Morphometric analysis of an Areal Watershed in Taubaté, SP, Brazil

Abstract

In 1991, the Integrated Water Resources Management System (SIGRHI) in the State of São Paulo adopted the watershed as a territorial unit for studies, integrated planning and sustainable development. The morphometric analysis of small watersheds, which involves the characterization of geometric parameters, relief, drainage network, combined with land use and occupation, may constitute an important model for environmental analysis of larger watersheds. This study aimed at characterization of the morphometry in the Areal river basin, in the city of Taubaté, São Paulo. The study found 1.89 km² of area, 7.44 km of perimeter and 3.11 km in length of the basin axis, which allowed the calculation of the compactness coefficient (Kc = 1.51), form factor, (F = 0.195) and circularity index (CI = 0.43), indicating that under normal precipitation conditions, this basin is unlikely to be flooded, due to the distance from the unit, leading to smaller concentrate outflow. The results obtained for the Maintenance Coefficient (Cm) indicate that 260 m² is required to maintain each meter of perennial channel. The high values of drainage density (Dd = 3.35 km.km-2) and sinuosity index (Is = 0.95) indicate that drainage channels in the area are rectilinear, thus suggesting the occurrence of a high surface runoff associated with high dissection. The analysis of land use and occupation revealed that, from the seven types of vegetation cover, the dominant cover in the Areal basin is 0.756 km2 (40%) constituted of pasture, while forest represents 0.580 km2 (31%), 0.321 km2 (17%) of the land cover is constituted of degraded forest. In terms of conservation, the Areal basin is conserved because, besides being located in the environmental preservation area of the Una River Basin, it is located at the meeting of two ecological corridors and is occupied by only 8 families of rural producers. On the other hand, the creation of approximately 120 head of cattle in the upper part of the basin and the existence of gully erosion of the order of 0.018 km2 (1%) of the basin area, which led to increased runoff and sediment concentration in the flat areas marginal to the creek of Areal. Based on morphometric analysis, it was concluded that: Areal watershed presents high capacity to form new watercourses and their relief characteristics, with low sinuosity channels and high values of altimetric amplitude, channel gradient, and density. Drainage, allied to the existence of erosive processes, favors sediment flow and transport. Conservation actions are required to control erosion.