ESTIMATES OF LAND USE AND LAND COVER CHANGE AND SOIL LOSS IN THE BRAZILIAN CERRADO THROUGH GEOTECHNOLOGY

Kleber Trabaquini1, Moisés Salgado Pereira Galvão2, Antônio Roberto Formaggio3, Lênio Soares

Galvão4

1, 2, 3, 4 National Institute for Space Research – INPE São José dos Campos – SP – Brazil

ABSTRACT

The Cerrado biome, with approximately 206 million hectares of Brazilian territory, is undergoing significant changes in recent decades due to land use and land cover changes. The agricultural activities are identified as the main sources of rapid reduction of native vegetation in the Cerrado in the last decade. The objective of this study is to evaluate changes in land cover that occurred in recent decades in a watershed located in the state of Mato Grosso, Cerrado biome, and therefore assess the soil loss from erosion, using the USLE model. Results showed that 15% of the area had already been converted into agriculture in 1980. Results for the years of 1990, 2000 and 2010 were 37%, 51% and 61%, respectively. The soil loss rates estimated from the USLE model showed that the average annual soil loss increased significantly with time.

Index Terms— Cerrado, remote sensing, land use and land cover, agriculture, soil loss.

1. INTRODUCTION The Cerrado biome, with approximately 206 million hectares, corresponds to 22% of the Brazilian territory comprising one of the most biologically rich savannas in the world [1]. However, it is undergoing significant changes in recent decades due to land use and land cover changes, which have been little documented. Therefore, systematic efforts are needed to better understand the dynamics between agricultural expansion and deforestation. The agriculture in the Brazilian savanna areas, which was considered a challenge in the past decades due to unfavorable soil composition, has become profitable after several studies carried out to introduce new techniques for soil remediation and to develop crop varieties adapted to this environment. As a result of the soil and crop management improvements, there was an expected replacement of the original savanna vegetation by the crops [2]. Estimates indicate losses ranging from 40 to 80% of natural vegetation [3][4][5]. The cattle ranching was the pioneer anthropogenic activity that provided the highest rates of deforestation in the Cerrado until mid-1985. In the last years, the agricultural practice became the main agent of change in this biome [6]. Agricultural activities such as cultivation of soybeans, corn and cotton are identified as the main factors of rapid agricultural expansion and consequent reduction of the native vegetation of Cerrado in the last three decades.

However, the practice of deforestation in this biome is sometimes accompanied by negative impacts on soil degradation. The dynamics of replacement of native vegetation with subsequent inappropriate land use, mainly by intensive agriculture, has caused large losses of soil through laminar erosion [7]. Degradation of large areas under agricultural use has been observed. In order to obtain estimates of soil losses, several authors [8][9][10][11] have used the Universal Soil Loss Equation (USLE) [12] to predict the medium and long-term soil loss by erosion based on time series analysis and on results from the planning to promote conservation practices to minimize losses at acceptable levels. Therefore, in this context, the present study aimed to evaluate land cover changes that occurred in recent decades in the Rio verde watershed, located in the Mato Grosso State (Cerrado biome). The objective was to evaluate soil loss from erosion using the USLE model and remote sensing and geographic information system (GIS) techniques.

2. METHODOLOGY The study area encompasses the Rio Verde watershed which is included in the Amazon basin as one of the tributaries of the Teles Pires River. The watershed is located in the state of Mato Grosso, between the coordinates 13°17´ and 14°20´ south latitude and 55°10´ and 55°50´ west longitude. Its drainage area is approximately 3,141.13 km2 (Figure 1). Based on the temporal analysis of satellite Landsat images, land use and land cover changes were evaluated. For this purpose, we used the following images and dates: Landsat-2/MSS (07/28/1980); Landsat-5/TM (08/02/1990); Landsat-5/TM (08/13/2000); Landsat-5/TM (07/24/2010). The detection of land use and land cover changes was performed by supervised classification of the images using the Bhattacharya distance, followed by small adjustments in erroneously classified areas whenever necessary. We defined only two classes for simplification: Agricultural Areas and Native Vegetation (Cerrado). The Agricultural class included crops and livestock, the latter represented by less than 1% of the study area. Fieldwork was performed in two campaigns: December 2010 and August 2011. After classification of Agricultural and Native Vegetation, we overlaid the map of vegetation of Seplan-MT [13] in the scale of 1/250,000 to better evaluate the deforestation trends over wooded savannas and ombrophilous forest areas. In order to estimate soil loss from erosion, we used the model USLE (A= R.K.L.S.C.P), which provides the results of soil loss (A) in t.ha-1.yr-1.

