Gross rainfall and its partitioning into throughfall, stemflow and evaporation of intercepted water in four forest ecosystems in western Amazonia
Introduction
The Amazonian rain forest seems to play an important role in the regulation of regional and global climate (Salati and Vose, 1984). Using a modelling approach, Eltahir and Bras (1993) concluded that the atmosphere in the Amazon basin is an open system and that the net input of atmospheric moisture into the basin is about 32%, about 68% of the gross input leaving the basin. They also found that the recycling ratio of the Amazon basin is about 25–35%. This ratio differs from those found by others, which were based on the erroneous hypothesis that the atmosphere over the Amazon basin is a closed system (Molion, 1975, Lettau et al., 1979, Salati et al., 1979). Eltahir and Bras (1993) furthermore concluded that deforestation of Amazonia would increase the surface temperature and decrease the heating of the upper troposphere, which would result in a reduction of precipitation. The foregoing illustrates that recent concern about tropical rain forest deforestation focuses on its impact on climate at regional and global scale (e.g. The Anglo Brazilian Amazonia Climate observation study ABRACOS, Gash et al., 1996; the Large Scale Biosphere–Atmosphere experiment in Amazonia, Nobre, 1996, Eltahir and Bras, 1993). Current research concentrates on global circulation models and on those parameters and fluxes that play a role in the global climate (e.g. Gash et al., 1996). Nevertheless, it is often argued that additional work is needed to explain and improve model predictions for impacts of deforestation (Bruijnzeel, 1990, Nobre et al., 1991). Evidently, local hydrological studies on undisturbed mature rain forests would provide base level information on initial conditions that might allow for an evaluation of the presumed influence of deforestation on regional and global climate. This paper concerns such local study.
In the Middle Caquetá (Colombian Amazonia), the structure and species composition of the forest vary considerably between the different landscape units (Duivenvoorden and Lips, 1995, Londoño, 1993). This causes equally large variations in temporal and spatial patterns of water fluxes in these units, leading to local differences in water and nutrient stocks in the various forest compartments (Vitousek and Denslow, 1986, Tanner, 1985). Thus, the partitioning of rainfall into throughfall and stemflow leads not only to a more diffuse input of water into the forest floor, but also to local concentration around the base of tree stems, which is known to induce spatial variability in soil properties and soil moisture conditions (Waidi et al., 1992). In tropical forests, the abundance of epiphytes, climbers and aerial roots renders this partitioning much more complicated than in temperate forests (Longman and Jenı́k, 1990).
It is well understood that in forested areas generally total evaporation is larger than in areas with shorter vegetation (e.g. grass) mainly due to the larger interception by the forest canopy (Bosch and Hewlett, 1982), which has been related to the large aerodynamic conductance of forest (Stewart, 1977). Additionally, there has been an increasing awareness that evaporation of intercepted rainfall has to be investigated separate from transpiration, especially in very humid areas (Hutjes et al., 1990, Shuttleworth and Calder, 1979). Furthermore, most studies on forest interception showed that this interception is closely related to gross rainfall amounts and characteristics. However, the influence of forest structure on interception is poorly known.
Most rainfall interception and water balance studies in Amazonia were executed in Brazil (Ubarana, 1996, Leopoldo et al., 1995, Lesak, 1993, Lloyd and Marques, 1988; Shuttleworth, 1988) and only a few in other parts of Amazonia (Hölscher et al., 1997, Jetten, 1996, Wright et al., 1992, Jordan, 1978). Thus, in Colombian Amazonia, which represents one of the most humid areas within the basin, very little attention has been paid to the hydrology of forest ecosystems and to the effects of forest structure on water dynamics.
This paper concerns a study designed to address this lack of knowledge by measuring rainfall and its partitioning after entering the canopy in four undisturbed rain forests in the Middle Caquetá, Colombian Amazonia. It focuses on the analysis of long-term hydrological measurements of rainfall, throughfall, stemflow, the resultant evaporation and the related structure of these forests.
Section snippets
The study area
The study area is in Peña Roja (Nonuya Indian community) near Araracuara, Middle Caquetá, Colombia, (0° 37′ and 1° 24′ S, 72° 23′ and 70° 43′ W; Fig. 1). Climate is classified as equatorial superhumid Afi (Köppen, 1936). The research sites are permanent undisturbed forest plots, used by the Tropenbos Foundation for its research. They lie approximately 200–250 metres above sea level and form a sequence from the lower terrace of the River Caquetá to the Tertiary sedimentary plain. Based on data
Materials and methods
The areas for the present study were selected as being representative for the natural vegetation in the main land units from this part of the Amazon basin. Three subplots were selected in the Tertiary sedimentary plain (SP) and two subplots in the high terrace (HT), the low terrace (LT) and the flood plain (FP), respectively, to measure gross rainfall above the forest canopy, throughfall and stemflow (Fig. 1). In 1992, approximately 3 km from the plots, in an open area of about 20 hectare (within
Forest structure
Average values of measured variables of forest structure in each forest ecosystem studied are presented in Table 1. The largest mapped crown area was 62.2 m2 and the smallest 2.4 m2, both in the high-terrace plot. For the trunk surface area, the correction factor of 0.5 was applied to trees with diameter larger than 0.1 m. The apparent inconsistency in the flood plain data, of a large tree crown area in combination with the lowest tree trunk surface area, is mainly due to the abundance of small
Discussion
Storage capacity values of the forests studied on the whole resemble the values found by Ubarana (1996) in the reserves Vale do Rio Doce and Jaru Duke in Brazil, which were based on linear regressions of throughfall against gross rainfall. However, Ubarana concluded that this method results in an overestimation of evaporative losses. Since our estimates of the canopy storage capacity were based on specific events for which it was assumed that evaporation was negligible, it might be that we
Conclusions
Of the gross rainfall of about 3400 mm y−1, most fell in small showers during the afternoon and at night. The overall average rainfall intensity was about 5 mm h−1. These rainfall characteristics largely explain the partitioning of rainfall into throughfall, stemflow and ensuing evaporation in the forests studied.
Water fluxes in the forest canopy of four forest ecosystems in western Amazonia have been quantified as a percentage of gross rainfall. Amounts of net precipitation reaching the forest
Acknowledgements
We are grateful to Dr John Gash from the UK Institute of Hydrology and to Dr Sampurno Bruijnzeel from the Free University, Amsterdam for their suggestions and corrections of earlier drafts of this paper. This work, forming part of a larger research project on water and nutrient cycling in Colombian Amazonia, was supported by the Tropenbos Foundation in Colombia and the Netherlands and by the Colombian Institute for Science and Technology “Colciencias”. Collected data (four years data on gross
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