Canopy hydrology and hydrometeorology:
First evidence of a structured and dynamic spatial pattern of
rainfall within a small humid tropical catchment
This study examined the spatial variability of rainfall across a network of 46
raingauges within a 4 km2 rainforest catchment within the interior of northeastern
Borneo. Correlation of seasonal rainfall totals with raingauge separation distance,
aspect and relief was undertaken. The analyses indicated a very high degree of spatial variability in seasonal totals
across a very small area, even in comparison with other regions experiencing
convective rainfall. Moreover, it is the first work in the humid tropics to show
that systematic, stochastic structure is present within rainfall over scales of
10's to 100's metres, and that these patterns change from the Southwest monsoon
(May-October) to the Northeast monsoon (November-April). Local associations
with aspect and relief are present, but explanation of the seasonal changes in
rainfall pattern over the whole 4 km2 catchment must relate to more complex
local topographic effects on the regional windfield. Bidin, K., and Chappell, N.A. 2003. Hydrology and Earth System
Science. 7(2), 245-253.
Sub-canopy rainfall and wet-canopy evaporation in a
selectively-logged rainforest, Sabah, Malaysia:
Understanding the impact of the selective removal of trees from a tropical rainforest
on the rate of wet-canopy evaporation and transpiration is critical to the assessment
of the impact of so called 'sustainable forestry' on local climate, and the water resources
potential of rivers. Accurate quantification of the changes in the wet-canopy evaporation
component is, however, difficult given the extreme heterogeneity of the vegetation patchwork
produced by commercial, selective logging.In order to address this issue for an area of lowland dipterocarp forest, selectively-logged
some eight years prior to the study, a network of 450 throughfall gauges, plus 22 gross rainfall
and 40 stemflow gauges, was installed within the 44-hectare Baru Experimental Catchment
(Sabah, Malaysian Borneo). Most of these gauges were located randomly within plots, themselves
stratified according to the six canopy classes.The results showed that more rainfall reached the forest floor beneath the undisturbed
remnants of rainforest (i.e. the protected areas), than those patches of canopy subject to
light or heavy impact. This may have been because the disturbed forest patches had a higher
rate of wet-canopy evaporation (i.e.12-18 % of gross rainfall) in comparison to the undisturbed
remnants (i.e. 7% of gross rainfall). Alternatively, the difference may, at least in part, have
been caused by the lower disturbed patches of vegetation being sheltered by the undisturbed
forest remnants, leading to the receipt of less rainfall on their canopy surfaces.
Bidin, K., and Chappell, N.A. 2003. Proceedings of the Water: Forestry and Landuse
Perspectives conference. Forestry Research Institute of Malaysia, Kepong. in press.
Net-rainfall and wet-canopy evaporation in a small selectively-logged rainforest
catchment, Sabah, Malaysia
In order to understanding the impact of the selective removal of trees from a tropical rainforest on
the rate of wet-canopy evaporation for an area of lowland dipterocarp forest, selectively-logged some eight years
prior to the study, a network of 450 throughfall gauges, plus 22 gross rainfall and 40 stemflow gauges, was
installed within the 44 hectare Baru Experimental Catchment (Sabah, Malaysian Borneo). Most of these gauges
were located randomly within plots, themselves stratified according to the six canopy classes identified. The
results showed that more rainfall reached the forest floor beneath the undisturbed remnants of rainforest (i.e., the
protected areas), than those patches of canopy subject to light or heavy impact. This may have been because the
disturbed forest patches had a higher rate of wet-canopy evaporation (i.e., 12 - 18 % of gross rainfall) in
comparison to the undisturbed remnants (i.e., 7 % of gross rainfall). Alternatively, the difference may, at least in
part, have been caused by the lower disturbed patches of vegetation being sheltered by the undisturbed forest
remnants, leading to the receipt of less rainfall on their canopy surfaces.
Bidin, K., Chappell, N.A., Sinun, W., and Tangki, H 2003. Proceedings of 1st International Conference on Hydrology
and Water Resources in Asia Pacific Region, vol 2, 13-15 March 2003, Kyoto, Japan. pp 659-666.
