SWAT Modeling of Nitrogen Dynamics Considering Atmospheric Deposition and Nitrogen Fixation in a Watershed Scale

Chung-Gil Jung,Seong-Joon Kim

doi:10.4236/as.2017.84024

Abstract

The Soil and Water Assessment Tool (SWAT) nitrogen (N) water quality model considers the artificial inputs associated with human activities, including point and nonpoint source pollution loads. Although SWAT has the ability to simulate atmospheric N deposition and fixation, they were not considered in the modeling research. N deposition from the air is an important and considerable pathway for the input of N species into watersheds and water bodies, causing soil and water body acidification and the leaching of N into surface and groundwater, resulting in eutrophication and degraded water quality. The goal of this study is to assess the effects of atmospheric and agricultural N loads on stream water quality at the watershed scale. For a 6642 km2 Chungju dam watershed, SWAT was calibrated for 4 years (2003-2006) and validated for another 4 years (2007-2010) using daily anthropogenic N data (sewage discharge pollutants and fertilizer) and monthly measured atmospheric deposition data for NO3ˉ, NH4+, and dissolved organic N (DON). At the watershed outlet, the Nash-Sutcliffe (1970) efficiency (NSE) of daily streamflow during the validation period was 0.74. The coefficient of determination (R2) of total N was 0.69 considering atmospheric deposition, whereas it was 0.33 when removing the deposition effect. The results of this study demonstrate the potential for using the N dynamics between the atmosphere and land for SWAT assessments of nonpoint source pollution and for modeling stream water quality.

Highlights

Soil and Water Assessment Tool (SWAT) is based on the concept of hydrologic response units (HRUs), which are the portions of a sub-basin that possess unique land-use/management/soil attributes
The SWAT model was used to simulate the discharge and total N (T-N) load in the Chungju dam watershed outlet for the 2003-2010 period, and the impact of the T-N load was evaluated to identify the effects of atmospheric, agricultural and sewage discharge N loads on the stream water quality at the watershed scale
The SWAT model was established using all available data on the N stores in the fertilizer, manure, fixation, sewage discharge N, and atmospheric deposition

Summary

Introduction

The large magnitude of this N production is problematic, as excess reactive N can be extremely detrimental to the functioning of various ecosystems [4]. The eutrophication process is accelerated by human activity in densely populated urban or agricultural regions, where point N sources discharged from sewage treatment plants supplement high levels of non-point N sources produced from vehicles or fertilization [5], which is referred to as cultural eutrophication [8]

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Conclusion