Environmental fragility and susceptibility mapping using geographic information systems : applications on Ribeirão do Pinhal watershed ( Limeira , State of São Paulo )

This paper presents the results of integrated environmental analysis of the Ribeirão do Pinhal drainage basin, undertaken with geographic information systems and spatial analysis techniques. The empirical analysis of environmental fragility methodology was used to identify areas that require more attention for improving environmental conditions. Because more than 60% of the study area has weak or very weak potential fragility grades, the natural characteristics of the basin may be considered appropriate. Regarding the environmental fragility, i.e. taking into account human actions, the basin has more than 50% of its area with weak or very weak grades. However, more than 40% of the study area has environmental fragility above their potential fragility grades. This situation indicates the presence of intensive land uses beyond natural landscape restoration processes. These sectors require that stricter territory management policies be implemented.


Introduction
One of the greatest challenges when selecting for analysis methods for environmental studies is dealing with a large set of data and information and producing diagnoses that can be interpreted in order to apply effective public policies in territory management and planning.In recent years, the use of Geographic Information Systems (GIS) in environmental studies has increased because of their great flexibility in managing and analyzing spatial information.In addition, improvements in the user interface have enhanced their utilization in diagnostic and environmental studies for professionals from several fields.
According to Ross (1994), analysis of natural and disturbed environments using geotechniques allows the understanding of study results by a wide range of professionals and even by people without technical training in areas related to environmental science.
The survey of physical and biotic resources that support anthropic action, such as topography, soils types and climate for environmental planning, should allow an integrated analysis, and thus identify the potential fragility of the natural system.
However, human action on the physical variables results in a series of processes that can lead to environmental degradation or, when performed within the limits imposed by the natural environment and the technology available, to the sustainable use of natural resources.Environmental fragility mapping seeks to determine the capability of the physical environment to support human intervention.
Underlying the mapping of environmental fragility is the methodology proposed by Tricart (1977), which suggests an ecodynamic classification of environments.Three categories are proposed in this approach, in which stable environments are characterized by dense vegetation cover to prevent the occurrence of mechanical processes of morphogenesis and are characterized by moderate dissection and absence of volcanism.In the intergrade environments there is a sensitive balance between morphogenesis and pedogenesis, characteristic of biogeographic transition zones.The strongly unstable environments have morphogenesis as the predominant feature in the dynamics of the landscape.That proposal will define the degree of sensitivity of the environment to natural and man-made phenomena (TRICART, 1977).
As stated by Ross (1997), Tricart's methodological proposal requires the inventory of either the natural or anthropic environment, and leads to a diagnosis that assumes an integrated evaluation and comparison of information.The procedures for the empirical analysis of the environments are described in Ross (1994).
The objective of environmental vulnerability study is to identify the most sensitive areas in a given territory and thus serve as a basis for guiding immediate action by policymakers interested in environmental management.
Thus the environmental fragility and potential fragility are compared to identify sensitive areas that require immediate attention.
For scales of greater detail, the use of slope maps is recommended to assess the relief.The limits of slope classes can be defined by land use capability surveys (LEPSCH, 1983) and geotechnical limits.
In assessing environmental fragility various degrees of protection are considered, which relates to different classes of land cover and their influence on soil losses according to studies of Bertoni and Lombardi Neto (1999).Types of vegetation that protect soil surface from the impact of raindrops and prevent runoff, such as perennial crops, forests and brushwood, show higher degree of protection.On the other hand, those types which expose soil surface or have low coverage are classified as less protective, such as annual crops and bare soil.Consolidated urban areas have high degree of watersealing, completely covering the soil surface, which results in high degree of protection.
Rosa and Ross (1999) present one of the first GIS applications for mapping environmental fragility, using map algebra and variables such as soil types, geomorphology, vegetation cover and slope maps to generate environmental and potential fragility maps.In a comparative study between the model of environmental fragility built both on relief dissection index or slope maps and the model based on Basic Territorial Units with GIS techniques, Spörl and Ross (2004) pointed out that assigning weights to different variables involved in the analysis is a subjective process.Donha et al. (2006) applied processes of multicriteria analysis and fuzzy logic to generate maps of environmental fragility, seeking to improve the assignment of weights to environmental variables and also incorporate distances of headwaters, rivers and dams as other relevant environmental factors.
To examine the relationship between urban sprawl and environment in the municipality of Santa Maria, Rio Grande do Sul State, Nascimento and Souza (2010) employed the methodology of mapping the environmental and potential fragility, incorporating geotechnical characteristics among the factors analyzed in GIS.In another territorial context, Cabral et al. (2011) mapped the environmental fragility with GIS techniques and used the Digital Elevation Model derived from radar interferometry from SRTM (Shuttle Radar Topography Mission) data to generate the slope map.
This research aimed to apply the methodology of environmental fragility mapping by using GIS techniques and spatial analysis to identify areas that require more attention in their management by the involved social actors, seeking preservation of environmental quality.

