Runoff Coefficient From Land Use Calculator

A composite runoff coefficient is an area-weighted mean of the coefficients assigned to each land cover present, and this tool applies representative values of 0.90 for impervious cover, 0.35 for agricultural land, 0.25 for grassland, and 0.15 for forest. Each individual coefficient compresses interception by foliage, depression storage in surface hollows, infiltration into the soil, and evaporation during the storm into a single fraction; the complement, 1 minus C, is what the catchment retains. That compression is why C is not a fixed property of a surface. It rises by roughly 0.05 to 0.10 on steep slopes, rises as hydrologic soil group shifts from sandy Group A toward clayey Group D, rises with wetter antecedent conditions, and rises for rarer design storms, which is why many manuals multiply C by a frequency factor of 1.1 to 1.25 for 25- to 100-year events and cap the product at 1.0. Published ASCE ranges give commercial and industrial zones 0.70 to 0.95, residential areas 0.30 to 0.75 depending on density, parks 0.10 to 0.25, cropland 0.20 to 0.50, pasture 0.15 to 0.45, and forest 0.05 to 0.25. Two cautions matter in practice. The Rational Method C is a peak-rate coefficient, not the volumetric ratio used to size water-quality treatment volume, where the Simple Method expression Rv = 0.05 + 0.009 times percent impervious is the usual tool; substituting one for the other misstates the design. And only impervious area hydraulically connected to the drainage network actually behaves near 0.90 - a roof that discharges onto a lawn or a road draining to a grass swale contributes far less, so weighting by total impervious percentage systematically overpredicts runoff in low-density development.

Empirical coefficients of this kind appeared as soon as engineers began sizing combined sewers directly from rainfall records. Emil Kuichling's 1889 study of Rochester, New York, tabulated the fraction of rainfall appearing as sewer flow across districts of differing building density, effectively the first field-derived runoff coefficients, and British practice following Lloyd-Davies's 1906 paper on sewer design used comparable percentage-impervious factors. Early tables were therefore descriptions of specific cities that were then generalised, which explains much of the spread in published values. The tables in modern use descend largely from the manual of practice on the design of sanitary and storm sewers issued jointly by the American Society of Civil Engineers and the Water Pollution Control Federation, first published in 1960, whose land-use categories and value ranges were reprinted in municipal drainage manuals for decades and revised in later ASCE manuals on urban stormwater systems. Two developments then sharpened them. The Soil Conservation Service's hydrologic soil groups A through D, developed for the curve-number method during the 1950s and 1960s, gave a defensible basis for varying C with soil texture instead of engineering judgment. Digital land cover data made the area weighting itself automatic: the USGS National Land Cover Database, first compiled for 1992 and updated on a multi-year cycle with 30 m impervious-fraction layers, allowed composite coefficients to be derived directly from a delineated catchment in GIS. Research on directly connected impervious area from the 1980s onward established why total impervious percentage overpredicts runoff, and separating connected from disconnected cover is now standard practice.