This study improves the estimation of the self-purification coefficient for runoff analysis of pollution load using geomorphological factors. Formerly, the assimilative capacity, K, was estimated using a single geomorphological factor, Horton's form ratio, Sf. Here, K was divided into two factors, namely, a watershed self-purification coefficient (k) and a watershed form ratio (Sf). The watershed form ratio, Sf, is the equivalent stream density and is considered an index of accessibility of the pollution load to the water body. Even though Sf had shown a clear reciprocal relationship with k, in agricultural areas, there is a limitation that the k, estimated by using an Sf only can't reflect the variability of land coverage characteristics and/or land use. In this study, a new geo-characteristic index (GCI), SR, which is composed of Sf and weighted flow accumulation ratio (Fr), is suggested. GIS and remote sensing technique were used to calculate Sf and Fr. Interestingly, a clear reciprocal relationship exists between k and SR, and this relationship seems to be stronger for agricultural areas, as urbanized areas have easier wash off due to the sewer network or paved surfaces.

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