Linkages between Watershed Characteristics, Hydrologic Responses, and Instream Conditions

Gale, Susan

ABSTRACT: The effect of watershed morphometry and land cover on watershed hydrologic responses and local instream ecological conditions at the watershed outlet were examined for sixty-nine watersheds across North Carolina. Two watershed hydrologic responses—the runoff ratio and stream flashiness— and multiple instream characteristics were significantly correlated with two environmental drivers: mean watershed slope (%) and total impervious area in the watershed (%TIA). Watersheds with a mean slope >10% had low %TIA, slope and runoff ratio were positively correlated, and stream flashiness was consistently low or moderate. Increasing slope was also correlated with improved instream conditions, including lower specific conductance, better instream habitat, less stressed biological communities, and a regulatory use support status of fully supporting. For watersheds with mean slope 4%, demonstrating a coupling of reach-level instream conditions and watershed-scale hydrologic function to a single stressor. The 4% TIA threshold is well below the existing regulatory requirements in NC, which requires remediation of stormwater impacts when TIA exceeds 24% TIA. These results suggest that significant degradation of watershed hydrologic responses and instream conditions may occur well below that threshold, and that the focus of stormwater management—minimizing the effects of peak flows—may be insufficient to address other changes that results from increases in watershed TIA, such as loss of infiltration capacity, groundwater recharge, and evapotranspiration. Basin morphometry, described using a basin form factor, correlated with stream flashiness and with metrics derived from the watershed width functions, though the strength of the correlations (Spearman ρ) was somewhat low. The results do suggest an existing linkage between watershed morphometry, the structure of the flow path network, and hydrologic responses, as has been proposed in the geomorphic instantaneous unit hydrograph theory. Stream flashiness was more strongly associated with more rounded basins that had width functions that were negatively skewed and exhibited higher kurtosis. The higher flashiness response in these types of watersheds was attributed to the hydrologic routing of a precipitation inputs during storm events through a relatively small number of hydrologic pathways near the watershed outlet over a shorter time period. These findings have direct impacts on watershed rehabilitation and management, and suggest that mitigating for deleterious impacts to instream conditions due to anthropogenic stressors, such as increased development, must necessarily address the concomitant modifications to the full range of basic hydrologic functions of watersheds.