Kawi Bidin

Recent trends in the intrinsic water-use efficiency of ringless rainforest trees in Borneo

Stable carbon isotope (δ13C) series were developed from analysis of sequential radial wood increments from AD 1850 to AD 2009 for four mature primary rainforest trees from the Danum and Imbak areas of Sabah, Malaysia. The aseasonal equatorial climate meant that conventional dendrochronology was not possible as the tree species investigated do not exhibit clear annual rings or dateable growth bands. Chronology was established using radiocarbon dating to model age–growth relationships and date the carbon isotopic series from which the intrinsic water-use efficiency (IWUE) was calculated. The two Eusideroxylon zwageri trees from Imbak yielded ages of their pith/central wood (±1 sigma) of 670 ± 40 and 759 ± 40 years old; the less dense Shorea johorensis and Shorea superba trees at Danum yielded ages of 240 ± 40 and 330 ± 40 years, respectively. All trees studied exhibit an increase in the IWUE since AD 1960. This reflects, in part, a response of the forest to increasing atmospheric carbon dioxide concentration. Unlike studies of some northern European trees, no clear plateau in this response was observed. A change in the IWUE implies an associated modification of the local carbon and/or hydrological cycles. To resolve these uncertainties, a shift in emphasis away from high-resolution studies towards long, well-replicated time series is proposed to develop the environmental data essential for model evaluation. Identification of old (greater than 700 years) ringless trees demonstrates their potential in assessing the impacts of climatic and atmospheric change. It also shows the scientific and applied value of a conservation policy that ensures the survival of primary forest containing particularly old trees (as in Imbak Canyon and Danum).

Long-term responses of rainforest erosional systems at different spatial scales to selective logging and climatic change

Long-term (21–30 years) erosional responses of rainforest terrain in the Upper Segama catchment, Sabah, to selective logging are assessed at slope, small and large catchment scales. In the 0.44 km2 Baru catchment, slope erosion measurements over 1990–2010 and sediment fingerprinting indicate that sediment sources 21 years after logging in 1989 are mainly road-linked, including fresh landslips and gullying of scars and toe deposits of 1994–1996 landslides. Analysis and modelling of 5–15 min stream-suspended sediment and discharge data demonstrate a reduction in storm-sediment response between 1996 and 2009, but not yet to pre-logging levels. An unmixing model using bed-sediment geochemical data indicates that 49 per cent of the 216 t km−2 a−1 2009 sediment yield comes from 10 per cent of its area affected by road-linked landslides. Fallout 210Pb and 137Cs values from a lateral bench core indicate that sedimentation rates in the 721 km2 Upper Segama catchment less than doubled with initially highly selective, low-slope logging in the 1980s, but rose 7–13 times when steep terrain was logged in 1992–1993 and 1999–2000. The need to keep steeplands under forest is emphasized if landsliding associated with current and predicted rises in extreme rainstorm magnitude-frequency is to be reduced in scale.

Runoff processes in Southeast Asia: role of soil, regolith and rock type.

Runoff processes govern the river hydrograph form, location of return-flow and biogeochemical water quality of tropical forest watersheds. This study reviews the literature on runoff processes from tropical rainforests and applies it to the situation in tropical Southeast Asia. The impact of clay mineralogy on permeability and thence water pathways within the soil, and the role of deep pathways with unconsolidated geological materials (regolith) or permeable rock (solid geology) are emphasised, and a new perceptual model, DELTAmodel, presented. Lastly, the implications of these findings for runoff processes within the Mekong Basin are discussed.