Material and methods
This study was conducted at the Ribeirão do Pinhal watershed, which most part is located at the county of Limeira, São Paulo State, Brazil (Figure 1).The watershed is delimited by the latitudes 22°15'S and 22°45'S and longitudes 47°30'W and 47°10'W.The basin drains land of the Depressão Periférica Paulista geomorphological province carved in sedimentary lithologies Tatuí and Irati formations of the sub-group Itararé, and in basic intrusive rocks (ALMEIDA et al., 1981).The landforms are characterized by gentle hills, more dissected in some sectors; the valleys are poorly restricted and river floodplains are rare and mainly along major river channels.This basin is an important water supplier for the city of Limeira, since it drains to the Tatu reservoir, water supplier of the city.
It is an area of predominantly rural land uses, with mainly sugarcane plantations and citrus orchards for orange juice companies, but also pig farming less often.Industrial plants are common along the main roads, and in recent years there was a significant increase in leisure farms.
The procedures for environmental and potential fragility mapping were applied for the Pinhal creek basin, covering an area of 30,100 ha.The procedures employed in this study are shown in Figure 2. The cartographic digital database of the project was produced by scanning topographic sheets on a scale of 1:10,000, provided by the Instituto Geográfico e Cartográfico.The vectorized layers were: road network, hydrography (lines and polygons) and altimetry (contour and elevation points).All procedures were developed in Ilwis GIS (WESTEN;FARIFTEH, 1997) and ArcGIS (ESRI, 1996).
The Digital Elevation Model (DEM) generation began with the interpolation of the contour and elevation points values.Filters were applied on the regular grid to generate a surface of continuous slope values, which was sliced into categories and fragility degrees were assigned according to Ross (1994) (Table 1).We carried out a semi-detailed (1:50,000 scale) soil survey of the watershed.Ranking of soil classes according to their fragility was based on soil conservation studies such as those described on Bertoni and Lombardi Neto (1999), considering as variables the attributes soil particle-size distribution (texture) classes, morphology, depth, textural discontinuity, among other elements determining soil fragility due to erosion.Soil mapping units fragility classes were defined according to Ross (1994), producing the soil fragility information layer (Tables 2 and 3).Soil textural differences within Oxisols allowed identifying different fragility degrees, since sandy or loamy soils or those with significant clay increase with depth (argillic-like horizons) received greater fragility grade than clayey or fine clayey ones.
The map of potential fragility resulted from a combination of the slope fragility and soil fragility layers onto a two-way table, as shown in Table 4. Visual interpretation of the color composite image by panchromatic band (5 m spatial resolution) and multispectral bands (23.5 m spatial resolution) from Resourcesat-1 satellite, complemented by field checking, producing the Land Use and Cover map.
Land use and cover were reclassified according to their protection degree, with higher grades given to classes with greater soil cover, which tends to reduce runoff and stabilize surface (Table 5).Protection refers to the degree of coverture that different uses provide to land surface, either preventing or enhancing the role of erosion processes.Land uses that keep soil covered most of the time with the lowest unprotected (uncovered) area provide best protection against raindrops, avoiding surface sealing, favoring infiltration and reducing runoff speed.
Potential Fragility and Protection Degree information layers were combined in a two-way table (Table 6), producing the environmental fragility map (ROSS, 1994).A diagnosis of vulnerable areas, i.e. those deserving priority attention by the players at the watershed, was also performed.This was done by combining potential and environmental fragility information layers in a two-way table.Areas showing increasing degree of environmental fragility as compared to potential fragility were classified as of emerging vulnerability; areas showing reduction in environmental fragility as compared to potential fragility were classified as of adequate vulnerability; areas of stable vulnerability were those in which there was no significant change of environmental fragility as compared to potential fragility, as long as they did not have a strong or very strong environmental fragility degree, in which case they were classified as of emerging vulnerability (Table 7).

