Factors Affecting Estimates of Average Watershed Slope

The sensitivity of average watershed slope, or Y-slope, to digital elevation model (DEM) grid size and source are examined by using high resolution light detection and ranging (LiDAR) data. The results confirm previous findings that Y-slope is inversely proportional to DEM grid size. The sensitivity of Y-slope estimates to grid resolution makes it less desirable as a slope metric than main channel slope, which has been shown to remain constant up to 90-m resolutions. A comparison of Y-slope estimates from LiDAR DEMs and U.S. Geological Survey (USGS) DEMs shows that those based on the USGS DEMs are systematically underbiased. Differences between slope metrics and estimation methods can be significant. Y-slope estimates that are based on an alternative contour-based method are comparable to the DEM-based estimates. However, both estimates of Y-slope differ considerably from the main channel slope. Developers of hydrologic response time formulas should adopt standard protocols for estimating slope metrics that consider DEM grid size and quality of source data. Users of such formulas should be dissuaded from interchanging slope metrics.