While using aerial photographs does not account for topography and thus tends to underestimate land area, it remains the most common method for mapping land use. Students should place tracing paper over an aerial photograph and then trace the watershed boundaries. Within the watershed boundaries, students should trace the outlines of different land use types within the watershed. They should look for three basic land use categories: wildlands, agriculture, and residential/commercial. Agriculture may include row crops, orchards, hay fields, and pasture lands. The residential/commercial category should include all major roads as well as house lots and commercial businesses. Wildlands will be tracts of uninterrupted woods as well as old fields or successional areas. Refer to Table 1. Students should assign a color code to each of the three categories and color their watershed areas accordingly.
Here are three methods students might use to determine the approximate proportion of each watershed that is in each land use category and to examine how land use has changed during the past 25 years. Challenge students to discover the best method.
Balance Method: Cut the tracing paper into land use units. Using a sensitive, well-calibrated balance (ideally it should read to 0.01g), weigh each piece of paper and then determine what percent of the total weight is represented by each of the three land use categories. By assuming that the paper weight is proportional to land area, students can find the percentage of land area that is devoted to wildlands, agriculture, and residential/commercial uses.
Grid Method: Lay a transparency with a square grid (0.5 cm by 0.5 cm works well) over the traced and colored watershed. Have students score each square of the grid as falling into one of the three land use categories.
Transect Method: Lay the ruler across the watershed in a random location and direction. Score every millimeter into one of the three land use categories and keep a tally. Repeat for two or three more transects. Add the results of all the transects.
Students can compare land use in the same watershed over time (1970 versus 1990) or they can compare contemporaneous land use in each of the three watersheds.
Graphing their land use data may help students summarize land use differences and changes. Two hypothetical graphs are shown below as examples.