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Water quality and unit nonpoint sources (NPS) pollution load from a forest area were studied in a mountainous watershed in Taiwan. The flow rates were measured with rectangular weirs and samples taken for water quality analysis in both non-rainy and rainy days for 2 years. The subroutine of the Hydrological Simulation Program--FORTRAN was used to simulate runoff for additional 3 years. Total annual loads of various water quality parameters were then estimated by a regression model. Most of the parameter concentrations are higher during the rainy days; their values are typically higher as compared to data from other undisturbed forest areas. Nevertheless, the concentration ratio of dissolved inorganic nitrogen to TN or PO4(3-) -P to TP shows TN or TP no correlations with the flow rates, whereas the concentrations of SS and TP are positively correlated with the flow rate. The fluctuation of annual load from this watershed is significant. For example, six major events of the entire year, for which the total duration is merely 6.4 days, contribute 42% of the annual precipitation and at least 40% of the annual NPS loads. The management for controlling the NPS pollution from this forest watershed is discussed.

First flush of stormwater runoff plays an important role in water quality management. Since stormwater runoff during the initial period of precipitation events carries a greater portion of pollution loads, appropriate strategies are needed for effectively controlling the non-point source loads (NPS). In the present study, case studies were performed in two industrial parks for correlating the relationship between pollutant mass and the runoff volume. Three precipitation events were used for calibration and verification of a deterministic model, Storm Water Management Model. The model was then used to simulate all rainfall events in a particular hydrological year to determine the relationship between interceptive amounts of initial runoff and their corresponding reduced portion of annual NPS loads. The results indicate that the first 6-8 mm of a storm runoff depth includes more than 60% NPS loads. If the depth is increased to 10 mm, 80% NPS loading can be contained. The interceptive volume for eliminating 80% NPS loading in Guan-Tian and Yong-Kong industrial areas are 7 and 12 mm, respectively.

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In this paper, the performance of water quality modeling is investigated in the analysis of the natural assimilative capacity (NAC) of a river system. The new data available, along with the appropriate integrated model, allow for the analysis of the NAC of the entire river. The integrated model developed here emphasized the implementation of the QUAL2E simulation model and the appropriate allocation of waste loads (optimization model) in a subtropical area. Simulations are calibrated and validated according to the data from the Water Quality and Flow Rate Monitoring Project using the QUAL2E model. On the basis of the optimal allocation model, the results of the NAC and allowable pollution loading in the Putzu River basin are discussed. The study provides a framework of the integrated model that can be used as a guideline to determine the NAC of subtropical rivers.