Analyzing Hydrological Transport Characteristics of Nonpoint Source Pollutants Using SWAT

Abstract

This study was conducted to simulate pollutant loads to stream reaches and to estimate its delivery downstream using the Soil and Water Assessment Tool (SWAT). The Chungju Dam basin of the Han River in South Korea was selected as the study area and divided into 11 sub-basins where hydrological and water quality data are available for the SWAT simulation. The calibration and validation revealed that the simulated stream flows and pollutant loads were in good agreement with the observations, where a '7-day window' approach was adopted for pollutants sampled and collected every eight days. The transport ratio defined as pollutant loads delivered to downstream as to upstream discharge loads may represent qualitative characteristics of pollutant movement along stream segments. For the period from 1980 to 2009, pollutant loads from the study area were simulated and resulting transport characteristics were evaluated in terms of rainfall amount, flow rate, and sub-basin reach. Transport ratio appeared to be relatively constant for larger rainfall events greater than 100 mm, while high variation was observed for smaller rainfall events. For 10 of 11 sub-basins, it was observed that the majority of pollutant loads runoff occurred during larger rainfall events. The exception was the Jechoen-A sub-basin, in which point source pollutants seemed to contribute substantial portion of nitrogen and phosphorus loads. Considering dominant contribution of larger rainfall events to pollutant loads runoff, the constant delivery ratio during large stream flow may represent the overall transport characteristics of the pollutants in stream reaches. Based on the transport ratio of each sub-basin, the overall pollutant transport ratio to the Chungju Dam was estimated. Downstream reaches showed higher delivery ratios than upstream areas, which may indicate that sub-basins of downstream have greater impacts on the overall basin pollutant runoff as compared to the headwater sub-basins. This study characterized overall pollutant transport that may be useful in indentifying critical locations for restoration in the Chungju Dam basin.