Here, a water quality assessment model (WQAM) is developed by non-linear regression as an alternative to physical watershed modeling in South Korea. Three cases and 10 scenarios are applied and reviewed to determine the most appropriate WQAM. The three cases are: (1) the area size allocation of sub-watersheds, (2) the watershed imperviousness ratio, and (3) the combination of the area size and imperviousness ratio. The 10 scenarios are: (1) impervious, (2) impervious + pervious, (3) impervious + rainfall, (4) impervious + slope, (5) impervious + rainfall + slope, (6) slope, (7) land usage, (8) land usage + rainfall, (9) land usage + slope, and (10) land usage + rainfall + slope. The best WQAMs are subsequently developed from the generated equations using statistics (R2, Adjusted R2, F-test, the Akaike information criterion and the Shapiro–Wilk test). In addition, the WQAM is verified using the Geum-Sum-Young River watershed. The percentage differences of biochemical oxygen demand (BOD), total nitrogen (T-N), and total phosphorus (T-P) are 31.66%, 8.08%, and 48.94%, respectively. The developed WQAM can be used in place of complex watershed modeling and to aid in the determination of the best restoration locations.

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