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FOREST DEGRADATION AND ALTERATION OF
RAINFALL REGIME IN THE BRAZILIAN AMAZON

Megumi Maruyama* and Yasushi Yamaguchi
Nagoya University, Nagoya, Japan

Abstract
Deforestation rate in the Brazilian Amazon has reached 15% in the last century and its influence on the regional climate is a matter of concern. In the Brazilian Amazon, rainforest was heavily cleared specially in the Arch of Deforestation over the last thirty years, due to the colonization projects of the Brazilian Government. The Brazilian Amazon has an average annual rainfall ranging between 2000 mm and 3000 mm, however, the microclimate of that region has turned to be drier, recently. In the present paper, time variations of observed rainfall of the Brazilian Amazon Basin was analyzed during the period between 1970 to 2001. The ANA (Agencia Nacional de Aguas) data had gaps in some observation years (months and days) however, alteration in rainfall trends could be noted during the last thirty years.

FOREST DEGRADATION

As a result, effects of the Amazonian deforestation showed a big reduction of the annual rainfall in many critical areas. The NDVI imagery of deforestation was compatible with the decreasing trend of rainfall within heavily converted areas. These facts suggest that deforestation may have a significant impact on the short time rainfall regime in that region.

INTRODUCTION
Deforestation is a matter of concern all over the world, as a result of conversion by the national projects, in many developing countries. Monoculture is also taking place in the Brazilian Amazon such as tree-plantations, soybeans, sugar-cane, among others. However, cattle pasture is the main cause of deforestation (. After conversion, the water cycles are interrupted by lack of sufficient water vapor. Natural forests are replaced by vegetation of lesser evapotranspiration: pasture is 1/3 of that of the Amazonian evergreen forest. Evapotranspiration plays an important role with regard to formation of rainfall clouds in that region. From the water balance equation, P = Q – E + ΔS (where P is precipitation, Q is runoff, E is evapotranspiration and ΔS is water storage ( = 0) ), evapotranspiration can be evaluated from the observed values of P and Q. As a result, deforestation affect the local climate, turning the region drier, specially in the states of the deforestation arch such as Maranhão, Tocantins, Pará, Mato Grosso and Rondônia.

FOREST DEGRADATION

Deforestation is causing reduced annual rainfall and longer and more severe dry seasons. A decrease in annual rainfall and longer dry seasons are affecting the structure and composition of the remaining forest. Relationship between the anthropological influence and rainfall during the period of the Amazonian development projects was studied in the present study. In some observation stations, the ENSO (El Niño Southern Oscillation) influence was clearly noted, with decreasing trends on rainfall in the critical years, however in some others not. The critical positive ENSO years were: 72~73, 76~77, 82~83, 86~87, 91~92, 93, 97~98, 02~03 with significant or not significant influence on the rainfall regime in the Amazon region (Malhi et al. 2004), (Aceituno, 1988) and (Richey et al. 1989). The present study tried to show in figures, the decreasing trend of rainfall in the Brazilian Amazon, analyzing rain gauge data of ANA (Agência Nacional de Águas = National Water Agency), and also showing the NDVI imagery of Landsat MSS, TM and ETM+ taken in 1986 to 2003, within the critical areas of deforestation of that region.

FOREST DEGRADATION FOREST DEGRADATION
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