Evaluation of Desertification Processes in Seridó Region (NE Brazil)

doi:10.4236/ijg.2013.45B003

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International Journal of Geosciences, 2013, 4, 12-17

http://dx.doi.org/10.4236/ijg.2013.45B003 Published Online September 2013 (http://www.scirp.org/journal/ijg)

Evaluation of Des er ti fic ati on Pro cesses in

Seridó Region (NE Brazil)

Reinaldo Antonio Petta1, Leila Vespoli de Carvalho2, Stefan Erasmi3, Charles Jones2

1Department of Geology, Federal University of Rio Grande do Norte, Brazil

2Geography Department, Uni versity California Santa Barbara

3Fakultät für Geowissenschaften und Geographie

Email: pe [email protected], [email protected], [email protected]

Received June 2013

ABSTRACT

This paper outlines procedures to analyze the desertification processes in the semi-ar id Seridó Region (NE Brazil). Us-

ing the Geosystem theory, the detection of desertification areas was based on environmental indices, digital image

processing in multispectral analysis and Geographic Information System (GIS). The first step was to treat the rainfall

data and NDVI satellite Modis, aiming at identifying areas which do not present vegetation cover, even during the rainy

seasons. The second step was to work on a regional scale using Lan dsat ETM + images (2000-2005) and data collected

in the field, as the evaluations of exposed surfaces, that together w ith MDT/SRTM-NASA and thematic maps, allowed

to classify the altitude and slope of the relief, soils type, different morphologies and geology, and correlate them with

the areas susceptible to desertification process. The integration of the georeferenced data, related to these indicators,

allowed the identification of five differ ent levels of susceptibility to desertification (very high, high, moderate, low and

very low), and the geographic domain of each class. Based on the analysis of the dynamics of the vegetation cover, we

can establish that the main results refer that there is a decrease of the biomass at the region, associated with the dense

caatinga vegetation areas, but more important, with the scrub and degraded areas.

Keywords: Desertification; Caatinga; MODIS; Landsat; NDVI; Seridó; Brazil

1. Introduction

The United Nations Conference on Environment and

Development, (Rio-92), officially defined as “Desertifi-

cation”, [chapter 12 of Agenda 21], as “land degradation

in arid, semi-arid and dry sub-humid areas resulting from

various factors, including climatic variations and human

activities”. The susceptibility to desertification process

according the Agenda 21 [1], affects about one third of

the Earth surface (about 36 million km2), or 70% of the

dry lands of the world (arid, semi-arid and dry sub-hu-

mid), exc luding hy per-arid areas and deserts.

Many developing countries suffer from problems such

as soil degradation and destruction of natural resources,

including Brazil, where the most vulnerable area is located

in the NE semi-arid region. This area exceeds 900,000

km2, and a 20 million population, corresponding 44% of

the Northeast and nearly 10% of the population of Brazil

[2]. This region is characterized by high evapotranspira-

tion, the occurrence of periods of drought, narrow soils

and reduced water retention capacity, thus limiting their

productive potential. All these combined elements lead to

very fragile ecosystem, which is worsening mainly due to

degradation of vegetation cover and mismanagement of

land, due to the predatory exploitation of the particular

biomass known as Caatinga, deforestation and burning.

The state of Rio Grande do Norte (RN) is the 4th nor-

theastern state (on eleven) with number of municipalities

included in the semi-arid desertification susceptibility

area. About 50% of these municipalities belong to the

Seridó Region, located in the most vulnerable area to

desertification, with an area of 6,836 km2 and concen-

trating the largest number of inhabitants (216.500) of this

region. The objective of this study is to identify and map

areas of Seridó Region showing advanced process of

desertification, using Remote Sensing and Geographic

Information Systems (GIS) to quantify and evaluate its

impacts and influences on the regional environment.

2. Characterization of the Area

The region is located over a Precambrian crystalline

basement and shows nucleus of desertification due to

geoecological fragility, accentuated by the forms of land

use, using a model of development whose economic base

is not tied to environmental restrictions. Processes of

environmental degradation and desertification in various

stages of development are recurring, especially in spots

R. A. PETTA ET AL.

discontinuous and fragmented spaces that permeate most

part of the Caatinga (Figure 1). The area has a semiarid

vegetation which more typical feature are represented by

a shallow ground cover composed mainly of grasses,

above which emerge the shrubs and trees with low or

medium-sized, with lots of thorny plants, from lop down,

interspersed with various cactuses and bromeliads. These

environmental conditions in the Seridó Region are natu-

rally susceptible to erosion, desertification and degrada-

tion of its natural resources, and are accentuated by the

inadequate management of human activities.

