Identifying rainfall-runoff pathways:
Effects of experimental uncertainty on the calculation of hillslope flow paths
Measurement uncertainty is a key hindrance to the quantification of water fluxes at all scales of investigation. Predictions of soil-water flux rely on accurate or representative measurements of hydraulic gradients and field-state hydraulic conductivity. We quantified the potential magnitude of errors associated with the parameters and variables used directly and indirectly within the Darcy - Buckingham soil-water-flux equation. These potential errors were applied to a field hydrometric data set collected from a forested hillslope in central Singapore, and their effect on flow pathway predictions was assessed. Potential errors in the hydraulic gradient calculations were small, approximately one order of magnitude less than the absolute magnitude of the hydraulic gradients. However, errors associated with field-state hydraulic conductivity derivation were very large. Borehole (Guelph permeameter) and core-based (Talsma ring permeameter) techniques were used to measure field-saturated hydraulic conductivity. Measurements using these two approaches differed by up to 3\9 orders of magnitude, with the difference becoming increasingly marked within the B horizon. The sensitivity of the shape of the predicted unsaturated hydraulic conductivity curve to ±5% moisture content error on the moisture release curve was also assessed. Applied moisture release curve error resulted in hydraulic conductivity predictions of less than ±0\2 orders of magnitude deviation from the apparent conductivity. The flow pathways derived from the borehole saturated hydraulic conductivity approach suggested a dominant near-surface flow pathway, whereas pathways calculated from the core-based measurements indicated vertical percolation to depth. Direct tracer evidence supported the latter flow pathway, although tracer velocities were approximately two orders of magnitude smaller than the Darcy predictions. We conclude that saturated hydraulic conductivity is the critical hillslope hydrological parameter, and there is an urgent need to address the issues regarding its measurement further.
Sherlock, M.D., Chappell, N.A., and McDonald, J.J. 2000. Hydrological Processes, 14, 2457-2471.
Parsimonious modelling of water and suspended-sediment flux from nested-catchments affected by selective tropical forestry
The ability to model the suspended-sediment flux (SSflux) and associated waterflow from terrain affected by selective-logging is important to the establishment of credible measures to improve the ecological sustainability of forestry practices. Recent appreciation of the impact of parameter uncertainty on the statistical credibility of complex models with little internal-state validation supports the use of more parsimonious approaches such as Data-Based-Mechanistic (DBM) modelling. The DBM approach combines physically-based understanding with model-structure identification based on transfer-functions and objective statistical inference. Within this study, these approaches have been newly applied to rainfall-SSflux response. The dynamics of the sediment system, together with the rainfall-riverflow system, were monitored at five, nested contributory areas within a 44-ha headwater region in Malaysian Borneo. The data series analysed covered a whole year at a 5-minute resolution, and were collected during a period some 5-6 years after selective timber harvesting had ceased. Physically-based and statistical interpretation of these data was possible given the wealth of contemporary and past hydro-geomorphic data collected within the same region.
The results indicated that parsimonious, three-parameter models of rainfall-riverflow and rainfall-SSflux for the whole catchment describe 80 and 90 % of the variance, respectively, and that parameter changes between scales could be explained in physically meaningful terms. Indeed, the modelling indicated some new conceptual descriptions of the riverflow- and sediment-generation systems. An extreme rainstorm having a 10-20-year return-period was present within the data-series and was shown to generate new mass movements along the forestry roads that had a differential impact on the monitored contributory areas. Critically, this spatially-discrete behaviour was captured by the modelling and may indicate the potential utility of DBM approaches for (a) predicting the differential effect of alternative forestry practices, (b) estimating uncertainty in the behaviour of ungauged areas and (c) forecasting riverflow and SSflux in terrain with temporal changes in rainfall regime and forestry impacts.
Chappell, N.A., McKenna, P., Bidin, K., Douglas, I., and Walsh, R.P.D. 1999. Phil. Trans. Roy. Soc. Lond. B., 354, 1831-1846.
Use of time domain reflectometry (TDR) techniques to monitor soil moisture conditions and slope movement
Subsurface water assessment (moisture status) in unsaturated soils is one of the most important parameters that influences slope instability. Research on the spatial and temporal dynamics of soil water has long been impeded by the lack of an automated technique for the measurement of soil water content in the field. Automating soil water content measurements by time domain reflectometry (TDR) technique has become a very popular method and is widely used. Further development on the TDR technology can also be used for long term monitoring slope movement at potential landslide areas. The objective of this paper is to review methods and case studies of measuring subsurface moisture content conditions in soils and the use of coaxial cables in monitoring slope movement using TDR automatic data logging system. Some advantages and limitations of using TDR techniques in soil studies are highlighted.
Tahir, W.Z., and Chappell, N.A. 1999. Second International Conference on Landslides, Slope Stability and The Safety of Infra-Structures, 27-28 July 1999, Hotel New Otani, Singapore, 1-10.
