Wavelet Analysis on NH3-N Pollution Index Changes of the Middle-Upper

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Procedia Environmental Sciences 2 (2010) 9–14

International Society for Environmental Information Sciences 2010 Annual Conference (ISEIS)

Wavelet Analysis on NH3-N Pollution Index Changes of the Middle-Upper Minjiang River during Last 6 Years Bo Yu a, Cheng-min Huanga*, Zu-han Liub, Bin Zhang b, Fa-chao Qin b

a College of Architecture and Environment, Sichuan University, Chengdu 610065,China b College of Land and Resources, China West Normal University, Nanchong 637009, China

Abstract

This paper explores and identifies the rules of the unsteady evolution of NH3-N in the upper and middle reaches of Minjiang River through multi-scale analysis on the weekly NH3-N data for the last six years provided by the water quality monitoring station, Minjiang River Bridge, Leshan City, Sichuan Province. The results of wavelet analysis indicate that the NH3-N index oscillates alternatively and differently at each time-scale in a 1 year cycle, and the density of NH3-N, subject to the runoff volume, the effects of Wenchuan earthquake and agricultural seeding and industrial construction, etc., is high in winter and spring, mild in summer and autumn.

Keywords : Wavelet analysis, NH3-N, the middle & upper reaches of the Minjiang River

The section between Dujiangyan City and Leshan City accounts for the middle reaches of Minjiang River which is one of the biggest branches of Yangtze River in its upper reaches, and locates in Sichuan Province, China (Figure 1). The middle and upper reaches of Minjiang River are exposed to intensive human activities and hence have serious water pollution. The pollutants of inorganic and organic released mainly from the excreta of human and animals, chemical fertilizers for agricultural application and industrial waste water, are the inorganic and organic elements of water which can cause eutrophication, and contaminate the fishes and certain hydrophytes. So it is important to know the unsteady evolution of nitrogen and ammonia pollution in its rules for pollution state identification and pollution control. The wavelet analysis works very well on analyzing the evolution features in time series at different scales[1-3] with proven applications in earthquake and geomagnetism[4,5], meteorology and climatology[6-8], slide disaster [9], and air pollution [10] studies and etc., but rare in analyzing the water quality. So, this paper takes 1-Dimension continuous wavelet analysis method to diagnose how the nitrogen and ammonia pollutants (the data of which is collected by the Minjiang Bridge water quality monitoring station, Leshan City, Sichuan Province) evolve in the time series, and reveals the rules of how it evolves at different scales, and locates the fluctuations, providing support for the decisions on water quality monitoring and protection.

* Corresponding author. Tel:ˇ86- 28- 8555 5685. E-mail address: [email protected]

Fig. 1. River systems in Sichuan Province, China

1 Data and methods

1.1 Data

This study has sourced NH3-N pollutant data published in the website of the Data Center of Ministry of Environmental Protection (http://datacenter.mep.gov.cn/) every week. This study covers weekly NH3-N index data for nearly 6 years which starts from the 1st week (abbreviated as w hereafter) of 2004 (specified as No 1) to the 51rd w of 2009 (as 310) and in total 310 pieces. Since the records of the3th , 27th ,28th , 29th ,30th w of 2004,the 7th w of 2005, the 18th w of 2006 and the 6w of 2008 are missing, so each is complemented by the algorithmic mean of the nearby 4 weeks (viz. two weeks before and two weeks after). To eliminate the boundary effect, periodical methods are used to expand the data. The data processing and wavelet transform are mainly realized by MATLAB7.0. The NH3-N index collected by the Minjiang Bridge Water Quality Monitoring station, Leshan City, Sichuan Province, can reflect how the middle and upper reaches of Minjiang River is polluted by the inorganic and organic.

1.2Method

Wavelet transform is local transform of time and frequency, which can extract information from signals effectively and carry out multi-scale zoom analysis on functions or signals through operation functions such as expansion and translation. The advantages of Wavelet analysis compared to the conventional time-frequency analysis lies in that it can decompose signals at any time-frequency resolutions, has favorable time-frequency multi-resolution functions and the characteristic of adaptability, and can focus on any details so as to observe changes in different time-scales [10-15]. 2 If \ ()tLR ( ) satisfies certain admissibility criteria:

|()|\Zˆ 2 Cd Z f \ ³ ||Z R

Ȍ(t) is called a wavelet; \ˆ()Z is Fourier transform of Ȍ(t). The continuous wavelet transform (CWT) is defined as the sum over all time of the signal multiplied by scaled, shifted versions of the wavelet function.