Under the system of climatic classification of Thorn-

thwaite [11], the climate of this region is semiarid, me-

gathermal, with little or no hydric surplus and concentra-

tion of evapotranspiration in the summer months equal to

26%, reaching an annual average of 1.464 mm. The high

temperatures are shown throughout the year, reaching an

average maximum of 28˚C (with picks of 37˚C) and a

minimum of 22˚C. These climatic aspects cover a high

rate of evaporation, contributing to a large hydric deficit,

which marks the intermittent seasonal surface water [13].

The water deficit results from the infrequent rainfall

and high potential evapotranspiration. The rain is con-

centrated in three to four months and with great variation

between sequences of years with periods of extreme dry.

Associated with variation of water deficit, there are soil

always very shallow that has low water stock capacity

and low permeability, lithological discontinuities in the

profiles, and salinity of the surface layers. The most im-

portant systems that generate precipitation are: Intertrop-

ical Convergence Zone (ITCZ), formed mainly by the

convergence of the northeast trade winds and southeast

trade winds which are considered primarily responsible

for the period of heaviest rains in the RN state.

The variability, intensity and positioning in the equa-

torial Atlantic are closely related to thermal conditions

that occur in the tropical oceans (Atlantic and Pacific).

The region is under the influence of the South Atlantic

anticyclone, which provides the stability of the time most

of the year [3].

Figure 1. Seridó region (NE Brazil).

Table 1 shows the aspects of the distribution of rain-

fall throughout the year. In Table 1 we can observe the

rainfall data for the period 1961-2010, collected in Sta-

tion of Patos (PB) (Lat-70.02”, Long-37. 27”).

We can observe the data of rainfall distribution show-

ing the rain concentration on March-May. Is very evident

characteristic of seasonal rainfall distribution of this re-

gion, varying in time and sp ace, setting up a rainy se ason,

irregular, varying from 3 to 4 months in the beginning of

the year, and a prolonged dry season in the final months.

These observations are important because they will orient

the selection of MODIS images that will mark the distr i-

bution of rainfall periods.

3. Date and Methods

The Modis NDVI were used to identify areas that do not

show vegetation cover, even during the rainy seasons. In

a second step, the Landsat ETM+ images (2000-2005)

were worked on a regional scale and checked with data

collected in the field, as the evaluations of exposed sur-

faces. A model of MDT/SRTM-NASA (2003) (Shuttle

Radar Topographic Mission) refined at a resolution of 30

meters using the Bicubic Spline interpolation and pre-

pared with the software SPRING, has allowed classify

the altitude and slope of the relief, different morpholgies

and correlate them with the areas susceptible to envi-

ronmental degradation (Figure 2).

A 16-day composite Terra/MODIS obtained image

during Feb. 1982 to May 2007 (121 full scenes of

MOD13Q1 data) was downloaded from NASA’s DAAC

web site (http://daac.gsfc.nasa.gov/data/). The MODIS

image was firstly geo-rectified and resized into 30 m

Table 1. Average rainfall (mm) (1961-2010) on the studied

area (INMET-Brazil).

Trimester Lower Limit Average Upper Limit

Jan-Feb-Marc 361 437 500

Feb-Marc-Apr 381 505 590

Marc-Apr-May 291 425 513

Apr-May-Jun 141 254 309

May-Jun-Jul 58 109 136

Jun-Jul-Aug 25 49 65

Jul-Aug-Spt 8 20 26

Aug-Spt-Oct 3 11 11

Spt-Oct-Nov 3 11 15

Oct-Nov-Dec 18 48 51

Nov-Dec-Jan 78 135 152

Dec-Jan-Feb 178 270 313

R. A. PETTA ET AL.

Figure 2. Relief, morphologies and correlate areas susceptible to environmental degradation.

resolution to match the TM image, and then, a relatively

atmospheric correction was applied to minimize the ef-

fect of atmospheric conditions difference between these

images. All procedures were performed using the GIS

and image processing, ArcGis and ENVI. To compose

the database, three sets of data were used: (1) the availa-

ble data in systematic mapping (1:100.000): altimetry-

landforms, settlements, roads, hydrography, (2) the the-

matic mappings on regional scale (between 1:100.000

and 1:500.000), such as hydrogeological potential, geol-

ogy, lithological resistance, soil types, class of land use,

underground water wells, mineral occurrence, (3) data

obtained from digital processing of remote sensing data,

such as levels of vegetation cover classes and fragmenta-

tion of vegetation cover.

3.1. NDVI MODIS Images

Vegetation change has been successfully observed at

scales ranging from local to global using the Normalized

Difference Vegetation Index (NDVI), derived from satel-

lite data [5-7,15]. The NDVI minimizes the effects of

topography and atmosphere, requires no prior knowledge

of ground conditions, and is sensitive to the amount of

photosynthetically active vegetation present [4]. The

change in the value of the NDVI between 1982 and 2007

obtained from imagery acquired by the MODIS satellite

was used for this indicator. Due to the fact of the sa tellite

MODIS present a constant imaging we could process a

complete series of images month to month. First they are

processed and analyzed the MODIS images with the

purpose of evaluate areas with vegetation deficit in the

dry and on rainy periods, selecting which ar eas wo uld b e

analyzed in a second moment with the Landsat images.

