Measurement of Total Nitrogen Concentration in Surface Water Using Hyperspectral Band Observation Method

Changjiang Liu,Xianlong Zhang,Xiangyu Ge,Yaxiao Qi,Ngai Weng Chan,Fei Zhang

doi:10.3390/w12071842

Abstract

Nitrogen overload is one of the main reasons for the deterioration of surface water quality. Hence, monitoring nitrogen loadings is vital in maintaining good surface water quality. Increasingly, the use of spectral reflectance to monitor nitrogen concentration in water has shown potentials, but it poses some problems. Therefore, it is necessary to explore new methods of quantitative monitoring of nitrogen concentration in surface water. In this paper, hyperspectral data from surface water in the Ebinur Lake watershed are used to select sensitive bands using spectral transformation, the spectral index, and a coupling of these two methods. The particle swarm optimization support vector machine (PSO-SVM) model, constructed on the basis of sensitive bands, is used quantitatively to estimate the total nitrogen concentration in surface water and subsequently to verify its accuracy. The results show that the bands near 680, 850, and 940 nm can be used as sensitive bands for estimation of the total nitrogen concentration of surface water in arid regions. Compared with the best estimation models constructed by sensitive bands selected using the spectral transformation or the spectral index alone, the best model based on the coupling of these two measures is more accurate (R2 = 0.604, Root Mean Square Error (RMSE) = 1.61 mg/L, Residual Prediction Deviation (RPD) = 2.002). This coupling method leads to a robust, accurate, and strong predictability model, and can contribute to improved quantitative estimation of water quality indexes of rivers in arid regions.

Highlights

Excessive nitrogen concentration is one of the main water pollution problems in Chinese rivers as well as rivers worldwide [1,2,3]
The black line in the figure indicates the critical value of the significance of correlation coefficient when the significance level is p = 0.01
The point above the critical value indicates that the band reflectance had a significant positive correlation with the total nitrogen concentration of water body, and the point below the critical value indicates a significant negative correlation

Summary

Introduction

Excessive nitrogen concentration is one of the main water pollution problems in Chinese rivers as well as rivers worldwide [1,2,3]. Excessive nitrogen in rivers feed algae causing it to grow faster than what most river ecosystems can handle. The result is the occurrence of eutrophication caused by large algal blooms which can consume almost all the oxygen in a river, producing so-called “dead zones” [4]. Rivers devoid of oxygen leads to hypoxia which kills virtually all aquatic organisms in the river, rendering the river to be declared as a “dead river”. Water 2020, 12, 1842 of the nitrogen concentration in inland waters is the intensification of industrial and agricultural activities, with their resulting effluents and pollutants discharging into waterways leading to severe ecological and environmental problems [5]. In China, it was reported that the Yangtze and Pearl river estuaries have been turned into dead zones, mostly due to nitrogen pollution of the waterways [6]

Objectives

Methods

Results

Discussion

Conclusion