Gross rainfall and its partitioning into throughfall, stemflow and evaporation of intercepted water in four forest ecosystems in western Amazonia

The partitioning of gross rainfall into throughfall, stemflow and evaporation of intercepted rainfall was studied in four forest ecosystems in the Middle Caquetá, Colombian Amazonia. Data on climate was collected automatically on an hourly basis during a five-year period. Weekly measurements of rainfall, throughfall and stemflow were carried out during a period of two years, while daily measurements, on an event basis, were carried out during two subsequent years. Throughfall, stemflow and evaporation in each forest were checked for correlations with gross rainfall characteristics, canopy gap fraction, tree crown area and bark texture. Canopy gap fraction differed between forests, ranging from 9% on the flood plain to 17% on the Tertiary sedimentary plain. Rainfall was rather evenly distributed over the year, with one dry period from December to February. 92% of the rain fell in single showers of less than 30 mm and most of the storms (56%) fell in less than one hour, during the afternoon or early night. Throughfall ranged from 82 to 87% of gross rainfall in the forests studied and varied with gross rainfall in all forests. It depended on the amounts and characteristics of rainfall, but differences in throughfall among forests, when comparing similar rainfall events, clearly indicated that throughfall also depends on forest structure. Stemflow contributed little to net precipitation (on average 1.1% of gross rainfall in all forests) and showed a power relation with gross rainfall. Correlations between stemflow per tree, projected crown area and bark texture were very poor as indicated by the low coefficients of determination. Evaporation during rainfall events exhibited a linear relation with rainfall duration and the ratio of evaporation over gross rainfall increased with forest cover (1-gap fraction) in the forests studied. The structure of the forests seemed to vary considerably and given its influence on rainfall partitioning it may explain both differences and similarities between results from this study and those from most other studies within Amazonia.