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Figure 1. Location of the study area in the Brazilian Mato Grosso State (a) and the Rio Verde watershed (b). The R factor (rainfall erosivity index, MJ.mm.ha-1.h-1) was defined based on the procedures described by [14] using the rainfall station data provided by Hydrological Information System of NWA (National Water Agency). The K factor (soil erodibility, t.ha.h.MJ-

1.mm-1) was obtained from the soil map by Seplan [15] for the two predominant soil types: dystrophic Red Latosoil and dystrophic Red Yellow Latosoil. The L and S factors (slope length and steepness of the field, respectively) were calculated with the USLE-2D software [16], based on the DEM (Digital Elevation Model) from the SRTM (Shuttle Radar Topography Mission). The C (soil use and soil management) and P (conservation practices) factors were calculated through the methodology from [17].

3. RESULTS

Figure 2 shows land use and land cover changes that occurred in the study area from 1980 to 2010. In 1980, 15% (489.36 km2) of native vegetation had already been converted into agriculture (Figure 2a). In the years of 1990, 2000 and 2010, the conversion was faster and reached values of 37% (1,178.50 km2), 51% (1,614.96 km2) and 61% (1,917.87 km2), respectively (Figures 2b, 2c, 2d). This rapid conversion can be explained primarily by the advance of the soybean cultivation. Because of the low price of the land and the topography favorable to mechanization, a great number of soybean producers from southern Brazil moved to central Brazil (Cerrado) in 1990. As a result, large soybean farms were consolidated [6][18]. The mechanized soybean areas were then one of the major agents of deforestation in Mato Grosso State at that time. The deforestation in Mato Grosso reached values up to 12,000 km2 in 2004 and decreased in the subsequent years (Figure 3). A similar deforestation pattern was observed in the study area, but the soybean production still increased even with reduction in deforestation after 2004 (Figure 3) due to improvements in management practices and the conversion of pasture into soybean areas. In fact, as shown in Figure 2, most of the Cerrado biome was strongly impacted by the agriculture between 2000 and 2010. Preserved areas are mostly associated with the riparian vegetation kept due to the environmental legislation for Legal Reserve and Permanent Preservation Areas (Forest Code - Law 4711 of 1965), as shown in Figure 2 (d).

Figure 2. Land use and land cover changes for the Rio Verde watershed in the years of 1980 (a), 1990 (b), 2000 (c) and 2010 (d). The color composites refer to TM bands 5(red), 4(green) and 3(blue). The relationships between the cropland expansion dynamics and deforestation have been discussed in previous studies [19]. The Rio Verde basin is located in a region of highly intensified agricultural production (Nova Mutum, Lucas do Rio Verde and Santa Rita do Trivelato municipalities). Much of the deforested areas are related to agricultural expansion, as shown in Figure 2, producing environmental impacts such as soil degradation. Figure 4 shows the rates of estimated soil loss from the USLE model grouped and represented by micro watersheds (0 to 23) of the study area from 1980 to 2010. The average of the R factor was 13,039.20 MJ.mm.ha-1.h-1, which is considered a very high rainfall erosivity index [20]. The K factor was 0.016 for the dystrophic Red Latosoil and 0.023 for the dystrophic Red Yellow Latosoil, which represents low erodibility [15] and relatively good resistance to erosion under natural conditions and appropriate management. The LS factor showed values lower than 1 considered as low [22], which is consistent with the flat terrain of the region and slopes ranging from 0 to 3%. The LS factor is directly proportional to the capacity of sediment transport. The highest values are found in regions of higher slopes with high probability of carrying soil sediments [22].

