Effects of land use and land cover change on the water cycle in the Amazon basin under a changing climate

Abstract

The Amazon rainforest, the world’s largest tropical rainforest, plays an important role on climate regulation by carbon fixation and cooling temperature throughout its high evapotranspiration rates. Nonetheless, the biome is pressured by deforestation on large part of its territory. The abandonment of deforested area due to low productivity and natural resources exhaustion, have resulted in a landscape characterized by a mosaic of several land covers, mainly the natural forest, pastures and secondary growth. This thesis aims to evaluate the seasonal and spatial variability of the fluxes of evapotranspiration over diverse Amazon land covers, with the purpose of investigating whether natural recovery counterbalances the effects caused by anthropogenic actions on the water cycle components. Besides that, the impact of future scenarios of land use and land cover change in combination with global climate change on river discharges and hydropower was also evaluated in an Amazon tributary by explicitly modeling the role of each land cover on evapotranspiration. Water and carbon fluxes were measured by eddy covariance and scintilometry besides other micrometeorological data, in three different land covers in Central and Southwestern Amazonia, in order to compare the effects of deforestation and natural regrowth on the water cycle components. The conversion of forest to pasture reduces the evapotranspiration. Two pasture sites analyzed present much lower evapotranspiration than in forests during the dry season, and with only one of them reaching the same rates of evaporation of primary vegetation during the wet season. Nonetheless, when pastures are abandoned due to low productivity, the secondary growth that replaces those areas reaches evapotranspiration 20% higher than that in primary forest at both dry and wet seasons. The gross primary productivity of the secondary vegetation is also higher than the primary in the wet season, but in a lower percentage (5% higher), and the same during the dry season. Besides the field data collection, this work is one of the first to consider the impacts of secondary forest in the hydrological modeling driven by different climate change scenarios (CC) and two future socioeconomic-based potential land-cover land-use change scenarios (LCLUC) on regional scale. The model projected a decrease on discharge under CC scenarios, and when deforestation scenarios without secondary growth are considered, the effects of CC are weakened. However, when secondary growth is also considered, the effects of CC are slightly enhanced. Results suggest that different aspects of environmental change, such as secondary growth, may affect water production and the sectors depending on it. For example, the potential energy production for Tucurui power plant, is projected to decrease of up to 58% under all land use change scenarios in 2041 to 2070 time slice. Secondary forest growth plays an important role in minimizing the impacts of land cover and land use changes from primary forest to pasture on the water balance in Amazonia, since the secondary forest contributes with higher transpiration than pastures, and even higher than in the primary forest. This indicates that secondary forests can contribute to the maintenance of moisture production of the Amazon region. In addition, secondary forest takes more carbon from the atmosphere than pasture sites, playing an important role on carbon uptake from the atmosphere together with primary forest, minimizing emission from deforestation.