THE NDVI WAVELET SPECTRAL ANALISYS DURING THE 2007, 2008
AND 2009 YEARS IN THE SÃO PAULO STATE REGION
Marcelo Schneider1, Joseph Harari 2, Humberto Barbosa3
Abstract  The objective of this research is the detection of oscillations of the
Normalized Difference Vegetation Index (NDVI) in the northern part of São Paulo
State (NSP) and São Paulo city (SP) regions. Wavelet analysis (WA) is applied for
the 2007-2009 daily dataset period. In this sense, the NDVI WA provides results from
the weekly to annual band and allowed the investigation of the spectra and scale
interaction. The results for the NSP region indicate there is a strong influence of the
annual cycle, followed by the semi-annual band (specially in Franca region). It is also
observed, in some cases, the interaction between the semi-annual, intraseasonal
and synoptic scales, particularly during extreme events. On the other hand, striking
results concerning SP city are the more energetic bands in the semi-annual to
intraseasonal scales. The WA turns to be useful for the formulation of conceptual
models also in the ecological field.
Index Terms  NDVI variability, spectral analisys, wavelets.
RESUMO: O objetivo desta pesquisa é a detecção de oscilações do Índice de Vegetação por
Diferença Normalizada (NDVI) na parte norte do Estado de São Paulo (NSP) e na cidade de
São Paulo (SP). Análise de Wavelets (AW) é aplicada diariamente para o período 2007-2009.
Nesse sentido, a AW apresenta resultados de uma semana à banda anual e permitiu a
investigação dos espectros com interação de escala. Os resultados para a região NSP
indicam que há uma forte influência do ciclo anual seguido pela banda semi-anual
(especialmente na região de Franca). Observa-se também, em alguns casos, a interação entre
os ciclos semi-anual, as escalas intrasazonal e sinótica, especialmente durante eventos
extremos. Por outro lado, os resultados mais importantes sobre a cidade SP são as bandas
mais energéticas encontradas nas escalas semi-anual e intrasazonal. A AW passa a ser útil
para a formulação de modelos conceituais também no campo ecológico.
INTRODUCTION
The São Paulo State region (SPSR) has relatively well defined climate characteristics,
with a wet period in the summer season and, on the other hand, a dry period in the winter
season. The climate, in a first approach, is highly dependent on the vegetation coverage
conditions. There is a feedback and an interaction between the soil, vegetation coverage and
the air circulation dynamics. It’s known that in wet (drought) conditions the vegetation
coverage tends to increase (decrease). Gurgel et.al [3] investigated the normalized difference
vegetation index over Brazil, and observed a pronounced annual cycle, particularly over
Cerrado region. The NDVI has proved to be a good proxy not only of the vegetation
conditions, but may be also related to the atmospheric conditions. The mapping and
1
Marcelo Schneider, Meteorologist at National Institute of meteorology, São Paulo, SP, Brazil, [email protected]
Joseph Harari, President of Regional Nucleus of Organization of Environmental Researches, Health and Safety, Rua Dom Pedro
II, 54, room 312, 11.010-080, Santos, SP, Brazil, [email protected]
3
Humberto Barbosa, Atmospheric Sciences Institute, laboratory of analysis and processing satellital images, Alagoas Federal
University, A.C. Simões Campus, BR 104 norte, Maceió, AL, Brazil, [email protected]
2
monitoring of NDVI is of fundamental importance in global vegetation studies and has been
widely used in remote estimates of evapotranspiration and surface temperature continental
[2].
In earlier studies mostly held in the atmospheric dynamics context [4]-[5] have identified the
importance of the multiple interactions in the consolidation of the major events using the
wavelet analysis. In this work we have the objective to investigate some spectral
characteristics, specially those related to the perturbation in the temporal evolution of the
seasonal NDVI cycle.
DATA AND METHODOLOGY
The wavelet transform analysis (WTA) is a powerful tool that describes the evolution
of an oscillation in the time-space domain. The WTA is used to verify the predominant
variability of the Normalized Difference Vegetation Index (NDVI) in the northern part of São
Paulo State (NSP) and São Paulo city (SP) regions. The NDVI daily dataset was obtained
from the LAPIS, a laboratory of image processing in Alagoas Federal University, Brazil. The
LAPIS develops (operationalize) the NDVI estimated with multispectral imagery from the
Meteosat Second Generation (MSG-2) satellite [1]. The images MSG-2 (SEVIRI) are
obtained by the receiving station of ICAT - established since April 2007. The daily index of
vegetation is estimated from the channels of the visible and near infrared of MSG-2
(METEOSAT-9) to calculate the NDVI images from the composition of images by the
maximum value.
