In: Wagner W„ Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010,1 APRS, Vol. XXXVIII, Part 7B 
INFERRING FLUCTUATIONS OF THE AQUIFER BY MONITORING THE AREA OF 
SMALL LAKES IN A BRAZILIAN SAVANNA REGION USING A TEMPORAL 
SEQUENCE OF 50 LANDSAT IMAGES 
Carlos H. Pires-Luiz and Philippe Maillard 
IGC, Department of Geography 
Universidade Federal de Minas Gerais 
31270-901, Belo Horizonte, Brazil 
carloshpl@ig.com.br, philippe@cart.igc.ufmg.br 
KEY WORDS: Multi-temporal, Water index, Water level monitoring, Interpolation, Hydrologic Budget 
ABSTRACT: 
Water availability is subjected to a complex dynamic involving fluctuation of the aquifer level, itself subject to climatic and edaphic 
factors as well as land use and land cover. Human pressure can have a drastic effect on the aquifer level, the effect of which are 
often only noticeable after years of continuous usage. In this article we are using a temporal sequence of 50 Landsat images to study 
a complex of small lakes in Northern Minas Gerais. Our objective is to quantify the fluctuations of the aquifer for the 1984-2009 
period by monitoring the area of these lakes bi-yearly and compare it with the hydric balance to understand its evolution. Having no 
measurements of the aquifer, we developed a methodology entirely based on remote sensing and meteorological data. Because these 
lakes are very small, the 30 m resolution of the Landsat data rendered very poor results. Based on the postulate that the water pixels 
behave like a smooth continuous surface, we increased the resolution to 5 m using minimum curvature interpolation. Two methods were 
tested for extracting the lake surface pixels: supervised classification and thresholding of the normalized difference water index. The 
results show that the interpolated Landsat data compared well with a high resolution Ikonos image of the same date and can improved 
the contouring of water bodies. For extracting the water pixel the classification approach performed better by about 15%. Statistical 
tests showed that the lakes have been systematically decreasing but that these changes cannot be attributed to climatic factors. 
RÉSUMÉ: 
La disponibilité en eau est soumise à une dynamique complexe impliquant la fluctuation du niveau de l’aquifère, elle-même soumise à 
des facteurs climatiques et édaphiques, ainsi qu’au couvert et à l’utilisation du sol. La pression humaine peut avoir un effet drastique 
sur le niveau des aquifères, dont l’effet n’est souvent perceptibles qu’après des années d’utilisation continue. Dans cet article, nous 
utilisons une séquence temporelle de 50 images Landsat pour l’étude d’un complexe de petits lacs dans le nord du Minas Gérais. Notre 
objectif est de quantifier les fluctuations de l’aquifère pour la période 1984-2009 par le suivi bi-annuelle de l’aire de ces lacs et de les 
comparer au bilan hydrique afin de comprendre leur évolution. N’ayant aucune mesures du niveau de l’aquifère, nous avons développé 
une méthodologie basée entièrement sur la télédétection et les données météorologiques. Comme ces lacs sont très petits, la résolution 
de 30 m des données Landsat donnait de piètres résultats. En se basant sur le postulat que les pixels d’eau se comportent comme une 
surface lisse et continue, nous avons augmenté la résolution à 5 m par interpolation de courbure minimale. Deux méthodes ont été testées 
pour l’extraction des pixels surface du lac: la classification supervisée et l’application d’un seuil sur l’indice différentiel normalisé de 
l’eau. Les résultats montrent que les données interpolées Landsat se comparent bien avec une image Ikonos de haute résolution de la 
même date et permettent d’améliorer l’extraction du contour des plans d’eau. Pour extraire les pixels d’eau, la classification supervisée 
a fourni des résultats à peu près 15% supérieurs. Des tests statistiques ont montré que les lacs ont systématiquement diminué, mais que 
ces changements ne peuvent être attribués à des facteurs climatiques. 
1 INTRODUCTION 
The watershed of the Peruagu River hosts two important pro 
tected areas totaling over 80,000 ha yet it suffers from strong hu 
man pressure for water which is the most sought resource in this 
semiarid zone. In particular, the Veredas do Peruagu State Park 
is apparently suffering from continuous lowering of its aquifer 
which is observable from the few small lakes inside the park and 
one larger lake outside. Although the phenomenon is quite ob 
vious to the local population, it still needed to be demonstrated 
in a scientific non-refutable manner. One such argument is that 
the lowering could be caused by local changes in the precipita 
tion and water balance (AW). Since no records of the level of the 
aquifer or the lakes are available for the past, we had to develop a 
methodology entirely based on historical remote sensing and me 
teorological data to unambiguously demonstrate and quantify the 
phenomenon. 
Although human occupation can be considered sparse, but since 
the Peruagu watershed is relatively small (1450/cm 2 ) and the re 
gion receives almost no precipitation during seven months of the 
year, we argue that the pressure of the irrigation for agriculture, 
eucalyptus plantations and the numerous wells that have been dug 
in the past 30 years is too great for the capacity of the watershed. 
A remote sensing multi-temporal approach was chosen to create a 
time sequence of images to monitor the size of the Peruagu lakes 
and Landsat images stood as the most logical choice for analyz 
ing the dynamics of these lakes for being the largest record of 
systematical remote sensing data available for civil use. Passive 
optical infrared images are also considered the most effective type 
of data for delineating water bodies since they absorb almost to 
tally the incoming radiation and produce a sharp contrast with the 
surrounding vegetation and soil (Bonn and Rochon, 1992; Jensen, 
2005).