text: There are number of methods available to complete a runoff footprint. The simplest methods involve using a runoff coefficient, which according to the State Water Resources Control Board of California is "a dimensionless coefficient relating the amount of runoff to the amount of precipitation received. It is a larger value for areas with low infiltration and high runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat land)." The runoff coefficients for different surface types on a site can be multiplied with the area for each surface along with the annual precipitation to generate a rough runoff footprint. If the runoff coefficient and areas of proposed stormwater green solutions like rain gardens and bioswales for the site are known, the reduction in overall runoff from these improvements can be estimated.
  More accurate runoff footprint tools exist. By using computer modeling and detailed weather data, complex runoff footprints can be made easy. The amounts of pollution in the stormwater runoff can be estimated, and the effects of combinations of stormwater green solutions can be assessed. The James River Association of central Virginia provides an [http://www.jamesriverassociation.org/get-involved/runoff-calculator/ online tool] where property owners in the James River watershed can generate a site-specific runoff pollution report. MyRunoff.org provides an online [http://www.myrunoff.org runoff footprint calculator] for property owners across the United States to estimate their baseline runoff and the reduction from different scenarios of rain barrels and rain gardens. The EPA launched the National Stormwater Calculator in July, 2013, which is a desktop application for Windows allowing users to model the annual impact of a range of stormwater green solutions.