First results of the MODIS cloud flagging attempts

with 5 masks (3 cloud + 2 shadow) produce large masked

areas. Further Investigations have to be carried out using

only 3 cloud + 1 shadow mask (shadow2) or using only

the cloud masks. Despite of the large coverage of masked

areas, these 5 masks cover the cloudy and shadowed

areas fits good results. After repeat this same procedure

with all the set of five years images and filtering of time

series data, we have the overview of this period and the

different season of dryness and rain. In this series of 121

Modis images from 1982 to 2007, it is possible to identi-

fy very clearly the positive peaks that mark the rainy

seasons and negative peaks that identify the dry seasons

(Figure 3).

The integration of data generated from the processing

of MODIS images allowed to id entify which areas of the

Seridó Region showed lack of vegetation even during the

rainy seasons and thus these areas were selected for the

next step which was centered on the processing of Land-

sat images to map and quantify the areas in the process of

desertification.

R. A. PETTA ET AL.

Figure 3. Standardized anomalies (Z-scores) of Normalized Difference Vegetation Index (NDVI) and rainfall data for North-

east Brazil between 1982 and 2007. The diagrams show the anomaly of a single month to normalized long term monthly mean

values (1982-2007).

3.2. Landsat 5-TM and Landsat 7 ETM+ Digital

Analysis

For mapping the desertification level of the Seridó Re-

gion, on the selected areas establish in the anterior im-

ages processing, were used images of the sensor

LANDSAT-7 ETM+, cut according to the coordinates of

the study area. These images were taken between Sep-

tember of 1999 and in the consecutive years, more pre-

cisely in March and Setember/2001, April and Octo-

ber/2002, September/2003 May and September/2004 and

April 2005. This aspect of multiple temporalities of the

images is important, because it allows the comparison of

the spectral response of the land features in conditions of

different humidity, as well as the monitoring of the evo-

lution of the patterns of anthropogenic activity in the

area.

The digital analysis was processed directly in the

software ENVI and the mapped classes were transformed

of the format raster for the vectorial format, being after

exported for the software ArcGIS where it was made the

last adjustments (in the mapped classes) in the intention

of obtaining the physiographic maps of Geology, Geo-

morphology, Soils and Vegetation (Figure 4(a), (b), (c)

and (d)) for this region, base of the identification of areas

with desertification process.

The normalized vegetation index (NDVI) was used to

identify unvegetated regions and generate the Desertifi-

cation Map (Figure 5). The field evaluation and the phy-

siographic aspects combine with the low values of this

index were used to represent in this map the areas with

stronger process of desertification. Comparisons were

made with composite images to derive the threshold, and

then unvegetated areas were identified.

4. Results

The map of the degraded areas susceptive to the deserti-

fication processes (Figu r e 5) it evidences in this condi-

tion of analyze, that has a great numbers of area located

on the center, northwest and north of the studied area,

that present serious levels of the desertification process.

The total of the areas where the phenomenon is verified

reaches 638 km2, that represent 27.26% of the total area

of Seridó Region. It is in these municipal districts of the

focused region where it is the largest extension of areas

in process of environmental degradation and susceptive

desertification process.

In Caicó District the spots that represent the degraded

areas are concentrated on the south, southeast and north-

east of the municipal area. The total of the areas com-

mitted by the degradation reaches 29.588 hectares, in

R. A. PETTA ET AL.

Figure 4. Maps of Geology (4A), Geomorphology (4B), Soils (4C) and Vegetation (4D).

Figure 5. Areas in process of desertification (dark spots).

R. A. PETTA ET AL.

other words, 17.59% of the total area of the municipal

district. The municipal district of Serra Negra constitutes,

in Medium Seridó, the least affected for the observed

environmental phenomenon, presenting dispersed scat-

tered spots in all its extension, with a larger concentra-

tion on southwest and northeast. In quantitative terms,

the degradation areas represent 7.434 hectares, corres-

ponding to 11.34% of its total area.

The special comparison was done of NDVI indexes

derived from Modis and current Vegetation NDVI on

Landsat. This study has shown that to compile the maps

of different images parameters of the same ecosystems,

including desertification depth (through special methods),

desertification yearly dynamics (through change/time

series analysis), vegetation monthly and seasonal index

dynamics (through NDVI change/time series analysis etc.

is possible to define and analyze spatial structure of

semi-arid ecosystems.

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