MAPPING OF EROSIVE FEATURES BY TYPE OF USE AND LAND COVER IN THE SUB-BASIN OF SÃO PEDRO RIVER, MACAÉ MUNICIPALITY – RIO DE JANEIRO STATE/BRAZIL Sara Regina de Araújo Neves – Geography Student (UFRJ) [email protected] Luiz Fernando Tavares Cardoso da Silva – Geography Student (UFRJ) [email protected] Hugo Alves Soares Loureiro – MSc Student (UFRJ) [email protected] Stella Peres Mendes – PhD student (UFRJ) [email protected] Prof. Antonio Jose Teixeira Guerra – PhD (Oxford); Professor of Geography (UFRJ) [email protected] Av. Athos da Silveira Ramos, 274. CCMN, Departamento de Geografia, Bloco I, LAGESOLOS - salas I1-009 e I1-011, Ilha do Fundão. CEP: 21.941.916 – Rio de Janeiro, Brasil ABSTRACT Soil degradation by erosion occurs as a serious environmental problem nowadays, due to soil mismanagement. Degraded areas mapping and studies related have been important to identify these areas and assisting the search for recovery and prevention measures. In Brazil, erosive processes occurrence is linked to soil inadequate use and management, aggravated by the absence of an integrated management plan by the government (Castro et al., 2010). These issues can be observed in the São Pedro river basin, which is one of the most important tributary basins of the Macaé river basin – very important on the northern of Rio de Janeiro State. The development of the mapping of erosive features and use and land cover map in the sub-basin of the São Pedro river, on 1:50,000 scale, and the relation among its results, established themselves as the research objectives. The erosive features were identified based on the software Google Earth and through the ALOS satellite images (2009), with the necessity for verification and confirmation of the features on the field. For mapping the features ArcGIS 9.3 was used, while for the preparation of the use and land cover map image bands of the sensor AVNIR-2 were processed in Spring 5.1.5 and ArcGIS 9.3. software. Through this analysis the occurrence of erosive features, there is predominance of the pastures, i.e., this type of use gives the appearance of sheet erosion features, rills and gullies. In addition, the use and land cover map shows that the sub-basin is well deforested. The original vegetation was replaced by grasslands, and to a lesser extent by agriculture (e.g. banana) and human settlement (urban areas) in various areas of the basin. INTRODUCTION Environmental degradation is increasingly present on the agenda of environmental debates. Among the various types of degradation is soil degradation. This type of degradation refers to the processes caused by human activities that adversely affect physical, chemical and biological nature, of one or more soil properties; these changes involving the deterioration of soil quality, which undermines their capacity (Morgan, 1981, in Bezerra, 2011; Hugo, 2006). The main cause of soil degradation is soil mismanagement, in rural and urban areas (Araújo et al., 2005; Morgan, 2005; Tonial et al., 2005; Guerra and Marçal, 2006; Mendes, 2007; Castro et al., 2010; Neves et al., 2011). Cunha and Guerra (2006) also claim that the very natural conditions coupled with inadequate soil management can accelerate its degradation. One type of soil degradation more severe and noticeable is water erosion. The adoption of the watershed as a planning and management unit of natural resources is important to evaluate the various elements of the landscape (climate, geology, topography, soil, population) in an integrated way, understanding the different processes. In terms of watersheds, their natural characteristics can contribute to slope erosion, and associated human activities cooperate in the environmental imbalance (Goulart, 1999). Thus, due to its importance in geomorphological studies and the insertion of the study area in a watershed, it was decided to adopt the term "sub-basin" as the scale of analysis for this work and for the mapping erosive features and their occurrence in accordance with use and land cover. It is in the highlands of the Rio de Janeiro State Macaé River in the town of Nova Friburgo, forming an outward drainage, in direction into the Atlantic Ocean. It is possible to verify on the drained lands by basin this river numerous signs of environmental degradation, resulting from centuries of human activities undertaken in the region, such as cattle raising and sugar-cane cultivation in the seventeenth century (Lima, 2008). São Pedro River sub-basin integrates Macaé River basin in its lower course. It is one of the main tributaries of the Macaé River basin. It has an area of approximately 420 km², and its divisors are limits with the municipality of Trajano de Morais, between the municipalities of Macaé and Conceição de Macabu. São Pedro River runs about 50 km, mostly inserted in Macaé municipality, intersecting Conceição de Macabu municipality in the lower course. The area is related to the environmental context of the Brazilian southeast region, marked by the evolution of the Serra do Mar. Its geomorphology is composed of scarped ranges, hilly domain and river plain areas. The scarped ranges are composed of crystalline rocks, with reworked morphology marked by structural control, with high slopes, with altitudes above 800 meters, the occurrence of talus deposits and rock outcrops, and they are areas susceptible to erosion and mass movements. This region is locally known as Serra de Macaé or Serra Macaense (CIDE, 1989; Cotrim, 2004). The hilly domain is characterized by agricultural and pasture