Spatio-temporal Variability in Rainfall and Wet-canopy Evaporation within a Small Catchment Recovering from Selective Tropical Forestry.

Bidin, Kawi (2001) Spatio-temporal Variability in Rainfall and Wet-canopy Evaporation within a Small Catchment Recovering from Selective Tropical Forestry. PhD thesis, Lancaster University.

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Abstract

Within tropical rainforest environments, rainfall pattern and canopy structure regulates the partitioning of water into wet-canopy evaporation and sub-canopy rainfall. These interrelated process then moderate atmospheric water vapour, plant water availability, runoff pathways and soil erosion. Forestry impacts on these atmospheric processes may, therefore, impact on a cascade of other environmental processes. This study, conducted within a 4 km2 experimental catchment in the interior of Northeast Borneo, that was recovering from selective timber harvesting, sought to identify the spatial and temporal structure of the local rainfall, and the impact of forestry on wet-canopy evaporation and lumped, water-balance components. A total of 450 throughfall gauges, 50 raingauges and 40 stemflow gauges were installed and digitally surveyed within the catchment, mostly within a 0. 44 km2 tributary area. Data from these instruments were then supported by those from rainfall recorders and river gauges, and an enumeration of the vegetation patchwork present at 8-years post-logging. Several approaches of statistical modelling were applied, and indicated that the rainfall during the 1997/8 drought-year was (1) highly localised in space, even for regions dominated by convective rainfall, (2) strongly moderated by the local undulating topography, with marked seasonal (monsoon) changes, and (3) delivered primarily as regular, short duration events of low intensity rainfall. The visually classified patchwork of canopy types (supported by a series of biophysical measurements), showed significant differences in rates of wet-canopy evaporation. Smaller quantities of sub-canopy rainfall were observed beneath the disturbed patches of vegetation, in comparison to those beneath undisturbed remnants of primary rainforest. This may have been caused by (i) a greater rate of wet-canopy evaporation, due to enhanced atmospheric turbulence and/or higher surface leaf densities, or (ii) disturbed forest blocks receiving less gross rainfall, due to sheltering by the higher undisturbed canopies. Modelling of the 8-year post-logging water balance data, indicated that both seasonal and inter-annual cycles (related to the El Nino Southern Oscillation) strongly affected the rainfall (P), riverflow (Q) and P-Q' dynamics. On removal of these cyclical components, the analysis indicated that there was no evidence of a change in evapotranspiration (strictly 'P-Q') with the 8-years of forest regeneration. Some of these results were unexpected, and underlined the need for a new emphasis on 'canopy hydrology' within rainforests managed for development and conservation.