“…From 1851 to 1931, several authors added an additional term (ofteñ F I G U R E 1 Time-history of key developments in rainfall-runoff models catchment slope) to the rational equation, based mainly on field data, to account for the nonlinearity of runoff because of variations in catchment area and slope in urban catchments. Kuichling (1889) appears to be first in the United States to adopt the time of concentration concept into analysis (Gregory, 1907). Except for special situations in urban drainage (see Reid, 1927 andRiley, 1931 who discuss the tangent method), adopting the time of concentration concept ensures the discharge estimated using the rational method is the maximum value for the given situation.…”