Mapeamento de áreas de soja em municípios da metade sul do estado do Rio Grande do Sul a partir de imagens de satélite

Abstract

Soybean cropped area rapidly increased over the last few years in southern Rio Grande do Sul State (RS), Brazil, which is very occupied by hydromorphic soils and irrigated rice. However, the soybean cultivation over those soils can greatly impact crop yield. The context in which cropped area maps are very important for agricultural planning purposes and environmental studies, the main aim of this study was to develop an objective methodology to identify and map soybean fields in municipalities of southern RS by using landsat-like images. Thus, eight images covering the municipalities of Bagé and Hulha Negra during 2013/14 crop year were used. In order to produce a reference map, soybean fields were first mapped by an unsupervised classification followed by a careful multispectral and multi-temporal visual interpretation. The four main images were simultaneous and individually analyzed to evaluate their mapping accuracy. To better understand the soybean crop season over the study area the spectro-temporal profile of MODIS Enhanced Vegetation Index was also assessed. Soybean reference map was also evaluated with soil, altitude and slope maps. Results showed that higher mapping accuracy (~95%) was obtained using images acquired in late December, mid-January, and early and late March and major contribution for soybean mapping was provided by the image acquired on early March. The analysis of Landsat and MODIS images indicated that for the soybean mapping in municipalities of southern RS it is necessary at least one cloud free image acquired every 40-50 days, from December to April and it is very important an image acquired between mid-February and mid-March. The majority of soybean fields was below 200 meters (55.7 and 48% for Bagé and Hulha Negra, respectively) and above 3% slope (90 and 99.2%, for Bagé and Hulha Negra, respectively). In Bagé, NEOSSOLO LITÓLICO, PLANOSSOLO HÁPLICO and LUVISSOLO HÁPLICO comprised about 70% of the soybean fields. In Hulha Negra, almost 78% of soybean fields were on the VERTISSOLO EBÂNICO, ARGISSOLO VERMELHO and CHERNOSSOLO ARGILÚVICO.