The effects of catchment and riparian forest quality on stream environmental conditions across a tropical rainforest and oil palm landscape in Malaysian Borneo

Abstract Freshwaters provide valuable habitat and important ecosystem services but are threatened worldwide by habitat loss and degradation. In Southeast Asia, rainforest streams are particularly threatened by logging and conversion to oil palm, but we lack information on the impacts of this on freshwater environmental conditions, and the relative importance of catchment versus riparian‐scale disturbance. We studied 16 streams in Sabah, Borneo, including old‐growth forest, logged forest, and oil palm sites. We assessed forest quality in riparian zones and across the whole catchment and compared it with stream environmental conditions including water quality, structural complexity, and organic inputs. We found that streams with the highest riparian forest quality were nearly 4 °C cooler, over 20 cm deeper, had over 40% less sand, greater canopy cover, more stored leaf litter, and wider channels than oil palm streams with the lowest riparian forest quality. Other variables were significantly related to catchment‐scale forest quality, with streams in the highest quality forest catchments having 40% more bedrock and 20 times more dead wood, along with higher phosphorus, and lower nitrate‐N levels compared to streams with the lowest catchment‐scale forest quality. Although riparian buffer strips went some way to protecting waterways, they did not maintain fully forest‐like stream conditions. In addition, logged forest streams still showed signs of disturbance 10–15 years after selective logging. Our results suggest that maintenance and restoration of buffer strips can help to protect healthy freshwater ecosystems but logging practices and catchment‐scale forest management also need to be considered.

A spreadsheet-based technique (lotus 1-2-3) for separating tropical forest storm hydrographs using hewlett and Hibbert's slope

The inclined line separation technique of Hewlett and Hibbert has been widely adopted to separate delayed flow from the total stream storm runoff. Presented here is the application of the technique to highly responsive storm hydrographs using a personal computer method based on a Lotus 1-2-3 spreadsheet. Using discharge measurements (in m3 s-1), catchment area (in km2) and time (in Julian days), the separation slope is adjusted on the monitor screen until the precise time at which the end of quickflow as storm runoff gives way to delayed flow may be established. The application of the inclined line method is compared with other separation techniques applied to the same dataset. The annual stream quickflow runoff for the study catchment was calculated by the four different separating lines - (i) best-fit curve, (ii) N-day after peak, (iii) inclined line and (iv) horizontal line - was 250, 312, 368, and 588 mm, amounting to 33, 31, 51 and 78 per cent respectively of the annual total stream runoff. Separation of flow by computer spreadsheet methods may be consistently applied throughout a dataset and therefore have a comparative advantage over more arbitrary techniques.

Modeling of Water Balance Components in a Small Island via a Numerical Model Application

Abstract PRAVEENA, S.M.; ABDULLAH, M.H.; BIDIN, K., and ARIS, A.Z., 2012. Modeling of water balance components in a small island via a numerical model application. A numerical model (SEAWAT-2000) was selected to estimate the water balance components in the low-lying area of Manukan Island. The water balance components of the current situation have shown that decreasing freshwater storage is due to irregularities in the patterns of recharge rate and overpumping. Scenarios 1 and 2 were selected for factors that affect the water balance components in the current situation. The water balance components in Scenarios 1 and 2 have illustrated sustainable management and utilization of groundwater resources in the study area. A valuable tool to restore groundwater storage has been demonstrated in Scenario 2, which represents the impact of future El Niño events. The best artificial recharge method must be selected with consideration given to other important factors. Extra meteorological and water usage data will aid sustainable management and utilization of groundwater resources, as well as further reduction of the pumping rate. The results of this study provide a management foundation for restoration of the groundwater resources of Manukan Island. The management foundation can also be used with other small islands that have similar hydrogeological conditions for the purpose of groundwater resource protection.

Simulating hourly rainfall occurrence within an equatorial rainforest, Borneo Island

Abstract The ability to simulate characteristics of the diurnal cycle of rainfall occurrence, and its evolution over the seasons is important to the forecasting of hydrological impacts resulting from land-use and climate changes within the humid tropics. This stochastic modelling study uses a generalized linear model (GLM) solution to second-order Markov chain models, as these discrete models are better at describing binary occurrence processes on an hourly time-scale than continuous-time approaches such as stochastic state-space models. We show that transition probabilities derived by the Markov chain method need to be time-varying rather than stationary to simulate the evolution of the diurnal cycle of rainfall occurrence over a Southeast Asian monsoon sequence. The conceptual and pragmatic links between discrete diurnal processes and continuous processes occurring over seasonal periods are thereby simulated within the same model.

