New Evaluation Method for the Achievement Rate of Target Water Quality in Nakdong River, Republic of Korea

Water quality in the four major river basins in South Korea has been managed by introducing total maximum daily loads (TMDLs) to achieve or maintain a target water quality (TWC); TMDLs can be obtained by discharging the allocated pollutant loads from the watershed. The Nakdong River basin is the most important source of water supply among these four river basins and consists of 41 watershed units. The current method for evaluating the rate of load management involves calculating the measured values over the past 3 years through a log transformation process and assessing whether the TWC is achieved after comparing the values with the reference TWC. The objective of this study is to compare and analyze the results of a flow duration curve (FDC) and load duration curve (LDC) evaluation based on the current method for TWC evaluation and the daily flow data measured at 8-day intervals (weekly) to present an efficient TWC assessment method for more relevant flow conditions. The daily flow data were from 1995 to 2004, and the flow data measured at 8-day interval were from 2005 to 2015. The target study areas included the 41 watershed units on the Nakdong River basin (14 main streams and 27 tributaries). Biochemical oxygen demand (BOD) and total phosphorus (TP) constituted the water quality indicators studied. LDCs were prepared using the estimated daily flow rate, and the measured flow data at 8-day intervals based on the BOD and TP revealed similar results. Thus, even the data measured at 8-day intervals are considered applicable for creating FDCs and evaluating LDCs without requiring complex tests, calibrations, and prediction processes using a watershed model based on accumulated data over many years.