use, with large areas of pasture, and development of small urban centers (Cotrim, 2004). In the areas of fluvial plain characterized by the presence of agriculture and pasture. Figure 2 illustrates, in general, the landscape found in the sub-basin São Pedro River. Figure 1. Location map of sub-basin São Pedro River. Figure 2. This picture illustrates, in general, the sub-basin landscape of the São Pedro river regarding the use and land cover: degraded pasture (terraces) and forest fragments. Rainfall is well marked throughout the year, causing a higher rain concentration in the summer months, exceeding 400mm (Nascimento et al., 2010). According to the mapping carried out by CPRM (2001, in English, RCMR - Research Company for Mineral Resources), 1:500,000 scale, the predominant soils in the watershed of the São Pedro River are Cambisoils, Litholic Soils, Oxisols, Aluvial Soils, Gleisoils and Organic Soils. The vegetation is characterized by grasses, which make up the landscape use for pasture, and forest, Ombrophilous dense type is fragmented and varied physiognomy. Problems such as rectification and damming of the channel, inadequate land management and removal of native vegetation, increase of natural grassland, and increased immigration process in the region over the years contributed to the framework of current degradation (Lima, 2007), especially soil erosion. The frequency and magnitude of erosive features become worrying, throughout the basin, since the degradation in some slopes becomes irreversible. Therefore, the elaboration of erosive features map and use and land cover map in the sub-basin São Pedro River, on a scale of 1:50,000, and the relationship between their respective results constitute the work objectives; its purpose is to catalog the different forms of erosion by identifying the types of use and incidence of sheet, rill and gully erosion features. THEORETICAL AND METHODOLOGICAL REFERENCE Soil erosion is a significant environmental problem in Brazil. Soil loss by removing sediment brings innumerable damages to human settlement, agricultural, landscape and ecosystems, mainly on the slopes. Therefore several authors ha studied erosion and his impacts in recent decades (Guerra and Mendonça, 2007; Netto and Sobreira, 2006; Lima, 2008; Bezerra, 2011) highlighting the importance of integrating structured on the management plan that addresses the socio-economic conditions and particularities of the environment. According Cunha and Guerra (2006), erosion is a phenomenon that occurs both from natural causes and human interventions in the environment. Oliveira (1999) describes that rills and gullies can be considered as incisions that result from the tendency of natural systems to achieve a state of balance between available energy and efficiency of the system to dissipate energy. Some soil uses are more likely to erosion as they take place on the areas for agriculture and pasture that are often marked by the presence of rills and gullies, originated when the capacity of water infiltration into the soil is exceeded, making the soil to suffer the action of sheet flow. These in turn can be deepen and broaden, raise bigger features that are gullies, classified as features with width and depth bigger than 0.5 m (Morgan, 1986). Selby (1993) and Guerra (2007) point out that the gullies often develop permanent erosive features in the landscape and it becomes difficult to control and their recovery when they reach that stage. Also according to Guerra (1999), permanent rills almost always develop into gullies, in other words, diffuse runoff until the formation of gullies there is a process chained, connecting multiple network channels in some landscapes. To help the identification of the areas under erosion process, remote sensing techniques have been used on the mapping and geographical studies. The use of images in flight level and orbital level has enabled the spatial analyze more integrated and at regular periodic intervals reliable. Remote sensing can be defined as a technique to obtain data for a particular object, areas or phenomenon; without that it occurs direct contact between the target and the sensor (Moraes Novo, 1992 in Barros, 2006; Luchiari et al., 2009). In this study, images were used to survey a use and land cover, and to recognize erosion features. For this purpose, the remote sensing products can be manipulated through digital image processing, that consist on using softwares, where various operations are made to improve the visual interpretation of the images. However, computational techniques are not fault-free and field surveys are essential, so surveys phenomenon and objects possible to represent. In geomorphology, for example, some processes are not recognized on remote sensing images. Together, remote sensing and Geographic Information Systems (GIS) have contributed to the comprehension of earth surface dynamics. Argento (2007) argues that without the GIS it becomes practically impossible the environmental projects development, so this presence on the geomorphologic mapping is indispensable. Thus, the identification of areas of erosive processes, usually due to incorrect use in some geomorphologic forms – as it occurs with agriculture and livestock – are important for environmental management and prevention of the degradation process. In this study, the mapping of use and land cover and erosion features identification the images from Japan satellite ALOS were carried out. There are three different sensors, but for this study