GIS adjustment of measured streamflow data from urbanized watersheds

Depending on the amount, spatial and temporal distribution, type, and land use prior to development, urbanization can alter the hydrologic response of a watershed to precipitation captured by U.S. Geological Survey stream gauges. Typically, urbanization increases peak discharge and can result in a significant non-stationary discharge signal depending on the urbanization characteristics. Also, if the urbanization occurs after the gauging period, the measured discharge data may no longer be representative of the urban watershed.; To investigate the hydrologic effects of this urbanization process high spatial and temporal resolution GIS based land use data are developed from three land use sources. Historical aerial photographs provide land use prior to development. GIS linked tax-map data provide the year of development for each parcel within the watershed, and generalized land use/land cover data provide current land use conditions. The combination of these three data sources provides parcel level land use data on an annual temporal resolution. Using these data, relationships between quantitative measures of urbanization and the presence or absence of statistically significant trends in the discharge series are developed, where the ratio of discharge to precipitation is used to reduce climate variability in the discharge data.; For discharge series that are non-stationary or no longer representative of the current watershed, a GIS adjustment procedure based on hydrologic modeling is developed to adjust the measured discharge data to represent specified fixed land use conditions and ensure their applicability for continued use in flood frequency applications. The specified fixed land use conditions can reflect current or future development conditions, and the gauged data can be from a watershed urbanized during and/or after the gauging period. The use of high spatial and temporal resolution GIS based land use data and hydrologic modeling enables the GIS adjustment methods to explicitly incorporate the spatial and temporal variability of urbanization at a level only approximated by existing adjustment methods. For applications where the GIS data are limited, an existing index flood adjustment method is modified such that the essence of spatial development can be incorporated. Several example applications of the GIS adjustment methods are provided to show the functionality of this approach.