INTEGRATING WEIGHTED FLOW ACCUMULATION AND AERIAL PHOTOGRAPHY FOR DESCRIBING OVERLAND FLOW PATHS: A CASE STUDY IN WESTERN NEW YORK

Abstract

Field measurements of overland runoff and aerial photographs were analyzed in Black Creek watershed (western New York State), to determine if a topographically based loading model could be used to identify overland flow paths in order to rank stream segments for prioritization for nutrient remediation. Discharge and phosphorous concentration were measured at twelve sites to compute the frequency of overland flow, approximate volume of overland flow and flux of phosphorous at each site. Using these metrics, sites were ranked according to their phosphorous contribution to the stream network. Although a nonparametric correlation of observed and modeled rankings suggests that flow accumulation did an inadequate job of ranking the sites, valuable management information on hydrologic connectivity was acquired by identifying where the model did not perform well. An analysis of the model showed that the performance of the weighted flow accumulation model was due primarily to its inaccuracy in estimating contributing area. The field and aerial assessments suggest that subtle anthropogenic changes to topography and hydrography, which altered surface flow paths, were the primary cause of this inaccuracy. These changes were not represented at the resolution of the USGS 10-meter DEM used in the model, either because the landscape modification took place after the source material for the DEM was created or the resolution was too low. In descending order of importance, topographic alterations include road and driveway berms, drainage ditches, stream straightening and stream squaring. One other modification type, tile drains, was important for changing the catchment area of overland flows. These features have an enormous impact on surface flow paths in the study area and were found to be important in controlling hydrologic 'connectivity' to the stream network. Identifying where these features impact surface flow paths is critical information in Best Management Practice (BMP) prioritization. Although the flow accumulation model did not accurately assess overland flow paths well enough to rank stream segments sufficiently for buffer prioritization, useful information was gleaned by overlaying the results of the model on aerial photographs and conducting field assessments. The ease with which this model approach can be carried out, requiring no calibration and needing spatial data that are available in practically all areas, make it a very effective tool for watershed planning. The study underscores the importance that anthropogenic alterations of the landscape have on hydrology and the need for better digital elevation products that represent these features.