Comparison of automated watershed delineations: Effects on land cover areas, percentages, and relationships to nutrient discharge

We compared manual delineations with those derived from ten automated delineations of 420 watersheds in four physiographic provinces of the Chesapeake Basin. Automated methods included commercial DEM-based routines and different parameterizations of four enhanced methods: stream burning, normalized excavation, surface reconditioning, and normalized reconditioning. Un-enhanced methods resulted in individual watershed boundaries with some gross discrepancies in watershed size relative to manual delineations (error rate of 0.22 > 25 percent difference compared to manual) and significantly different watershed size distributions (Mann-Whitney U p = 0.012). Integrating mopped streams through enhanced methods substantially improved correspondence with manual watersheds (error rates of only 0.08-0.02 > 25 percent difference). Analysis of cropland area among methods showed a significant difference between manual estimates and un-enhanced estimates (p = 0.049) that was corrected using enhanced algorithms, Subsequent analysis of percent cropland revealed that measurements of land cover proportions were not always affected by delineation errors. However, differences were large enough to influence regressions with stream nitrate-N at the 90 percent confidence level within one physiographic province. Enhanced delineations produced statistical relationships between percent cropland and nitrate-N concentrations consistent with manual delineations. The results provide support for enhanced automated watershed delineation within the Chesapeake Basin and suggest that normalized excavation can be an effective augmentation of existing stream burning and reconditioning procedures.