Climate cycle and land-use change sensitivity of tropical hydrological systems: A precursor to GCM land-use change simulations
Global Circulation Models (GCMs) are increasingly used to assess possible impacts of tropical land-use change on evapotranspiration and the feedback to precipitation. Evaluation of these scenarios requires an understanding of (i) the natural cyclicity and trends in the driving processes (e.g., precipitation, air temperature and net radiation), and (ii) the land-use change impacts that have been observed within relevant landscape-scale studies. This study examines the relative impact of
several types of forest cover change on water balance and evapotranspiration in South East Asia, against (a) the natural dynamics and stochastic elements inherent within time-series of precipitation and evapo-transpiration, and (b) uncertainty in our ability to characterise these dynamics. The very limited number of studies addressing water balance changes as a result of tropical forestry were examined using QAA and other statistical procedures. The Dynamic Harmonic Regression (DHR) model, recently developed at Lancaster, was then used to characterise the time-series dynamics. This state-space stochastic model, was used to detect and estimate cyclic components related to monsoon cycles, short- and longer-term El Niņo Southern Oscillation cycles, cycles resulting from Walker cell migration, as well as longer-term drifts in the precipitation and evaporation data from studies undertaken across SE Asia. These results demonstrated (i) some of the features of precipitation and evapotranspiration time-series for SE Asia that GCM simulations might be evaluated against, and (ii) the magnitude of land-use change impacts that such GCMs might be expected to forecast.
Chappell, N.A., and Tych, W. 2002. UGAMP Newsletter, 25, 11.
Land-use change impacts on SE Asian rainfall: Identifying dominant temporal characteristics of rainfall time series
This poster describes a component of a study addressing the affects of land-use change in South East Asia on the regional climate. To date, most tropical climate modelling studies have focused on Amazonia and Africa, leaving a dearth of work within SE Asia. As this region is more strongly affected by maritime conditions and has a very different assemblage of land-use practices, the sensitivity to change may be different to that experienced elsewhere within the tropics.
Specifically, the poster evaluates a new modelling technique developed at Lancaster that robustly identifies the cyclical and trend characteristics in time-series data. Such characteristics within tropical rainfall data includes the monsoons, short- and longer-term El Niņo Southern Oscillation cycles, cycles resulting from Walker cell migration, and longer-term drifts in the climate. The algorithm is called the Dynamic Harmonic Regression (DHR) model, and its ef.ciency in modelling rainfall time-series from sites across SE Asia is evaluated. Interpretation of the strength and periodicity in the disaggregated rainfall data is then made. Implications of these analyses in the evaluation of the accuracy of General Circulation Models (GCMs) in the simulation SE Asian climates is then discussed. The more reliable are the temporal characteristics of GCMs applied to SE Asia, the greater will be the faith attributable to GCM land-use change scenarios.
Fowell, M., Chappell, N.A., Mackenzie, A.R. and Tych, W. 2002. UGAMP Newsletter, 25, 16.
Modelling rainfall and canopy controls on net-precipitation beneath selectively-logged tropical forest
Understanding spatio-temporal patterns in rainfall received beneath tropical 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 canopy 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 temporal patterns in above-canopy or gross rainfall, and (b) the rate of wet-canopy evaporation associated with each type of canopy structure created by selective-forestry. For this study, over 400 raingauges were installed within a 10 km2 area of lowland dipterocarp forest affected by selective-forestry some 9-years prior to this work. Gauges were located beneath various canopy types and within large openings. The spatial distribution of gross rainfall (monitored within the openings) was modelled using variography, while the effects of different canopy types on sub-canopy preciptation was analysed by comparing 6-month totals. The temporal distribution of gross rainfall over an 11-year record collected at the same site (Danum Valley Field Centre) was modelled with Data-Based-Mechanistic (DBM) approaches. These DBM approaches were also applied to the rainfall time-series of the two adjacent meteorological stations; all three gauges being contained within a 5,000 km2 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 periodicity of the El Niņo-Southern Oscillation (ENSO) was present within all of the region's rainfall records, though marked differences in annual and intra-annual seasonality were apparent. The preliminary variogram modelling indicated that a deterministic drift was present within the local-scale gross rainfall data, probably related to local topographic effects. Notwithstanding 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 dipterocarp forest allowed 91 percent of the gross rainfall to reach the ground as throughfall. These rates were, however, reduced to between 80 and 86 percent beneath representative plots of moderately impacted to creeper-covered, highly damaged patches of forest.
