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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.
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