Semi-arid and arid areas exhibit great temporal variability in water availability.In some of these regions,a one-or two-day rainfall is followed by intervening dry periods of variable length.In recent decades,many rivers of the semi-arid portion of the Jequitinhonha Basin have been undergoing a progressive discharge reduction,mainly of their base flow,the Arauaí River is one of them.In order to understand this transformation,a long-term analysis of the annual water balance of the Arauaí River Basin was performed,the results of which are presented herein.Satellite images,hydrometeorological and river discharge data were analyzed;mean and variance tests were conducted to determine temporal homogeneity.Historical pluviometric data analysis shows no corresponding precipitation reduction and temperature undergoes only a slight increase in the same period.On the other hand,evaporation is extremely high,higher than precipitation during most of the year,leaving almost no water for infiltration(aquifer recharge) and runoff.Furthermore,the Arauaí headwaters of its tributaries are now occupied by a monoculture,Eucaliptus sp.,used for paper production.Because of the decreased fluvial discharges,its lowlands,usually used for agriculture and pasture,are abandoned and partially eroded. Semi-arid and arid areas exhibit great temporal variability in water availability. In some of these regions, one or two-day rainfall is followed by intervening dry periods of variable length. In recent decades, many rivers of the semi-arid portion of the Jequitinhonha Basin have been undergoing a progressive discharge reduction, mainly of their base flow, the Arauaí River is one of them. In order to understand this transformation, a long-term analysis of the annual water balance of the Arauaí River Basin was performed, the results of which are submitted herein. Satellite images, hydrometeorological and river discharge data were analyzed; mean and variance tests were conducted to determine temporal homogeneity .istorical pluviometric data analysis shows no corresponding precipitation reduction and temperature onlyes slight increase in the same period.On the other hand, evaporation is extremely high, higher than precipitation during most of the year, leaving almost no water for infi ltration (aquifer recharge) and runoff.Furthermore, the Arauaí headwaters of its tributaries are now occupied by a monoculture, Eucaliptus sp., used for paper production.Because of the decreased fluvial discharges, its lowlands, usually used for agriculture and pasture are are and partially eroded