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Figure 3. Deforestation in Mato Grosso in km2 from 1988 to 2010 (Source: Prodes [21]); Planted area of soybeans in the Rio Verde watershed in km2. The P and C factors, calculated using the methodology described in [17], were consistent when compared with other studies [8] [9] [10] [11]. We observed that the average annual soil loss had higher rates where there was major dynamics of deforestation. The losses of 12.5 t.ha-1.yr-1 observed for deep well-drained soils are tolerable and were classified as moderate losses [23]. The assessment of soil loss tolerance is crucial for the choice and adoption of practices that minimize soil degradation. Soil loss tolerance can be defined as a maximum rate of erosion that may occur without reducing productivity while keeping the cost of production viable [24]. With the exception of the sub-basins numbered with 0, 1, 3, 14 and 16, losses in the Rio Verde watershed were considered low with average rates less than 12 t.ha-1.yr-1.

Figure 4. Soil loss (t.ha-1.yr-1) estimated from the USLE model for all watersheds (0-23) in the Rio Verde basin for the years 1980, 1990, 2000 and 2010. Figure 5 shows the conversion of forest into agriculture in the watersheds of numbers 3 and 4. In 1980, when most of the watersheds were covered by natural vegetation, the USLE model estimated a soil loss of 1.94 t ha-1.yr-1. In 2010, with the almost complete removal of the natural vegetation of Cerrado, the model indicated a loss of 12 t.ha-1.yr-1. In areas where natural vegetation has been preserved, there was low loss, as found in the watershed of number 12, where rates did not exceed 6 t.ha-1.yr-1 in 2010. The preserved natural vegetation is considered a decisive factor for the control of soil erosion. The presence of vegetation has as main advantages a higher water infiltration, a reducing runoff and, thus, a reducing soil loss [25].

Figure 5. Dynamics of forest conversion into agriculture and soil loss for the micro-watershed 3 and 4 for the years 1980 (a), 1990 (b), 2000 (c) and 2010 (d). Finally, we concluded that satellite imagery and the integration of remote sensing and empirical models, such as USLE applied in large areas and in environments with highly dynamic land use changes, allowed a rapid assessment of the deforestation. Data integration facilitates the understanding of the conservation state and degradation of the environment. They provide support to define recovery plans and conservation areas where the agriculture activities may become an agent of soil degradation in watersheds. ACKNOWLEDGEMENTS

We thank the Brazilian Research Councils: CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, contract 304928/2011-9) and FAPESP (São Paulo Research Foundation, contract 2009/01894-0) REFERENCES [1] Klink, A.C.; Machado, R.B. Conservation of the Brazilian Cerrado. Conservation Biology, v. 19, n. 3, p. 707–713, 2005. [2] Brannstrom, C.; Jepson, W.; Filippi, A.M.; Redo, D.; Xu, Z.; Ganesh, S. Land change in the Brazilian Savanna (Cerrado), 1986–2002: Comparative analysis and implications for land-use policy. 2008. Land Use Policy, 579–595. 2008. [3] Ratter, J.A.; Ribeiro, J.F.; Bridgewater, S. The brazilian Cerrado vegetation and threats to its biodiversity. Annals of Botany 80: 223-230, 1997. [4] Joly, C.A.; Aidar, M.P.M.; Klink, C.A.; Mcgrath, D.G.; Moreira, A. G.; Moutinho, P.; Nepstad, D.C.; Oliveira, A. A.; Pott, A.; Rodal, M.J.N.; Sampaio, E.V.S.B. Evolution of the Brazilian phytogeography classification systems: implications for biodiversity conservation. Science and Culture, 51(5/6): 331-348. 1999. [5] Sano, E.E.; Rosa, R.; Brito, J.L.S.; Ferreira, L.G. Land cover mapping of the tropical savanna region in Brazil. Environmental Monitoring Assessment, p.2780-2792. 2009. [6] Jepson, W. A disappearing biome? Reconsidering land-cover change in the Brazilian savanna. The Geographical Journal, vol.171, no. 2, June 2005, pp. 99–111.

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