Delineating source areas for runoff in depressional landscapes: Implications for hydrologic modeling

A digital terrain analysis of the Huron River watershed, using a new spatial analytic procedure, suggests that many areas do not contribute surface runoff directly to the Huron River. These areas are depressions and internally drained valleys of glacial origin. Understanding gained from this analysis should significantly improve the design of lumped-parameter hydrologic models of undulatory glacial terrains. The analysis also shows that man-made drainage features can greatly increase runoff by connecting parts of the watershed that were previously topographically isolated. Consequently, the current use of contemporary subbasin divides is an inaccurate means of calculating surface runoff in southeast Michigan, because of the incorporation of areas that are not directly contributing runoff to the main river system. For example, 63.1% of the Huron River watershed is unconnected. An example is presented for Mallets Creek, showing how the new procedure can automate definitions of overland flow planes and catchments in a hydrological model. This model is then used to evaluate the sensitivity of runoff to the introduction of storm sewers that change the potential contributing source area. Results suggest that anthropogenic drainage features have doubled the effective size of Mallets Creek watershed.