Mapping of natural and simulated risks of erosion and erosion expectation in a sub-basin for conservation agricultural planning
ISBN 978-85-88783-11-9
Authors
1Demarchi, J.C.; 2Zimback, C.R.L.; 3Piroli, E.L.
1UNESP Email: julio.demarchi@gmail.com
2UNESP Email: czimback@gmail.com
3UNESP Email: piroli@ourinhos.unesp.br
Abstract
The erosion and degradation of agricultural lands are caused by land uses and managements in disagreement with the natural support capacity and the attributes of the soil. As a consequence of the irrational land use, the annual soil loss by erosion in São Paulo State, Brazil, for example, is nearly 194 million ton, of which 48.5 million ton reach the water bodies in form of sediment, occasioning silting and pollution (BERTOLINI et al., 1993). This scenario implies the need of a proper land use planning, which may be carried out by means of a diagnosis of the current erosion process and the definition of proper farming crops and management techniques, according to the natural potential of erosion. This paper aims to determine and map the natural risk of erosion, the simulated risk of erosion and the erosion expectation of the Ribeirão das Perobas sub-basin, located in Santa Cruz do Rio Pardo, SP, in 2011, by means of Remote Sensing and GIS techniques. Such indexes derive from Revised Universal Soil Loss Equation (RUSLE), developed by Renard et al. (1997). The natural risk of erosion consists in the ratio between tolerable soil loss and the natural potential of erosion multiplied by the conservation practices (factor P of RUSLE) (DONZELLI et al., 1992). The higher its value, the lower the natural risk of erosion. It also indicates the maximum value of the factor C (land use and management) admissible so that the soil loss does not exceed the tolerance. The simulated risk of erosion represents the ratio between soil loss under certain conditions of land use, management and conservation practices, and the loss tolerance of the same soil (CAVALIERI, 1998). The erosion expectation indicates the degree of erosion that is expected in a certain place regarding the land use, management and conservation practices adopted, and consists of the subtraction of a given CP factor by the tolerable CP factor, derived from the division of the tolerable soil loss by the natural potential of erosion (VALÉRIO FILHO, 1994). The soil survey, soil loss tolerance and soil erodibility were elaborated by Demarchi and Zimback (2014) by means of soil sampling in the field, the technique of photopedology and the equations of Mannigel et al. (2002). The rain erosivity and the factor LS (slope length and slope) were determined by the softwares “netErosividade SP” (MOREIRA et al., 2006) and USLE-2D (VAN OOST; GOVERS, 2000). The maps of land use and conservation practices of January, 2011 were elaborated through ALOS and Landsat-5 TM satellite images. For natural risk of erosion calculation, the whole sub-basin was considered to adopt the contour tillage, the simplest conservation practice used in Brazil. The production of maps and GIS operations were performed in GIS Idrisi Selva. The map of natural risk of erosion indicated that 56.21% of the study area has “extremely low” to “low” erosion risk and present flat to medium slopes, being adequate to crops of less protection to the soil, whose minimum factor C is 0.048. The classes “very high” and “extremely high” are almost inexistent, and the classes “moderate” to “high” represent 43.64% of the total area, being adequate to pasture and forestry. The map of simulated risk of erosion revealed that 71.37% of the sub-basin presents the soil loss under the tolerance, but soil loss was greater ten times than the tolerance in 6.64% of its area. Areas of steep slopes, whose soils have low tolerance to erosion and/or are cultivated with crops of low protection to the soil, such as cassava, soybean, fruits or bare soil, presented higher simulated risk of erosion, above the tolerance. The map of erosion expectation indicated “absent” erosion risk in the same area whose simulated erosion risk is under the tolerable limits, because of the appropriate land use, management and conservation practices. The index was “low” in 14,37%, “medium” in 12.72% and “high” in 1.54% of the area due to more intensive cultivation practices.
Keywords
land use planning; GIS and Remote Sensing; watershed