Funcionamento hidrológico de nascentes em microbacias instáveis, Barra de Guaratiba - RJ

Abstract

The characterization and surveying of water supply from Barra de Guaratiba, RJ might be an important variable to environmental planning, as they come from 88 small watersheds (0,02 to 38 ha) with low capacity of water storage at the hillside with: medium declivity of 60% and shallow soils from the classes Cambissolo, Argissolo e Neosolo Litólico, with slopes varying its altitude from zero to 345 meters, with vegetation predominating at the medium and high portion of hill and the urban occupation at the bottom part (4.380 people). Annual rainfall (1.300 mm) is well distributed along year but it might occur tropical tempests. All these characteristics conferee high functional instability to ecosystems, where waters high velocity generates diverse erosive processes: sheet erosion, grooves, landslides, undermine process and displacement of rocks that can deposit at drainages, modifying local hydrological conditions. These combined factors conferee low administration capacity to the watersheds, being one of the reasons for reforestation of 96,3 ha by the Rio de Janeiro City County, enclosing 69% from springs water capiture areas. It was registered 28 water springs and 11 of them are located on six watersheds and are responsible for water supply to 150 families. Springs where classified by hydrological processes responsible for water adiministration on: Calhas entulhadas - water storage on river bed filled up with sediments (24 units); Encosta - storage by a slope subsoil structural controls (3 units); and Poço - deep water bed (1 unit). An encosta spring was studied in more details. It showed a Direct Influence Area (AID) of 300 m2, medium outflow of 5,00 l/h (dry period) and 7,36 l/h (rainy period), showing that micro-topographic conditions can determinate its perennity in function of speciffic hydrologhical processes. Spatial variation of soil humidity at AID indicated that saturation of recharge points occur at different regions and deepths, where soil texture explains partialy this variation. Distribution of the vegetal sponttaneous regeneration rectyfied humidity results. On areas with high humidity levels Shannon index reached H = 4,75 and species where distributed more equaly with a Simpson index of 0,96. Total Richness for regeneration was 91 species and Abundance of 458 individuals. Canopy cover varied little (from 79 to 81%) along the year, indicating constacy on water offer. Superficial soil layers (first 15-20 cm) are not dense, optimazing springs recharge. Soil Layer get more dense away from spring and with clay improve, acting as humidity reserve areas. Areas near the spring have low density sandy soils, facilitating water transmition and spring recharge. We could observe the formation of micro pipes that might be contribuing to springs water suply at the instable watersheds.