Mapping Runoff Flow Paths and Their Pollution Contribution Potential: The Impact of Crops as a Land Cover Category

Elaine Langone Center, Forum

High resolution land use and land cover (LULC) and digital elevation models (DEMs) can be used to map stormwater runoff flow pathways, and to estimate, based on land use, the likelihood of the flow path to contribute pollutants to a stream. Pioneered by Jeffrey Allenby and Conor Phelan at the Chesapeake Conservancy, this is an example of Precision Conservation--the use of high-resolution remotely-sensed data to identify priority sites for conservation. For each cell in a watershed, all the upstream cells that contribute overland flow to that cell are counted in two ways. The “unweighted” count is simply the number of contributing cells, and the “weighted” count sums up a weighting factor attributed to each contributing cell, based on the cell’s LULC classification. For example, forests (weight 2) have a lower potential for contributing pollution than parking lots (weight 10). The weighted and unweighted contributing areas for each cell are combined into an index called NDFI, which theoretically varies from -1 to 1, with larger values indicating greater potential to contribute pollution. This study centers on whether NDFI values are impacted by inclusion of crops as a LULC category, because many available LULC maps lump crops and other low vegetation into a single category. A simple rule-based procedure was used at Bucknell during the summer of 2015 to develop a LULC for the Buffalo Creek watershed near Lewisburg, PA. This map includes both low vegetation (weight 5) and crops (weight 7). A second map was developed in which crops were given the same weight as low vegetation (weight 5). Differences between the two estimates of NDFI are greatest for smaller channels in agricultural regions of the watershed.