ab s t r ac tTo improve the water quality of the four major rivers in Korea, Ministry of Environment of Korea (MOE) has introduced the Total Maximum Daily Loads (TMDLs) management system since 2002 and BOD was selected as one of the target materials during the first period (2004)(2005)(2006)(2007)(2008)(2009)(2010). For an effective watershed management, it is necessary to have one or more quantitative measures that can be used to evaluate the relationship between pollutant sources and their impacts on water quality. Such measurable quantities are termed water quality indicators. Once an indicator is selected, target values for that indicator must be established to distinguish between the impaired and unimpaired state of the water-body. Various factors such as available data, application, management conditions and cost will be considered for the selection of an appropriate watershed management indicator. This paper introduces various factors required for choosing target water quality indicators and establishes reasonable target values during the second TMDLs period (2010)(2011)(2012)(2013)(2014)(2015).

We investigate the Japan's Master Plan of Comprehensive Sewerage System (JMPS) and Lake Biwa basin sewerage and suggest future development direction of the Watershed Sewerage System Maintenance Plan in Korea enforced on February 2, 2013. The JMPS is designed for compliance with the environmental standard for water quality under the Environmental Policy Act. The effluent standards applied in the master plan of Lake Biwa's Sewerage Plan for the Lake Biwa is tougher than the national standards. Therefore the Lake Biwa Baisn Sewerage System was the first in Japan to adopt facilities that perform advanced treatment for nitrogen and phosphorus removal. BOD, SS, T-N and T-P concentrations of discharge water of sewage are 0.9, 0.6, 5.5, 0.06 mg/L, respectively. Especially removal efficiency for BOD is 99.5 percent. It is necessary to study the diversification of the evaluation criteria, cost minimization approach, subsidy system improvement, economic concept of discharge load adjustment system and establishment of basin sewerage concept for the development of the basin sewerage plan in Korea.

The Natural Resources Conservation Service‐Curve Number (NRCS‐CN) method has been used widely for estimation of direct runoff. However, the determination methods of CN values for land covers from the National Engineering Handbook Chapter 4 (NEH‐4) are inadequate due to uncertainty raised in determining the of the hydrologic soil group, which could change from reported soil survey data due to soil compaction and other human activities. To overcome these drawbacks, an asymptotic CN (ACN) regression equation for the relationship between measured streamflow and rainfall for gauged river basins (ACN‐RB) was developed and used in various studies. The ACN‐RB determines various CN values depending on the amount of rainfall in an event. The ACN‐RB has limitations in application to ungauged river basin because the ACN‐based CN values do not consider various land covers. Accordingly, 13 land cover‐based asymptotic CN regression equations (LC‐ACN‐REs) were developed and evaluated in this study by comparing estimated direct runoff with measured data. The estimated direct runoff using the LC‐ACN‐REs, which were classified as “standard types”, matched observed direct runoff well, as evidenced by a Nash‐Sutcliff efficiency (NSE) value of 0.78 for Jungrang A basin and a NSE value of 0.73 for Tancheon A basin. Copyright © 2016 John Wiley & Sons, Ltd.

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