Efeitos do desmatamento no comportamento hidrológico da sub-bacia da UHE de Estreito

Abstract

The hydrological cycle of water is characterized by a natural phenomenon of closed circulation between the earth's surface and the atmosphere. On its way towards the surface, precipitation passes, through atmospheric conditions, descending onto the surface with a vegetation cover that retains part of this precipitation for its morphophysiological activities. Researchers conclude that the reduction of forest in a river basin can cause an increase in surface runoff, promoting an increase in maximum flows, as the removal of the original vegetation will result in less water interception. Therefore, monitoring the study area is essential to prevent and minimize the impacts of possible disasters, such as floods, landslides, water contamination and droughts. The main objective of this work was to evaluate the impact of deforestation on the dynamics of the hydrological response over 37 years in the SB-ESTREITO (Estreito Sub-basin), which is part of the Tocantins river basin up to the Estreito hydroelectric plant, with an area of approximately 285,167.06 km², mainly covering the States of Tocantins, Goiás and Maranhão, entirely inserted in the cerrado biome. The period 2012–2022 was also considered for this research. Trend methods were applied to the variables' precipitation, flow, Hydrological Response (RH) and Outflow Increment Rate (TID). The aim was to check whether they were showing an increase or decrease. Subsequently, the variables were related to accumulated deforestation. RH and TID are reported by scientists to be associated with deforestation dynamics, indicating that forest removal generates a rapid response in surface and lateral runoff values due to the decrease in interception and infiltration processes after forest removal. In total, SB-Estreito lost approximately 26.6% (49,872 km²) of its original vegetation present in 1985. The results indicate a tendency for the period 1985–2022 to decrease precipitation data (which could compromise the water potential of the basin), flow and TID, while for RH data there were no trends. The TID and flow values for the 37 years showed a slower response in water flow. Another situation is that deforestation values only reached more than 20% of loss of original vegetation in the basin in 2022; by 2021, the loss was around approximately 17%, and in APPs the removal of forest was around 17% of the original coverage (1985-2022). The main characteristic of SB-Estreito is the construction of several hydroelectric complexes, transforming the river into a succession of dams. In order to observe whether the construction of Estreito had any impact, the years 2012–2022 were selected. 2012 is marked by the inauguration of the Estreito plant and the launch of the new forestry code. No hydrometeorological variable showed a trend for the period 2012–2022, however, RH showed an increase from 2018 onwards and accompanying deforestation, having its highest value in 2022 (the year in which suppression reaches more than 20%; some researchers conclude that the decrease in vegetation increases water runoff as long as the deforested area is greater than 20%); that is, in this specific year we had more water turning into flow. The TID predominated as a standard behavior, for the last decade, a more controlled flow rate, perhaps related to the inauguration of the plant from 2012 onwards. Based on the data collected, it is concluded that the flow and TID variables did not present a faster flow. For the period of 37 years, and for the last decade (2012-2022) no hydrometeorological variable showed trends. In addition to deforestation, other factors seem to influence the hydrological behavior of the basin in question, such as hydroelectric plants (UHEs), which are altering the natural flow of the river and directly modifying the flow behavior.