Modelling rainfall and canopy controls on net-precipitation beneath selectively-logged tropical forest

Understanding spatio-temporal patterns in rainfall received beneath tropica l forest is required for eco- hydrological modelling of soil-water status, river behaviour, soil erosion, nutrient loss and wet-canopy evaporation. As selective-logging of tropical forest leaves a very complex mosaic of canop y types, it is likely to add to the spatio-temporal complexity of this sub- canopy or net precipitation. As a precursor to addressing this problem, the analysis presented here will examine the two dominant biophysical controls on sub-canopy precipitation. These controls are: (a) the spatial and tempo ral patterns in above-canopy or gross rainfall, and (b) the rate of wet-can opy evaporation associated with each type of canopy structure created by se lective-forestry. For this study, over 400 raingauges were installed within a 10 km(2) area of lowland dipterocarp forest affected by selective-forest ry some 9-years prior to this work. Gauges were located beneath various can opy types and within large openings. The spatial distribution of gross rain fall (monitored within the openings) was modelled using variography, while the effects of different canopy types on sub-canopy preciptation was analys ed by comparing 6-month totals. The temporal distribution of gross rainfall over an 11-year record collected at the same site (Danum Valley Field Cent re) was modelled with Data-Based-Mechanistic (DBM) approaches. These DBM ap proaches were also applied to the rainfall time-series of the two adjacent meteorological stations; all three gauges being contained within a 5000 km( 2) region of Eastern Sabah in Malaysian Borneo. Strong diurnal modulation was apparent within gross rainfall for the inland rainforest site, with a distribution consistent with a dominance of local convective rain cells. A similarly strong cycle coincident with the periodi city of the El Nino-Southern Oscillation (ENSO) was present within all of t he region's rainfall records, though marked differences in annual and intra -annual seasonality were apparent. The preliminary variogram modelling indi cated that a deterministic drift was present within the local-scale gross r ainfall data, probably related to local topographic effects. Notwithstandin g the need to remove this drift, the work indicated that spatial models of gross rainfall could be identified and used to interpret similar models of net-precipitation. During the ENSO drought-period monitored, the lowland di pterocarp forest allowed 91% of the gross rainfall to reach the ground as t hroughfall. These rates were, however, reduced to between 80%-86% beneath r epresentative plots of moderately impacted to creeper-covered, highly damag ed patches of forest.