HYDROLOGICAL PROCESSES AT 2 SUBTROPICAL FOREST CATCHMENTS - THE SERRA-DO-MAR, SAO-PAULO, BRAZIL

The Cunha Forest Hydrological Laboratory was established in the Serra do Mar, Sao Paulo, Brazil to achieve some understanding of the hydrolo gical processes and the effects of forest cover on these processes in the headwater areas. Stream-gauging from two subtropical forest catchm ents (56.0 ha and 36.7 ha) covered with the Mata Atlantica commenced i n 1982. Measurements of crown interception, surface runoff from a hill slope and estimation of soil water storage within the catchments quant ified individual components of the hydrological processes. Ten years o f field measurements and hydrograph analysis show that about 15% of an nual rainfall is intercepted by the forest cover and returns directly to the atmosphere while 85% of the rainfall reaches the forest Boor, w here it infiltrates and remains in the soil to feed subsurface flow an d baseflow or transpiration. Humid conditions obtain throughout the ye ar and surface runoff is a rare occurrence on forested hillslopes. Sto rmflow is generated from wetland source areas adjacent to streams and from seepage from hillslopes. The total volume of stormflow is only 11 % of annual rainfall. Fifty-nine % of annual rainfall is stored in the soil mantle and Bows via subsurface routes to streams as baseflow thr oughout the year. This sustained flow of streams is one of the most im portant hydrological features and is controlled by such basin characte ristics as the physical properties of the soil, the depth of the soil mantle and the vegetative cover. Soil evaporation and transpiration we re estimated as 15% of annual rainfall by the water balance equation. Thus, the annual hydrological budget for the catchments is 70% streamf low and 30% evapotranspiration. Riparian areas are also places of stor mflow production as well as of soil water and groundwater storage of w ater derived from hillslopes as interflow. An assessment of the area, depth of sediment and porosity of the riparian areas helps in understa nding the runoff processes in low-order catchments in the Serra do Mar . (C) 1997 Elsevier Science B.V.