LONG-TERM WATER BALANCE IN SMALL CATCHMENTS IN THE ATLANTIC FOREST OF SOUTHEAST BRAZIL

The Atlantic Forest (Mata Atlântica) supplies millions of people with fresh water. Long-term studies of its catchments are essential for an understanding of the hydrological processes involved. A study of the annual water balance maintained in small catchments of the Walter Emmerich Forest Hydrology Laboratory (LHFWE), Serra do Mar State Park, Brazil, was carried out over a period of 20 years (catchment A), 26 years (catchment B) and 30 years (catchment D). The basic hydrological equation P = Q + ET ± ∆S was used. Rainfall (P) was measured with tipping bucket rain gauges installed in clearings. Streamflow (Q) was measured in gauging stations equipped with continuous stage recorders. Soil water storage change (∆S) was considered equal to zero. Evapotranspiration (ET) was calculated by the difference between P and Q. Average annual rainfall of the catchments was high with a mean of 1960 mm and wide interannual variability. The average streamflow was 1432 mm, corresponding to 73% of the rainfall, indicating remarkable water yield in the catchments. The average annual evapotranspiration was 32.2% for catchment A, 24.5% for B and 24.4% for D. These percentages are smaller than those obtained in studies carried out in other tropical forests, including the Atlantic Forest biome.