1/2 tb \ ab, ()ta | |\ ( ) a (2) Bo Yu et al. / Procedia Environmental Sciences 2 (2010) 9–14 11

tb Wab( , ) ft (),\\ () t | a |1/2 ft () ( ) dt fa,b³ a R (3) Wab(,) where a, bę R, a>0. The parameters a and b are the translation and scale, respectively, and the values f of the CWT are the wavelet coefficients. The Gauss wavelet is used here. 1 , xt xa a 2 MM )( aRxa z 0,, (4) a 2 RLf )( , the transformation of Gauss wavelet is: xt )( 2 wav x 1 2 ³ M )()()( dttatffT ³ ))(( 4SD dtexttf (5) R 4D SD aa R 2 Result analyses

2.1 The time series of NH3-N

See Figure 2 for the time series of the density of NH3-N (mg/L) contained in the water supervised by the Minjiang Bridge water quality monitoring station, Leshan City, Sichuan Province. The extent of NH3-N is jointly determined by the emission intensities of pollution sources and hydrologic conditions, etc. The pollution index signals are unstable of which the time series is chaotic, nonlinear and multi time-scaled.

Fig. 2. Weekly change of NH3-N index for the last 6 years

In the last 6 years, water quality has reached the State Standardċ˄˚1.0˅. But there are some fluctuations, for example, the index fluctuated obviously from May, 2004 to May, 2008, however, have been appeased ever since May, 2008.

2.2 NH3-N index changes at multi-scales

Figure 3 shows the periodical oscillation of weekly NH3-N index at different time-scales for the last 6 years. The intensity of oscillation signals is represented by the wavelet coefficient: in the gray-scale image, the smaller/bigger the grey value, the bigger/smaller the wavelet coefficient and NH3-N index, and the more/less serious the pollution is, respectively. In Figure 3, the upper area indicates periodic oscillations within larger time scales and the lower, smaller. Under given time and scale, the wavelet coefficient map can offer rough explanation on how the pollutant density is distributed along the time scales, viz. the NH3-N index evolves differently at different scales and the small-scaled evolutions are embedded in large-scaled ones, contributing to a more complex structure of pollution evolution. Detailed evolutions at chosen scales are illustrated in Figure 4 which shows that the weekly NH3-N index of last 6 years at a proximately one year (52 weeks) time scale forms a curve which follows a pattern of “high – low – high – low – flat”. To be specific, the NH3-N is relatively serious in the early 2005, it dramatically reduces to the lowest level in the summer of 2005 and maintains a mild state with fluctuations until bouncing back to a serious state in the winter of 2005 and maintains the high level until the spring of 2006. Relatively large fluctuations of NH3-N index are observed in the winter and spring seasons that have higher pollution, and it reduces and become less fluctuant in 2006. After this, the index is noticeably reduced (even in the highly polluted seasons of winter and spring) despite the former change pattern is maintained, followed by a relatively flat alternation of rise and falls (Figure 4).

12 Bo Yu et al. / Procedia Environmental Sciences 2 (2010) 9–14

Fig. 3. The time-scale distribution of NH3-N index for the last 6 years

Fig. 4. Wavelet coefficients of NH3-Npollution index at some scales

Note: a, b, c, d, e, f stands for the Gauss wavelet coefficient curve of NH3-N index at time scale of 53 w(approx. 1 year), 26 w (approx. 5 months), 13 w (approx. 3 months), 68 w(approx. 2 months), 4 w(approx. 1 month) and 1 w.

As for all the above scales, the peaks (local max, relatively serious pollution) are basically occur in spring and the trough (local min, relatively mild pollution) are often in summer, which also indicates the pollution is more serious in spring and less in summer.

