Comportamento hidrológico de pequenas bacias hidrográficas pareadas com campo e eucalipto no bioma Pampa.

Abstract

With the growing demand for forestry, especially eucalyptus and pine plantations for the production of wood, pulp and resins, concerns have increased about the impacts on the conservation of water resources, especially in relation to water consumption. Thus, this study aims to evaluate the flow regime in different watersheds with eucalyptus species and native grasslands located in a region characteristic of the Pampa Biome in the state of Rio Grande do Sul. The focus is on nine small watersheds located within the municipalities of Alegrete, Eldorado do Sul, São Gabriel and Pinheiro Machado. Flow permanence curves were performed, analysis of the behavior of the flows in the period before and after the cut. The base flow index was calculated by the Lyne and Hollick method, using a digital filter. Piezometers were used to estimate the water table in São Gabriel. For the rainfall-runoff relationship the number curve (CN) method was used. The analysis of hydrological behavior for São Gabriel determined that the basin with E. saligna has more constant flow throughout the period studied, producing more runoff during precipitation, while the basin with E. benthammii presents greater variability in flow, and produces less runoff. For the separation of runoff the values of recession coefficients were close to 1.0 indicating the potential for water retention in the basin, the values of the correlation coefficients were mostly higher than 0.9, indicating that the recession curves fit well to the exponential model. The runoff coefficients presented extremely low values, less than 0.01. The CN values varied from 26 to 100, being these associated with the combinations of land use and occupation area with the area of the respective hydrological soil group. For all watersheds the variation of CN values observed is significant, ranging from 26 to 100 for forested watersheds and from 62 to 100 for field watersheds. For the municipality of São Gabriel, the specific flow rates in the basins with eucalyptus were influenced by the stage of development of the plants. The basin with E. saligna produces more runoff during precipitation, while the basin with E. benthammii produces less runoff. The flow permanence curves indicated that in periods of drought, land use with eucalyptus plantations made more water available downstream than the native field in the basins located in Alegrete, Eldorado do Sul and for the BEs in the municipality of São Gabriel. The basins with native grasslands have more surface runoff, and less water storage in periods of drought. Groundwater level variation after precipitation events differ as a consequence of interception and evapotranspiration. The CN method based on separate rainfall-runoff events obtained approximate mode, mean, and median values, suggesting a normal distribution.