only two were used: AVNIR-2 and PRISM. The first has four spectral bands, equivalent to blue, green, red and near infrared, has 10 m spatial resolution and 8 bits radiometric resolution, while the second has 2.5 m spatial resolution, one panchromatic band and 8 bits radiometric resolution. For mapping of use and land cover were used the software Spring 5.1.5, based on the three visible bands (red, green and blue) of the AVNIR-2 sensor. It is important to note that further tests will be made with the infrared band. It has been chosen the semi-automatic classification, where were made the segmentation, with similarity values of 10 and 10 area also, training and classification. Classification was made by Bhattacharya method, whose measures the average distance between the probability distributions for spectral classes (SPRING TUTORIAL, 2009). After this, were made final adjustments on the map through ArcGIS 9.3 software. The mapped classes were adapted according the work objective, distributed between, forest, agricultural and pasture areas, exposed soil, anthropic urban areas, rocky outcrops and other (including water and shadow). The recognition of the erosion features was made from a visual analyzes of the fusion images of the PRISM and AVNIR-2 sensors. The fusion image has allowed improving the visual aspect, allowing to recognize more relief details than analyzing the images separately. As a support field work was carried out and viewing in Google Earth, in order to eliminate the doubts. Once identified the points of erosion in the landscape, the correlations were made between the frequency of eroded areas and classes of use and land cover map. RESULTS By analyzing the location of different points of erosive features on the use of map and ground cover, it was found that there is a direct relationship between the type of use/cover and the occurrence of erosive features. This fact was observed both in the ALOS satellite image (2009), in field work, and Google Earth, where these features are located preferentially along the area of human settlement, where it gives the highest incidence areas cleared for grazing. It was identified 125 erosive features, classified as sheet erosion, rill and gully. These were identified during field work and visual analyzes of satellite images. The feature with the highest frequency was found gully type, with 49.6%, in other words, practically half of the total erosive features in the subbasin. Table 1 shows the total number of occurrence of erosive features and in percentage (%). Figure 3 shows these erosive features seen on the field. Erosive Features Type of erosion Amount Sheet Erosion 37 Rill 26 Gully 62 Total 125 % 29.6 20.8 49.6 100 Table 1. Number of occurrences of erosive features (total and percentage). Figure 2. Rill erosion, gully erosion and sheet erosion – in the sub-basin São Pedro River. Figure 3. Location map of erosive features identified. The use and land cover map (Figure 4) shows that São Pedro sub-basin has had its vegetation well cleared and, consequently, very degraded. The area associated with pasture and crop represents 47.3% of the total area, while the forest area has 46.2% coverage. However, it can be argued that the latter was overestimated by the model used in the software SPRING 5.1. Another aspect to highlight concerns the classification of agricultural and pasture areas, which are almost entirely dominated by pasture use and much reduced share for agriculture. The steepest areas are the best preserved due to the difficulty of settlement, unlike the smooth plains and gentler relief, along the main drainage channels, where human activities occur. Table 2 presents the percentages of the classes and area of use and land cover. Use and Land Cover Classes Km² Forest 193.5 Agricultural and Pasture 198.3 Others 14.1 Exposed Soils 1.0 Anthropic Urban Area 4.2 Rocky Outcrops 8.1 Total 419.2 % 46.2 47.3 3.4 0.2 1.0 1.9 100 Table 2. Classes of use and land cover in the sub-basin of the São Pedro river. Figure 4. Use and coverage map of the sub-basin São Pedro River. By overlaying the points of erosive features and the use and coverage map, it was verified that they are all under agricultural and pasture use (Figure 5). This shows that specific types of land use for agriculture and for grazing mainly provide the appearance of erosive features, leading to accelerated land degradation. Figure 5. Location map of the erosive features in each type of use and land cover in the sub-basin São Pedro river. CONCLUSIONS 1- Field work is indispensable for recognizing and confirming degraded areas. 2- The work of erosive features mapping through satellite imagery lacks a standard methodology and we have had to adopt an empirical method, in other words, using more visual recognition of features in the field than in the images themselves. 3- ALOS (2009) image was limited faced with visual possibilities of recognizing erosive features, it is extremely important to use Google Earth. 4- The concentration of erosive features in the grazing areas clearly demonstrates that the type of land use affects the development of erosion, if no preventive measure is taken. REFERENCES ARAUJO, G.H.S.; ALMEIDA, J.R.; GUERRA, A.J.T. (2005). Gestão ambiental de áreas degradadas. Rio de Janeiro: Bertrand Brasil, 320p. AGM-10, WMO/TD-No. 1428: Portland, Oregon, p. 127-146. Disponível em: http://www.wamis.org/agm/pubs/agm10/agm10_12.pdf. ARGENTO, M. S. F. (2007). Mapeamento Geomorfológico. 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