Evaluation of watershed land-use changes and hydrologic impacts

Timothy Eaton

Queens College CUNY

This exercise is designed for undergraduate students or beginning graduate students taking a course in hydrology or hydrogeology. The course requires students to have taken college physics and algebra, but not necessarily calculus.

Students must be familiar with concepts of area and volume, and be proficient with unit conversions. They must also have mastered water-table mapping and contouring skills.

This exercise is designed to follow lecture material about rainfall, runoff and infiltration, water-table mapping and Darcy's Law.

1. Better understand the relationship between land use and runoff or infiltration.
2. Develop and strengthen analytical and mathematical skills by quantifying volumes based on precipitation depths.
3. Recognize and compare differences in discharges over different time periods.
4. Provide experience in making empirical calculations that lay the foundation for eventual field data collection.

1. Understand sources of uncertainty in quantifying hydrologic processes.
2. Evaluate the relative size of those uncertainties and effects on results.

This assignment is best presented in two lab periods to represent the "before" and "after" scenarios of land use and their impact on hydrology. It requires documentation in map and/or airphoto form of land use in a specific watershed at two times: historical and modern. Historical USGS topographic maps from the 19th century were used in this case, along with digital orthophotos for the modern-day scenario. Some means of quantifying subareas within the watershed is also needed, either using software (ArcGIS) or transparent overlays and boxcounting from a translucent grid would work.

For each of the sets of documentation: historical and modern, the students follow the USDA-NRCS TR55 empirical procedure to estimate event runoff depths and peak estimated discharge from the watershed. An area-weighted curve number (CN) is calculated based on tabulated categories of land use. Some judgment is involved in adapting the tabulated land use categories to the specific watershed used, and selecting an appropriate statistical average rainfall event to use. The sum of Darcy's Law calculations of discharge along streamtubes to a surface stream or estuary provides a groundwater discharge value over time for comparison. Each of these parts of the activity provides opportunities for the instructor to discuss uncertainties and sources of error.

Note that although software allowing TR-55 analysis exists, it is simpler and more instructive to have students use the paper method and forms in the manual.

Learning outcomes are based on the following:

1. Results and analysis in lab reports that correspond to these exercises. There is no "right answer," but all students' calculations should be similar. It is instructive to compare all results as a class to illustrate uncertainty in the calculations and method as a whole.
2. Ability of students, as evaluated during class exams, to recognize what controls infiltration and runoff in a watershed.
3. Ease with which students can recognize the significance of order-of-magnitude calculations and understand the usefulness but inherent limits of such analysis for future lab exercises.

USDA-NRCS TR-55 Urban Hydrology for Small Watersheds manual
http://www.wsi.nrcs.usda.gov/products/W2Q/H&H/docs/WinTR55/user_man_2002.doc

Historical USGS Maps for New England and NY from the UNH Library:
http://docs.unh.edu/nhtopos/nhtopos.htm

United States Digital Map Library (Rootsweb.com)
http://www.rootsweb.ancestry.com/~usgenweb/maps/

United States Geological Survey EarthExplorer
http://edcsns17.cr.usgs.gov/EarthExplorer/ ( This site may be offline. )