Three selected areas in the NSP will be compared with the SP city, all areas have also
meteorological stations where variables as rain and humidity, in stance, may be used in
another paper to compare with the results that we obtained here.
FIGURE. 1 THE SÃO PAULO STATE AREA, WITH THE FOUR FRANCA, VOTUPORANGA,
CATANDUVA E SÃO PAULO SELECTED REGIONS WHERE NDVI 2007-2009 DATASET WAS
CALCULATED.
The WTA method used here is adapted from the Torrence and Compo [6] and is
applied for the 2007-2009 daily dataset period. In order to obtain the phase of the NDVI
signal during the dry and the wet season it was calculated the real part of wavelet coefficients.
A scale averaging was performed to clarify the influence of the respective phase on both the
semi and annual bands.
The missing data are averaged with the nearest values in order to keep the daily
resolution. As will be seen later, below the one week interval there is a weak variability in the
NDVI. So results interpretations, in fact, based in the “NDVI reaction” are consistent with the
wavelet scalogram showed in the Figures.
RESULTS AND DISCUSSION
The results for Franca (FIGURA 2) show the main amplitude of the wavelet coefficients
between the semi and annual bands. As expected, the energy (expressed here as the
alternating high and low phase of the wavelet coefficients) highlights the annual cycle in the
NSP region. The semi-annual cycle, in turn, may be considered as an important modulation
band in the signal of the raw Franca series. According to the filtering method, viewed as the
alternating positive and negative amplitude of the signal in the 100-230 (say semi-annual) and
230-450 (annual) bands, the semi-annual band shows more amplitude in the end of the 2007
year, period of a sharp increment in the NDVI index to establish the beginning of the 20072008 wet season. It’s important to note that generally the values of NDVI increase markedly
in late spring and early summer.
FIGURE. 2 THE NDVI ANALYSIS OF THE FRANCA REGION. THE RAW NVDI
DAILY SERIES OF THE 2007-2009 PERIOD (UPPER), THE SCALOGRAM OF THE
WAVELET COEFFICIENTS WITH THE CONE OF INFLUENCE (MIDDLE) AND THE
100-230 AND 230-450 DAYS SCALE AVERAGE (LOWER)
Regarding the Votuporanga results it may be considered similar to the Franca region,
specially in the semi to annual broad band oscillation. Once again the interaction of the low
NDVI index in these two bands are determinates to have the minimum of the raw index. In
these period of sharp transition regime it may be observed that also the intraseasonal to
submensal oscillations have a relatively impact on the NDVI raw signal.
From a more rigorous examination concerning Votuporanga scalogram arise a possible
intermediate oscillation, near 100-120 days, besides of the semi-annual band.
In a raw view, some general aspects discussed earlier, appear in the Catanduva region ,
except for the lack or the flat behavior of the semi-annual oscillation after November of 2008,
as seen by directly in the scalogram figure or in the the filtering method.
The metropolitan area of SP, mainly constituted by an agglomeration of building, explains
how the NDVI has the lowest values in comparison with the others areas. So, the spectral
results (FIGURE 3) are different, specially those related to the annual cycle, due to the dataset
don’t capture the highs and lows of the index during the year. In spite of that, it’s possible to
see a connection between the submensal till the semi-annual band held between 2007-2008
years.
FIGURE. 3 THE SAME AS FIGURE 2, EXCEPT FOR THE NDVI ANALISYS OF THE
SÃO PAULO REGION.
CONCLUSIONS
Some predominant scales of oscillation in the 2007-2009 years have been identified.
The results for the NDVI index in the NSP region reinforce there is a strong influence of the
annual cycle, followed by the semi-annual band (specially in Franca region). It was discussed
with particular interest the sharp transition of the NDVI index in the 2007-2008 period. It was
observed a clear interaction between the annual, semi-annual and submensal scales. It has
been detected that this may occur particularly during extreme events. On the other hand,
striking results concerning the urban metropolis of SP city are the more energetic bands in the
semi-annual to submensal bands. These spectral results should be more investigated in future
work because they are closely dependent of the NDVI evolution signal. The WTA turns to be
useful for the formulation of conceptual models also in the ecological field.
REFERENCES
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