Changes in large rainstorm magnitude-frequency over the last century in Sabah, Malaysian Borneo and their geomorphological implications

Increases in the frequency of large to extreme rainfall events are widely predicted with global warming, but evidence from the humid tropics is hampered by a paucity of long-term data. This paper assesses changes in daily rainfall magnitude–frequency and their geomorphological consequences in the equatorial environment of northern Borneo using (1) rare daily rainfall data series for 1906–2012 assembled from archival and current sources and (2) geomorphological process data from the Danum Valley area of eastern Sabah. Rainstorm changes are explored using (1) analysis of frequencies of daily falls above threshold values, (2) extreme-value analysis comparing differences in locally weighted scatterplot smoothing (LOESS) best-fit curves for successive 20-year periods and (3) a novel approach constructing graphs of long-term change in daily falls of 1- to 10-year return period derived from overlapping 20-year LOESS curves. Substantial increases, some statistically significant, since 1980 (intensifying since 1999) are detected at most stations in (1) the frequency of daily falls ≥50, 100 and 200 mm and (2) the magnitudes of daily falls with return periods of 0.2–5.0 years. The scale and temporal patterns of historical change vary with (1) coastal aspect and (2) different parts of the return period spectrum. The period 1921–1940 exhibited higher rainstorm magnitude–frequency than 1952–1979 and demonstrates the dangers of simple deductions derived from post-1960 data comparisons. Geomorphological responses indicated by long-term monitoring at Danum (eastern Sabah), where there has been a marked upswing in heavy rainfalls since 1999, include a threefold increase in slopewash rates, more frequent high flows and higher sediment loads. The very high soil pipe sediment yields, rates of pipe roof collapse and stream channel extension recorded from 2002 to 2012 in pipe systems at Danum may be a consequence of this upswing. Increased landsliding is also a likely consequence, exacerbated in logged, agricultural and urban terrain.

Numerical simulation of seawater intrusion in Manukan Island, East Malaysia

Purpose – This paper aims to define the current and potential extent of seawater intrusion in Manukan Island under different scenarios of varying recharge and pumping rates. The calibrated model was also used to predict the extent of seawater intrusion in low lying area of Manukan Island for two years with all conditions assumed to remain the same as those in December 2009.Design/methodology/approach – Different scenarios of varying recharge and pumping rates based on threats received by Manukan Island were investigated. El‐Nino events and overpumping are represented by varying recharge and pumping rates. Simulation was done using SEAWAT‐2000, the latest modeling software available in groundwater modeling that couples flow and transport together.Findings – The seawater‐freshwater mixing ratio moves landwards after two years of simulation in Scenario 1. In order to control overpumping in this study area, Scenario 2 has resulted in backward movement of the 1.4 percent seawater‐freshwater mixing ratio toward...

A spreadsheet (Lotus 1-2-3) based technique for analysing storm suspended sediment data with particular reference to logging disturbance in tropical forests

The procedure describes a simple and functional method using a commercial spreadsheet (Lotus 1-2-3) to calculate water and sediment yields from measured or given data sets. Suspended sediment concentrations are located on a storm hydrograph and concentrations for unsampled points on the hydrograph are estimated using the principle that the change in concentration between two sampled points will be proportional to the change in discharge between the same two points. The relationships are then solved by a cross-correlation equation using simple formulae in the form of triangle-square equations to calculate water and sediment yields between each sample time. The technique is applied to a 51 month data set from a long-term monitoring programme assessing the impacts of commercial logging operations in Sabah, East Malaysia. Yields for the monitoring period derived by the spreadsheet technique are compared with results from the application of more traditional discharge-sediment rating techniques. In the undisturbed catchment, yields derived from some rating equations compare favourably with the spreadsheet technique. However, in the disturbed catchment, rating techniques proved less applicable because of the continuously changing nature of the catchment in relation to vegetation recovery, exacerbating variation and scatter in the data set. The application of the spreadsheet technique provides detailed information at an individual storm level; however, working with the method on long-term discharge records requires a significant commitment of time as compared to the straightforward application of rating equations