Efeitos do Desmatamento em Larga-Escala na Hidrologia da Bacia do Uraim, Amazônia (Effects of large-scale deforestation on hydrology of Uraim basin, Amazonia)

A conversao de florestas tropicais e um grande problema ambiental, com impactos no clima e nos regimes hidrologicos. Diversas pesquisas apontam que o desmatamento em larga-escala da Amazonia poderia levar a alteracoes no clima, se refletindo em reducao de chuvas. Assim, o objetivo aqui e analisar o impacto da mudanca de cobertura vegetal na precipitacao e na vazao de uma bacia de grande-escala na Amazonia, a qual ja sofreu desflorestamento extenso. A Bacia do Rio Uraim, localizada no Para, foi entao selecionada porque originalmente era uma area de floresta, mas atualmente ja experimentou desflorestamento da ordem de 65% de sua extensao. Usando serie historica de dados, testes estatisticos z-teste e t-teste foram realizados para verificar as diferencas hidrologicas entre dois periodos (representando condicoes distintas de cobertura vegetal). Os resultados indicam que a precipitacao media anual do periodo desmatado e aproximadamente 25% menor que para os anos com floresta (p=0,04). Como consequencia da diminuicao de chuvas ha uma reducao de 22% na vazao do canal (p=0,006). No que diz respeito aos regimes da bacia os resultados tambem indicam reducao de 65% na precipitacao durante a estacao seca (p=0,0001) e de 36% na vazao durante os meses de estiagem (p=8,9E-8). Esses resultados sao particularmente relevantes uma vez que confirmam com dados de campo a ocorrencia de decrescimo na chuva como resultado de desmatamento em larga-escala, fenomeno apontado ate entao apenas como previsao, obtida por meio de simulacoes, para cenarios futuros. A B S T R A C T Conversion of tropical forests is a major environmental problem, with impacts on climate and on hydrological regimes. Several researches point that Amazon deforestation on a large scale could lead to climate changes, reflecting in reduced rainfall. So, objective here is to analyze the impact of vegetation cover changes in precipitation and streamflow of a large-scale catchment in Amazon which has already undergone extensive deforestation. The Uraim River Basin, located in Para, was then selected because this was originally a rainforest area, but currently has already experienced deforestation in the order of 65% of its extension. Using long-time series data, classical statistical z-tests and t-tests were performed to verify the hydrological differences between two periods (representing different conditions of land cover vegetation). The results indicate that mean annual precipitation of deforested period is approximately 25% lower than the values for forested years (p=0.04). As consequence of decreasing rainfall there is a 22% reduction in the channel´s discharge (p=0.006). Regarding basin´s regime the results also indicate a 65% reduction in precipitation during dry season (p = 0.0001) and a 36% reduction in streamflow during low-flow months (p = 8,9E-8). These results are particularly relevant once confirm with field data the occurrence of decrease in rainfall as a result of large-scale deforestation, phenomenon pointed until now only as forecasts obtained by various simulations of future scenarios . Keywords: land cover change, hydroclimatology, precipitation, streamflow.