| dc.description.abstracteng | On many levels, there is a lack of understanding regarding the impacts of deforestation on water resources and soil properties in the Amazon-Cerrado ecotone, where the Amazonian agricultural frontier is mostly present. Both the large spatial extent of this region and the wide diversity of environmental conditions requires extensive field-based data collection to allow comprehensive process characterization and to improve hydrological modelling parameterization. Furthermore, the Cerrado biome, where most of the deforestation in this region has occurred, is often not integrated into the studies regarding the Amazon deforestation.
To contribute to fill this knowledge gap, I, in the context of the CarBioCial project (Gerold, 2017), conducted several hydrological and soil analyses in areas of Southern Amazonia that have been rapidly deforested. To that end, two macro-catchments were selected, one in the Amazon biome (Jamanxim River basin, 37,403 km2) and one in the Cerrado biome (das Mortes River basin, 17,556 km2), both located on the Amazon agricultural frontier. In both the das Mortes and the Jamanxim River basins, paired micro-catchments under different land use and land cover, i.e., native vegetation (rainforest or cerrado vegetation) vs. pasture for extensive cattle ranching, and a cropland area were selected to characterize the changes in hydrology and soil hydro-physical properties due to these contrasting land uses. The general objectives of this Ph.D. research were to: a) analyze trends in discharge and water quality in streams of macro-catchments in the Amazon and Cerrado biomes; b) determine soil hydro-physical properties and quantify streamflow and evapotranspiration from adjacent micro-catchments whose major difference is the LULC; c) quantify stream CAN concentrations and output fluxes during prevalent baseflow and stormflow conditions to improve the understanding of carbon and nutrient drivers in low-order streams; d) assess the soil hydro-physical and chemical properties, as well as water quality of cropland and riparian vegetation areas of a catchment in a typical large-scale commercial cropland system.
The analyses showed that land-use and land-cover change alters water quantity of large rivers in the Amazon and Cerrado biomes. These changes are more pronounced as an increase in the low flows, which are mainly maintained by hundreds of small streams that have baseflow as a dominant discharge condition. In these small catchments, catchment physiographic parameters play an essential role in the hydrological responses in both Amazon and Cerrado biomes, and the native vegetation conversion into pastures substantially change the water balance of these catchments. This proved to drive an increase in the baseflow in low-order streams. The changes due to soil hydro-physical degradation (e.g., increased bulk density and reduced soil porosity) cause increases in short-lived events as peak flows, as observed in the pasture catchment in the Amazon biome. The decrease of evapotranspiration is also another critical driver in the water balance in this region, because pastures could not maintain evapotranspiration rates as high as the native vegetation. All these changes are connected to other aspects of the environment. As this thesis shows, the difference in the hydrological fluxes increases the carbon and nutrient fluxes. In this context, the stormflow is a substantial hydrological pathway for carbon and nutrient losses, especially in areas where rainfall intensities exceed the infiltration capacity rates. On account of these impacts, the conservation of riparian zones appears to be one of the land management strategies that will mitigate further implications in the hydrochemistry of rivers. This study indicates that, indeed, riparian zones have a complex ecosystem of plants and soil properties that directly improve the water quality of flows towards the streams. However, the long-term implications of deforestation in the Amazon and Cerrado in these riparian zones are still unknown.
This Ph.D. research connects several themes under discussion regarding the environmental changes in the Amazon and Cerrado biomes. The outcomes of this thesis provide solid research directions for further studies in these biomes, which should focus on the subsurface water flows and the role of the fragmented vegetation patches, such as the riparian zones, in these regions. | de |