Dinâmica hidrossedimentológica de pequenas bacias hidrográficas florestais

Abstract

Areas cultivated with eucalyptus forests have increased in Southern Brazil, and the effects on hydrosedimentological processes are not well known particularly at the watershed scale and in watersheds where land use has been changing. The Soil and Water Assessment Tool (SWAT) model has been applied in a few forested watersheds to evaluate and predict effects of commercial forest cultivation or land use change in hydrological processes, whilst erosion processes has not been the main goal. The present study aimed (i) to evaluate the effects of eucalyptus plantation on hydrosedimentological processes, and to evaluate the efficiency and limitations of the Soil and Water Assessment Tool (SWAT) model to simulate streamflow and sediment yield (daily and monthly) in nested eucalyptus watersheds and in two paired watersheds with eucalyptus and grassland, and (ii) to identify the contribution of the bank channel, unpaved roads and eucalyptus stands sediment-sources for two different size fractions of sediment (< 0.063 mm and 0.063-2 mm) deposited along the drainage network, and suspended sediment collected in the outlet of small watersheds. SWAT was used to evaluate hydrological and erosion processes for two nested forest watersheds (watershed-0.98 km2 and sub-watershed-0.39 km2) and two paired watersheds in Pampa Biome, one with grassland (1.10 km2) and other with eucalyptus (0.83 km2). Measured streamflow and sediment yield at the watersheds outlets was used to evaluate model sensitivity to selected model parameters, and for calibration from 2009 to 2013 for nested watersheds, and from 2009 to 2013 for paired watersheds. Time series plots and standard statistical measures were used to verify model predictions. For nested watersheds, predicted monthly streamflow was very good during calibration for the watershed and for the sub-watershed, respectively. Predicted monthly sediment yield was very good and satisfactory for the watershed, and unsatisfactory for the sub-watershed. Simulations for daily time-scale were satisfactory to predict streamflow and sediment yield for the watershed. For the sub-watershed, predicted streamflow was satisfactory , but sediment yield was unsatisfactory . For paired watersheds, predicted monthly streamflow was good during calibration for the grassland watershed and for the eucalyptus watershed. Predicted monthly sediment yield was satisfactory for the grassland watershed, and very good and satisfactory for eucalyptus watershed. Daily simulations were satisfactory to predict streamflow in both grassland and eucalyptus watersheds. The results suggest that the SWAT model is a promising tool to evaluate hydrological processes in Brazilian watersheds. However, more field work with continuous monitoring is required and empirical equations of SWAT must change to better represent the processes that predominate in each watershed and to represent the hydrological and erosion processes in sub-daily time-scale and, especially, for small watersheds where the time of concentration is less than one day. Sediment-sources was evaluated using geochemical properties, determined by inductively coupled plasma mass spectrometry and X-ray fluorescence analyses, and the data used to calculate proportional contributions of sediment. Source contributions were determined at points spatially-distributed along the main channel of the watershed. Source determination for in-stream sites was done using samples collected in one spot to evaluate source-contribution of area upstream of this site of interest to indicate how different sources dominate at different downstream locations. To examine whether different size-fractions shared similar origins, two size fractions of both source and suspended samples including fine (<0.063 mm) and coarse (0.063 2 mm) particles were analyzed. Fingerprinting approach indicated particle-size and location of sources within a watershed are major factors affecting the measured contribution of sources for coarse and fine sediments. The closer a sediment sampling site is to a potential source, the more likely this sediment-source will dominate the sampled material, especially for coarse particles. Based on the classification analysis, the relative contribution of each source to eroded sediments was bank channel, i.e. from the stream network. Modeling results and sediment-source identification can provide an improved understanding effect of land use or change in land use on sediment yield in watershed scale, which are useful and may be used as a management tool of natural resources.