In: Wagner W., Székely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B
Because the lakes under investigation are relatively small, the
resolution of Landsat images is somewhat marginally acceptable
when considering the mixed pixel problem. The fact that these
water bodies are smooth continuous surfaces let us postulate that
mixed border pixels have a predictable behavior and could be sub
sampled using some interpolation technique.
The objective of this article is to infer the dynamics of the fluctua
tions of the water level of the aquifer through the past monitoring
of the successive receding and inflating of the open water sur
face of six lakes found in the Veredas do Peruagu State Park and
surroundings. To achieve this goal it was necessary to:
1. create a valid methodology for the extraction of the open
water surfaces of these lakes from the historiacl series of
past Landsat images knowing that there would not be any
validation data for the past records;
2. establishing the relationship, if any, with the AW computed
from meteorological data.
2 MATERIAL AND METHODS
Because our objectives have a twofold aspect, our methodology
was also split. On one hand we needed to define a reliable ap
proach to extract systematically the contours of the lakes in the
long series of Landsat images given that we would only be able
do validate the data for two of these images (using one high res
olution image of 2006 a one geodetic survey of 2010). On the
other hand we want to use the data extracted from these lake con
tours to cross with AW data to verify our hypotheses that the lakes
surfaces are receding at an alarming rate.
2.1 Study Area
The study area (Figure 1) is located in Northern Minas Gerais
- Brazil, a savannah region that can be marginally classified as
semiarid with less than 900 mm of rain per year. The lakes un
der study are all inside or within the vicinities of the Veredas do
Peruagu State Park. The hydrographic network is part of the
Peruagu River Basin being a left tributary of the San Francisco
River. Rainfall is unevenly distributed during the year and is
mostly concentrated between November and March. The whole
region is mostly flat with deep soils composed mostly of sand and
less than 15% of clay that have a low capacity of water retention.
Figure 1 : Location of the study area in Northern Minag Gerais.
The lakes themselves are small with the largest having an aver
age area of around ten hectares. Six lakes are analyzed in this
article; they are from the largest to the smallest: Lagoa Formoza
Lagoa Azul, Ladoa da Sede, Lagoa do Meio, Lagoa Tres e Lagoa
da Pista (now dry) (Figure 1. Although there has been a few hy
pothesis to explain the genesis of these lakes and their relative
alignment, no conclusive results were ever presented. The last of
these lake (Lagoa da Pista) had open water until 2000 but has
dried up and is now but an intermittently saturated herbaceous
round field. Unofficial reports by the local population all outline
the gradual decrease of the open water surface of most of these
lakes but no actual study was ever undertaken.
Until the 1970s the region was occupied by small family groups
descended from the Indian tribe Xacriaba. In the middle of that
decade the Brazilian government offered subsidies and incentives
to companies that were willing to invest in eucalyptus plantations
for wood supply. This was also the beginning of a much denser
occupation of the area by workers and farmers. The impacts of
the plantations were reflected in the decrease of biodiversity, both
in terms of fauna and flora, and also by an increased pressure on
water resources. Plantings occurred until the early 1990’s, then
the companies abandoned the planting of eucalyptus in the region
due to the low productivity of the plantation that was not well
adapted to the natural conditions. The region was recognized as
having unique biological characteristics and the Brazilian author
ities created a national park (Cavernas do Peruagu) and a state
park (Veredas do Peruagu) to protect the natural beauties and the
archeological heritage (rock paintings) of the Peruagu watershed
(Maillard et al., 2009). Although the area is now protected by law,
the effect of the previous uses can still be observed and the area
surrounding the parks still suffer from human pressure, especially
on water.
2.2 Data and data pre-processing
Landsat images. Images from Landsat-5 TM were chosen for
the obvious reason that they constitute the largest multi-temporal
image bank existing today. Landsat-5 has been continuously col
lecting image data for the past 26 years. The period considered
by this research starts in 1984 and ends in 2009. In all, 53 im
ages were acquired from Landsat-5 TM and two from Landsat-7
ETM+, all for the orbit/scene 219/70 (World Reference System).
Table 1 shows the exact dates for the images. The dates of the
image correspond ideally to the end of the wet season (first im
age) and the end of the dry season (second image) but had to be
slightly shifted in cases where images were either of low quality
(clouds) or unavailable.
Year
jSt
image
2 nd
image
Year
2st
image
2 nd
image
1984
13/jun
13/oct
1998
20/jun
26/oct
1985
31 /may
06/oct
tl 999
19/mar
11/sep
1986
15/mar
09/oct
2000
24/apr
15/oct
1987
02/mar
12/oct
2001
24/mar
01/oct
1988
21/apr
30/oct
2002
20/apr*
13/oct*
1989
Excluded
Excluded
2003
20/jul
08/oct
1990
10/mar
20/oct
2004
01/apr
24/sep
1991
30/apr
07/oct
2005
04/apr
13/oct
1992
18/may
23/sep
2006
20/jun
30/sep
1993
18/mar
12/oct
2007
Excluded
03/oct
1994
22/apr
12/aug
2008
24/feb
05/oct
1995
24/apr
02/oct
2009
14/mar
06/sep
1996
26/mar
20/oct
2010
4/may**
1997
09/feb
07/oct
Table 1 : List of Landsat images (* indicates Landsat-7, the rest
are Landsat-5; ** the 2010 image was only used to validate the
lake contour extraction method).
The images had to be geometrically and radiometrically corrected
and an atmospheric compensation also had to be applied. The