Chappell, N.A., Bidin, K., and Tych, W. 2001. Plant Ecology, 153, 215-229.
Spatio-temporal variability in rainfall and wet-canopy evaporation within a small catchment recovering from selective tropical forestry
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.
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 atmosphere turbulence and/or higher surface of 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), overflow (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.
Bidin, K. 2001. Unpublished PhD thesis, Lancaster University, UK
Struktur ruang dan masa (spatio-temporal structure) taburan curahan hujan kasar dan bersih kawasan hutan, Pusat Luar Lembah Danum, Sabah
Aktiviti pembalakan secara terpilih, disusuli dengan proses regenarasi mengubah kadar dan bentuk penyejatan semula air hujan yang mececah kanopi merupakan unsur utama dalam imbangan pencegatan kawasan hutan (canopy-interception budget). Kesan ini selanjutnya bertanggungjawab untuck mengubah: (a) olakan setempat curahan hujan (iaitu sebahagian dari imbangan kerpasan terus), (b) kelengasan tanih, dan (c) keadaan aliran/banjir sungai. Dengan menggunakan data dari rangkaian-rangkaian tolok hujan yang pada kebiasaannya dipasang dalam keadaan 'jarang/sedikit' bagi sesuata kawasan tadahan berhutan, struktur ruang dan masa bagi kerpasan kasar dan bersih sememangnya sudah komplek boleh menyebabkan angaran keatas banjir atau model imbangan air menjadi jauh meleset. Oleh itu dalam kajian ini, rangkaian yang 'banyak' melibatkan 500 tolok hujan telah dipasang dalam kawasan hutan seluas 10 km2, meliputi hutan hujan yang terdiri dari mozek-mozek hutan asli dan beberapa kategori kanopi yang sedang melalui proses pemuliharaan selepas gangguan pembalakan. Variasi kerpasan kasar dan bersih yang amat ketara bagi kedua-dua kawasan terbuka dan berhutan melibatkan struktur ruang dan masa adalah dibincangkan.
Bidin, K., Chappell, N.A. Sinun, W., and Dalimin, M.N. 1999. Proceedings of the CFFPR conference
Water: Forestry and Landuse Perspectives, Forestry Research Institute of Malaysia (FRIM), Kepong, 31 March to 1 April 1999.
Geostatistical structure of the canopy evaporation from selectively logged forests in Borneo
Selective harvesting of primary rain forest, followed by regeneration, changes the rate and pattern of canopy re-evaporation which is a key element of the canopy-interception budget. These effects then propagate to modify: (a) local convective rainfall (i.e., part of the direct precipitation budget), (b) soil moisture and thus drought stress, and (c) the flood behaviour of rivers. Given typically sparse raingauge networks, the spatio-temporal complexity of direct and canopy-intercepted precipit-ation can result in highly inaccurate precipitation estimates for flood or water-balance modelling.
Within this study, a dense network of 500 direct and net precipitation gauges were installed within a 10 km2 region covered by a mosaic of primary, lightly-impacted and highly-damaged rain forest. Twelve months of measurements from this intensive network were combined with 10-15 years of gross precipitation records for Eastern Sabah (Malaysian Borneo), and analysed using a combination of time-series and geostatistical modelling.
We have been able to identify: (a) diurnal, seasonal and ENSO patterns within the direct precipitation, (b) the spatial extent of events of these characteristic periods, (c) the impact of spatio-temporal patterns in direct precipitation on components of the canopy-interception budget, and (d) systematic variations in canopy re-evaporation between different characteristic elements of the disturbance mosaic.
Chappell, N.A., Bidin, K., and Tych, W. 1998. Tropical Forest Canopies, European Science Foundation Tropical Canopy Programme Final Conference, Oxford, 12-16 December 1998.
