Analysis of Pollutant Transportation processes with Large Time-Scale at at a Watershed Level

What is it about?

The pollutant transport equilibrium in a watershed can be analyzed on a large time scale, and land-use export coefficients can be calculated directly under certain hydrologic and transport conditions, by ignoring hydrologic and transport processes at small space and time scales on hydrologic response units. In this study, the water environment system of a watershed was deconstructed into three parts (source, source-sink, and runoff transport) to construct a pollutant transportation equilibrium model on a large time scale. A watershed with an annual source-sink accumulation of zero was defined as a completely transported watershed; therefore, we derived a completely transported equilibrium equation. The problem of seeking the land export coefficient was converted into a problem of seeking the optimal solution of linear programming, which can be estimated according to the variation in pollutant output processes. The feasibility of the solution can be analyzed using multi-year stochastic rainfall processes. The model was used to analyze the transport equilibrium of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) upstream of the monitored cross-sections in a watershed, which covered 3145.66 km2.The land export coefficients were calculated according to the model. The model calculations indicated that the watershed was completely transported during perennial years. The calculated export coefficients of COD, TN, and TP for farmland, primary vegetation, and urban land were within the range of general empirical values. The calculated maximum accumulations of COD, TN, and TP were 0.19×107, 0.063×107, and 0.049×106 kg, respectively, for perennial rainfall.

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Photo by le Sixième Rêve on Unsplash

Photo by le Sixième Rêve on Unsplash

Why is it important?

This study developed a pollutant transport model with large time scale to provide a simple method for calculating the land-use export coefficient and analyzing the pollutants transport process. It can be used as a framework for analyzing the aquatic environment and pollution control at a watershed level.

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