Bo Yu et al. / Procedia Environmental Sciences 2 (2010) 9–14 13

2.3 The extremes of NH3-N index

The NH3-N index at nearly one-year scale (approx. 52 weeks) evolves in a one year cycle and by reading the local extremes of the wavelet coefficients at such scale, the dates of maximum or minimum pollution occurs can be accurately located. Fig. 5 shows that the troughs often occur around July every year viz. the period from June to July each year with least serious NH3-N; and the peaks often occurs from this winter to the next spring, viz. the period from December to the next March, with most serious NH3-N. To be specific, the NH3-N index went down to the bottom around July 1, 2004, before July 1, 2005, after June 1, 2006, before July 1, 2007, before July 1, 2008, before July 1, 2009, after July 1, 2004, after July 1, 2005, after July 1, 2006, after July 1, 2007,And likewise, the dates when the NH3-N reached to the peak can also be identified. The NH3-N for the Minjiang Bridge, Leshan City, Sichuan Province will reach its worst from December to the next May each year and get better in the summer and worse again at the end of the second half year.

Fig. 5. Local extremes of wavelet coefficients at the scale of 52 weeks

3 Discussion

The evolution of NH3-N pollution index for the Minjiang Bridge, Leshan City, Sichuan Province is obviously affected by the runoff volume, the May 12 Earthquake and the agriculture production. The rainy season for the middle & upper reaches of Minjiang River lasts from May to October and the dry season is from November to next April. The rainfall of summer and autumn accounts for about 80% of rainfall of the entire year and the other 20% are from winter and spring. Rainfall differs largely between years. The minimum annual precipitation is less than 900 mm and the maximum is more than 2000mm. The temperature and rainfall are rather sensitive to the altitude. The runoff can be observed in terms of dry season and rainy season. In the dry season, the runoff of Minjiang River is small and the density of NH3-N is high, contrary to the rainy season. In the middle and upper reaches of Minjiang River, rapes, wheat, sweet potatoes and beans are planted in October and November as well as paddy in February and March. During these seeding seasons, a lot of nitrogen fertilizer is applied to cropland and finally discharged into the Minjiang River, leading to a rise of NH3-N density. On May 12, 2008, an earthquake measuring 8.0 Richter scale hit Yingxiu Town, Wenchuan County, which claimed numerous lives and properties, and crushed industrial and agricultural construction in the stricken area. Crops cultivation and industrial output are limited due to the damage of farmland and factories, resulting in a dramatically reduced application of inorganic and organic fertilizer and hence the amount of fertilizer discharged into the Minjiang River. So the NH3-N pollutants were diluted and the pollution index was reduced subsequently. The wavelet analysis can localize the time and frequency simultaneously, by means of which, the fine structure of the time series of air pollution can be focused on, viz. being able to demonstrate the accurate, detailed frequency of air pollution change at each scale. The NH3-N index evolves in terms of multi-layered temporal structures and the stages and sequence of the evolution can be accurately analyzed by wavelet analysis. The wavelet analysis differs from other analysis methods in the way of studying the cycle, local extremes and change-points of the air pollution evolution at each scale, and it is both feasible and effectively. The study results are valuable references for 14 Bo Yu et al. / Procedia Environmental Sciences 2 (2010) 9–14

understanding the features of air pollution and conducting air pollution monitoring and predication. This paper studies NH3-N, however, the time series of such pollutants as PH, DO, NH3-N are also worth studying.

4 Conclusions The NH3-N pollution index for the Minjiang Bridge, Leshan City, Sichuan Province, has a multi-scaled evolution which varies at different time scales and in a main cycle of about 1 year. Judged by the alteration of the severity of pollution, now the water under study is subject to a relatively serious pollution of NH3-N and such state shall persist for a while. Attentions must be paid. The evolution of NH3-N pollution for the Minjiang Bridge, Leshan City, Sichuan Province is affected by the runoff volume, the May 12 Earthquake and the agriculture production etc, and the pollution is “serious in winter, and mitigated in summer”. The method of wavelet analysis is both scientific and effective in studying the time series of NH3-N pollutions as well as the rules of evolutions of other pollutants.

Acknowledgements

The authors are very grateful to the constructive amendments proposed by Professor Peng Gao from Syracuse University, U.S. and grateful to the anonymous reviewers and thank for the financial support from Program for New Century Excellent Talents in University of China (Grant No. NCET-08-0379) and National Basic Research Program of China (Grant No. 2006CB701401).

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