Simulation of water drainage of a rain forest and forest conversion plots using a soil water model

The water budgets were estimated for a rain forest plot and two conversion plots (cut and burn) in a rain forest region of eastern Amazonia, Brazil, using a soil water model (SilVlow) to estimate the drainage and soil water storage compartments. A quantification of drainage was required to determine element leaching associated with conversion. The plots were equipped with fast recording tensiometer fields down to 5 m depth. Soil water tension and meteorological data were recorded at 15 min intervals. Monitoring was maintained for 18 months from July 1992 to December 1993. The special feature of the model is that soil parameter functions (matric potential/ volumetric moisture content and matric potential/ hydraulic conductivity) are introduced in tabular form, and can be changed deliberately during a fitting procedure adjusting simulated to measured matric potential. This fitting requires a vegetation-free period, which was provided by the conversion plots after clear-cutting the forest. For this fitting procedure, high-resolution recording was essential. Actual evapotranspiration in the soil water model is derived from potential (Penman) evaporation using a matric potential-dependent transpiration reduction function. The adjustment of these parameter functions was performed on plots under vegetation, also by fitting. The annual rainfall of 2479–2706 mm (depending on time interval chosen), fell short of the long-term average of 3000 mm. The forest intercepted 15% of the rain while total evapotranspiration was about 1350 mm. Drainage decreased from 1484–1733 mm at 110 cm to 1130–1331 mm at 500 cm suggesting a water uptake by roots of 350–400 mm from the depth zone 110–500 cm. On the conversion plots that were planted with eucalypts but regularly weeded, drainage at 500 cm depth amounted to >90% of rainfall.