Results and discussion
Since delineation of the potential fragility polygons were very close to those of soil class polygons, we assumed that spatial distribution of potential fragility was mainly associated to the mapped soil classes at the watershed, with relief taking a secondary role to this fragility.Thus, areas at the southern and northeast/eastern part of the watershed, where Lithic Udorthents predominate, had very strong potential fragility, followed by the central watershed portion with strong potential fragility, where Inceptisols predominate.Ultisols, Oxisols with clayey or very clayey kandic horizons and steeper-slope loamytexture Oxisols were associated to medium potential fragility.
Relief shape was responsible for increasing fragility in areas of low-fragility soils, which produced greater fragmentation in polygons of weak and medium potential fragility as compared to polygons of soil classes.Increasing fragmentation of the vulnerability polygons makes it difficult by the decision makers to generalize recommendations and to plan conservation strategies.In areas where intrinsic soil fragility is high, relief does not have relevant role on determining potential fragility.
Due to favorable intrinsic soil properties, very weak fragility predominates on Oxisols, such as those of the northern/northwestern watershed dividers.Toward drainage channels, as long as sloppiness increases enhancing runoff and erosion processes, potential fragility degree increases to medium or weak (Figure 3).Anthropic activities, by modifying the land uses, lead to changes in morphodynamics, represented by the environmental fragility.In the south, northeast and northwest parts of the catchment, the medium and strong classes of environmental fragility predominate, but they can be found in some small areas of very strong environmental fragility as well.Sugarcane, cultivated on Oxisols, is the main activity in these areas, but in the south and northeast parts of the catchment, the Entisols can be found and can represent high erosion risk when cultivated without adequate soil management.
In central parts of the catchment the lowenvironmental fragility classes dominate.In these areas, citrus and perennial crops provide better soil protection, even if cultivated on Ultisols and Inceptsols.(Figure 5).A certain increase of the environmental fragility was observed in these areas and it is associated with increased slope and a resulting increase in erosion.However, as can be seen in Table 8 and Figure 7, and considering that the anthropic uses that provide good protection of the soil slightly reduces the surface area occupied by the class of very strong environmental fragility, an increase was observed in medium environmental fragility classes.This shows that, despite the apparent suitability of land uses in the basin, there is an increase in the potential fragility of the land.This characteristic is associated with more intensive land uses in relation to the potential fragility.Those are deep soils with less clay and relief characterized by long and gently hillsides.Such soils are predominantly occupied by sugarcane.In its early stages, sugarcane has intense mechanization, resulting in low soil protection, increasing runoff and consequently soil erosion.
In order to guide the planning and rational use of the territory it was produced a map of environmental vulnerability (Figure 8).This map identifies the emerging areas where there was an increase of the potential fragility according to the land use type.In these locations land use has caused the greatest impact on natural resources, over the tolerable limit.In appropriate areas, the classes of land use have protected the ground surface, reducing soil loss and providing stabilization of morphodynamic processes.In stable areas, the degree of protection of the ground surface has the same degree of limitation of the potential fragility.

Cultivation
of sugarcane promotes environmental changes that go beyond the support capability of the natural resources from these areas, even in areas of low vulnerability (Figure 8).In sectors where relief and soil are more susceptible to erosion, uses that are consistent with these environmental restrictions could be identified, such as reforestation and perennial crops, resulting in more stable environments.
More than 40% of the area is classified as emerging, and have land use that cause pressure on natural resources.In these areas actions should be implemented for adequate management of natural resources (Table 9).However, the current situation can be considered delicate even with more than 55% of the area showing appropriate uses or showing uses that are less detrimental than those supported by the environment The trend of replacing citrus growing areas with sugarcane cultivation, even considering the higher operational requirements of sugarcane, indicates that the current environmental conditions, which are generally considered favorable, are changing towards more intensive uses (MORAES et al., 2008).

Conclusion
In the study area, considering the processes of external morphodynamics, environmental conditions are satisfactory.In general, current land uses are adequate to the natural features of the area.
Fragmentation of homogeneous areas of high fragility potential due to slope may limit land use planning.Areas of low fragility potential turn out to be areas of medium environmental fragility due to inadequate land use.
Critical areas of strong to very strong fragility and oncoming land use changes at the watershed, such as those with expansion of sugarcane, increasing pressure on natural resources, may require government and community to implement actions towards sustainable development.

Figure 1 .
Figure 1.Location of the study area.

Figure 2 .
Figure 2. Flowchart of the procedures.

Figure 3 .Figure 4 .
Figure 3. Potential fragility map of Ribeirão do Pinhal watershed.The studied watershed has larger areas of very weak and weak potential fragility and smaller areas of medium and very strong potential fragility.Significant occurrences of land with strong fragility (Figure4) are mostly associated to Inceptisols and Lithic Udorthents, which occur mainly on relatively steep slopes but also on narrow convex summits.

Figure 5 .
Figure 5. Environmental fragility map of the watershed.

Figure 6 Figure 6 .
Figure6shows the distribution of environmental fragility classes where large areas classified as weak and very weak environmental fragility are observed.The very strong environmental fragility class occurs in small areas, enhancing the apparently inappropriate use of natural resources.

Table 1 .
Fragility degrees attributed to slope categories.

Table 2 .
Fragility classes of soil mapping units.

Table 3 .
Description of soil mapping units.

Table 6 .
Environmental fragility classes resulting from combination of potential fragility and protection of land use classes.

Table 7 .
Vulnerability classes resulting from combining environmental and potential fragilities.

Table 8 .
Potential and environmental fragility.

Table 9 .
Distribution of classes of susceptibility.