Evaluating concentrated flowpaths in riparian forest buffer contributing areas using lidar imagery and topographic metrics

Authors: WALLACE, C. - Pennsylvania State University; McCarty, Gregory; LEE, S. - University Of Maryland; BROOKS, R.P. - Pennsylvania State University; Veith, Tameria - Tamie; Kleinman, Peter; Sadeghi, Ali

Submitted to: Remote Sensing
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2018
Publication Date: 4/17/2018
Citation: Wallace, C., McCarty, G.W., Lee, S., Brooks, R., Veith, T.L., Kleinman, P.J., Sadeghi, A.M. 2018. Evaluating concentrated flowpaths in riparian forest buffer contributing areas using lidar imagery and topographic metrics. Remote Sensing. 10(4):614. https://doi.org/10.3390/rs10040614.
DOI: https://doi.org/10.3390/rs10040614

Interpretive Summary: According to USEPA, the Chesapeake Bay is one of the most polluted water bodies in the United States. The decline in water quality is largely attributed to an overabundance of sediment, nitrogen, and phosphorus in surface runoff, particularly from agricultural lands. In 2010, the USEPA established a Total Maximum Daily Load (TMDL) program for the entire Bay to ensure that all pollution control measures needed to fully restore the Bay’s water quality are in place by 2025. Riparian forest buffers are considered a substantial component of the best management practices promoted under the TMDL program. These buffers are typically comprised of trees, shrubs, and grasses that intercept surface runoff, trap sediment, and subsequently reduce nitrogen and phosphorus before they enter the stream. Riparian buffers are believed to be most effective when surface runoff enters the riparian zone as dispersed sheet flow. However, under convergent flow conditions, where surface runoff occurs in concentrated flowpaths, the riparian buffers’ effectiveness is reduced. In this study, we examined two methods (topographic openness and flow accumulation) for evaluating the impact of concentrated flowpaths in riparian buffer contributing areas and used the most appropriate method to demonstrate the role of concentrated flowpaths on reducing the effective contributing area of riparian forest buffers. The flow accumulation technique worked best in medium to high and high relief areas such as those in the Piedmont and Appalachian Ridge and Valley physiographic provinces, while topographic openness worked best in low relief areas such as the coastal plain. Results of this study provide insight for the conservation managers to consider the occurrence of hydrologic bypass features when designing and maintaining riparian buffers to protect stream water quality.

Technical Abstract: Riparian forest (CP22) buffers are implemented in the Chesapeake Bay Watershed to trap pollutants in surface runoff thus minimizing the amount of pollutants entering the stream network. For these buffers to function effectively, overland flow must enter the riparian zones as dispersed sheet flow to facilitate slowing, filtering, and infiltrating of surface runoff. The occurrence of concentrated flowpaths, however, is prevalent across the watershed. Concentrated flowpaths limit buffer filtration capacity by channeling overland flow through or around buffers. In this study, two topographic metrics (topographic openness and flow accumulation) were used to evaluate the occurrence of concentrated flowpaths and to derive effective CP22 contributing areas in four Long-Term Agroecosystem Research (LTAR) watersheds within the Chesapeake Bay Watershed. The study watersheds include the Tuckahoe Creek watershed (TCW) located in Maryland, and the Spring Creek (SCW), Conewago Creek (CCW) and Mahantango Creek (MCW) watersheds located in Pennsylvania. Topographic openness identified detailed topographic variation and critical source areas in the lower relief areas while flow accumulation was better at identifying concentrated flowpaths in higher relief areas. Results also indicated that concentrated flowpaths are prevalent across all four watersheds, reducing CP22 effective contributing areas by 78% (in TCW), 54% (in SCW), 38% (in CCW) and 22% (in MCW). Thus, to improve surface water quality in the encompassing Chesapeake Bay Watershed, the implementation of riparian forest buffers should be done in such a way as to mitigate the effects of concentrated flowpaths that continue to short-circuit these buffers.