.

Source: Susan Hoskins, Institute for Resource Information Sciences, Cornell University.