Cost apportionment for a storm-water management system: Differential burdens on landowners from hydrologic and area-based approaches

Communities deciding how to equitably distribute the cost of storm-water management systems need an understanding of how selection of a particular apportionment approach alters the cost distribution between various segments of the community. With the use of a geographic information system (GIS), differential impacts by apportionment approach were assessed for the Cuppy-McClure Watershed, Indiana, an area undergoing conversion from agricultural to residential and industrial uses. A per-acre charge approach is simple to construct and administer and gives a spatially uniform cost, but does not account for differential storm-water generation between different land uses. A single storm event runoff coefficient approach increases the cost load on land areas that generate more storm-water runoff from rare, extreme rainstorms, in particular, land uses with extensive impervious surfaces. For the single-storm approach, cost per unit area in the study area ranges from 25% below the per-acre charge level to 291% above the per-acre charge level. A long-term runoff impact approach increases the cost load on land uses that generate more runoff from the natural range of storm events for an area. It gives the greatest differential impact between land uses, ranging from 51% below the per-acre charge level to 484% above the per-acre charge level. Runoff-based approaches to fee apportionment shift financial burden to the landowners responsible for generating storm water runoff. Assigning fiscal responsibility on the basis of impact level encourages impact-minimization strategies, such as including hydrologic analyses in land-use decisions. Different spatial arrangements of the same land uses in a watershed can significantly alter the level of hydrologic impact, thus impact-based fee apportionment also encourages inclusion of spatial organization and environmental concerns in land-use planning.