Nitrogen Retention Effects under Reservoir Regulation at Multiple Time Scales in a Subtropical River Basin

Meibing Liu,Lu Gao,Ying Chen,Haijun Deng,Xingwei Chen

doi:10.3390/w11081685

Abstract

Reservoirs are an important nitrogen sink as a result of their retention effect, but their retention performance may vary with hydrologic conditions with time-varying characteristics, which also change them from being a sink to source over time. This study uses a coupled modelling system (Soil and Water Assessment Tool (SWAT) and a two-dimensional hydrodynamic and water quality model (CE-QUAL-W2) to analyze the nitrogen retention effect and influential factors at annual, monthly, and daily scales in Shanmei Reservoir in southeast China. The results showed that there was a positive retention effect of total nitrogen (TN), nitrate-nitrogen (NO3-N) and ammonia nitrogen (NH4-N) in most years, with average retention rates up to 12.7%, 7.83% and 26.17%, respectively. The reservoir serves mainly as a nitrogen sink at an annual scale. The monthly retention performances of TN and NO3-N were observed during the wet season (April–October) with higher water temperature and lower velocity, while a release effect occurred during the dry season (November–March). For NH4-N, which is prone to nitrification, the retention effect lasted longer, from May to December. The daily nitrogen retention process changed more dramatically, with the retention rate varying from −292.49 to 58.17%. During the period of dispatch, the regulated discharge was the primary factor of daily retention performance, while the hydraulic residence time, velocity and water level were all significantly correlated with nitrogen retention during the period without dispatch.

Highlights

A large number of water conservation projects have been instigated worldwide to retain river water behind dams, and these have significantly changed hydrological characteristics, physical transformations, nutrient transfers, and transport flux of rivers [1,2]
For TN and NO3 -N, during the wet season (April–October), the reservoir played the role of a nitrogen sink with nitrogen retention, while during the dry season (November–March), it acted as a source with nitrogen release
An assessment of nitrogen retention effect under reservoir regulation at multiple time scales in Shanmei Reservoir in southeast China has been performed with a coupled modeling system of SWAT

Summary

Introduction

A large number of water conservation projects have been instigated worldwide to retain river water behind dams, and these have significantly changed hydrological characteristics, physical transformations, nutrient transfers, and transport flux of rivers [1,2]. Nutrients in reservoirs are primarily removed from water by certain processes that include sediment burial, temporary storage in biomass, and denitrification [3,4,5]. This may reduce the amounts of nutrients transported downstream. Nitrogen is the material basis of the biogeochemical cycle and the main element of eutrophication. The impacts of reservoirs on nitrogen retention within watersheds may include the reduction of the nitrogen transported downstream and the slowing of the trend of eutrophication in estuarine waters [6]. Estimating the nitrogen retention effect in a reservoir is of great importance for controlling eutrophication and improving the aquatic environment

Objectives

Results

Discussion

Conclusion