The forest and the hydrological management strongly influence the hydrological regime. One of the main causes of disturbance is the clearcutting of the natural vegetation in order to convert the forest into agricultural areas and to set up commercial and industrial areas. In the specific case of the Amazonian Forest, great extensions of the native forests have been clearcut for years with the purpose of establishing agricultural and pasture areas which tend to be abandoned after few years of use. Several studies have shown the direct influence of the vegetation on erosion processes, nutrient dynamic, water source protection, water quality and production. The vegetation removal changes the evapotranspiration, the photosynthesis and the precipitation rates, affecting the availability of water. Since the development process in Brazil always occurs in association with native vegetation removal, the forest and the hydrological managements aiming a sustainable development are not only important, but also necessary. The main objective of this work was to evaluate the deforestation impact on the hydrological response dynamics at Ji-Paraná River Basin (32,860 km2), in Rondônia during 23 years. The impact is compared to a preserved forest area at Sucunduri River Basin (13,677 km2), in Amazonas. Annual forest cover maps were generated for both river basins and analyzed together with time series of streamflow, precipitation, estimated evapotranspiration, hydrological response and rising limb rate. The results do not indicate any trends in the period between 187 and 2001, in spite of the increasing deforestation in the Ji-Paraná River Basin. Deforestation rates in the same period summed 55%, this included the removal of more than a half of the native vegetation at PPA,s of rivers smaller than 10 meters width. Nevertheless, the rising limb rates and the hydrological response are associated to the annual deforestation rates at Ji-Paraná River Basin. This indicates that the forest clearcutting generates a fast response in both the superficial and lateral flow values, due to the decrease of interception and infiltration processes after the forest removal. The human occupation dynamics in Rondônia is characterized by constant forest burns, by a low compactation of the soil due to no mechanized agriculture procedures and sparse cattle growing, and by the presence of many secondary forest areas. That seems to compensate and balance the system, contribuing to soothe the deforestation effects and to the maintainance of the hydrological and energetic balance. The water and energy volumes that circulate in meso and large scale basins, mainly in Amazonia are very big. The hydrological effects of the land use and cover changes are not as evident as in small basins. Despite this compensation, the residence rate for Ji-Paraná River Basin presented oscillations since 1985, when the total deforestation reached 20%. This indicated instability in the basin, probably owing to land cover changes. This study allows us to conclude that the deforestation really has an immediate influence on the dynamics of the hydrological response, on the rising limb rate and on the residence rate. However, no trends were observed in all hydrological time series that may indicate a climatic change linked to deforestation. The identification of the scale at which the deforestation could cause climatic changes must be further investigate.
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