Multi-scale estimation of effective permeability within the Greenholes Beck catchment
This study addresses the problem of spatial variability and scale effects through a combination of fieldwork and modelling. A catchment has been instrumented to allow measurement of rainfall, stream discharge, piezometric levels and soil moisture content. This was supplemented with a variety of point-scale measurements of soil permeability. Because of the acknowledged problem of relating point-scale values to the catchment-scale techniques for the estimation of hillslope-scale permeability through the use of kinematic wave theory, pulse-wave and tracer experiments have been developed and applied to several hillslope transects. The catchment-scale rainfall-runoff data was first subjected to error analysis and then modelled using a combination of recession curve analysis, TOPMODEL theory and Data-Based Mechanistic modelling.
The upscaled point-scale estimates of permeability were found to give values lower than those provided by the hillslope-scale methods of analysis. This was attributed to the effects of macropore flow. The hillslope-scale estimates were in turn lower than those derived from the catchment-scale analysis. This is attributed to simplification of catchment processes within the catchment-scale model structure.
Ultimately, this work has illustrated the need to undertake multi-scale experiments and modelling in order to develop greater understanding of catchment hydrological processes.
Lancaster, J.W. 1999. Unpublished PhD thesis, Lancaster University, UK
Linking soil type with runoff processes: examples from two tropical rainforest catchments in Southeast Asia
The links between catchment geomorphology and hydrology in tropical catchments is poorly understood. Available literature for the forested humid tropic catchments suggests that hillslope flow pathways vary considerably, attributable to such factors as soil type, catchment geomorphology, lithology and rainfall characteristics. We report on a study from two tropical rain forest catchments in Southeast Asia that seek to characterize contrasting water flow pathways through different soil types: the Ferric Acrisol of the Jungle Falls catchment (Singapore), and the Haplic Alisol of the W8S5 catchment (Malaysia). Potential gradient data over the upper 90 cm of the soil profile were derived from 25 multi-depth tensiometers installed across a distribution of 5x5 m plots. These were combined with hydraulic conductivity data, to determine the magnitude and direction of the Darcian flux. Tracer experiments were also conducted within the plots, whereby 5.8 kg sodium chloride was applied as a line source directly across the upper portion of each plot. Our results indicate that flows within the Ferric Acrisol were predominantly vertical to depth, despite evidence of near-surface saturation during intense rainfall. Flux was predominantly lateral through the near-surface A and B1 horizons of the Haplic Alisol despite the fact that there was no significant difference between the measured saturated hydraulic conductivity of the two horizons (this may be explained by ring permeametry error, which was highly sensitive to the artificial boundary conditions imposed on the excavated soil core). Notwithstanding, we show that the contrasts in flow behavior between the two catchments is related to the very different hydraulic conductivity distributions associated with the physical and chemical composition of the soils.
Sherlock, M.D., Chappell, N.A. and McDonnell, J.J. 1998. American Geophysical Union Fall Meeting, San Francisco 1998. Paper H72E-03.
Multi-scale permeability estimation in a tropical catchment
Physically based and spatially distributed modelling of catchment hydrology involves the estimation of block or whole-hillslope permeabilities. Invariably these estimates are derived by calibration against rainfall-runoff response. Rarely are these estimates rigorously compared with parameter measurements made at the small-scale.
This study uses a parametrically simple model, TOPMODEL, and an uncertainty framework to derive permeability at the catchment-scale. The utility of expert knowledge of the internal catchment dynamics (i.e., extent of saturated area) in constraining parameter uncertainty is demonstrated. Model-derived estimates are then compared with core-based measurements of permeability appropriately upscaled. The observed differences between the permeability estimates derived by the two methods might be attributed to the role of intermediate scale features (natural soil pipes). An alternative method of determining block permeabilities at the intermediate or hillslope scale is described. This method uses pulse-wave tests and explicitly incorporates the resultant effects of phenomena such as soil piping and kinematic wave migration. The study aims to highlight issues associated with parameterising or validating distributed models, rather than provide a definitive solution. The fact that the permeability distribution within the Borneo study catchment is comparatively simple, assists the comparisons. The field data were collected in terrain covered by equatorial rain forest. Combined field measurement and modelling programmes are rare within such environments.
Chappell, N.A., Franks, S.W., and Larenus, J. 1998. Hydrological Processes, 12, 1507-1523.
Ring permeametry: design, operation and error analysis
Assessment of slope stability, soil management or contaminant transport problems usually requires numerous, yet accurate point-measurements of permeability. This technical note describes a new method for the rapid field assessment of permeability in near-surface soils or unconsolidated sediments. The procedure is known as 'ring permeametry' and is an ex situ core-based method giving measurements which can be guaranteed to be stratum-specific, unlike measurements from some in situ techniques. The potential sources of precision and bias error within the method are quantified and their effect on the uncertainty of permeability estimates is illustrated.
Chappell, N.A., and Ternan, J.L. 1997. Earth Surface Processes and Landforms, 22, 1197-1205.
Plot scale hydrometric and tracer characterisation of soil water flow in two tropical forest catchments in Southeast Asia
This research has endeavoured to characterise soil water flow vectors across a distribution of plots within two undistributed tropical rain forest catchments in Southeast Asia. The research focused on the potentially contrasting flow pathways through two different soil types: the Ferric Acrisol of the Jungle Falls catchment (Singapore), and the Haplic Alisol of the W8S5 catchment (Sabah, Malaysia). The research also sought to determine the uncertainty associated with the flow characterisation methodologies. Flow pathways were determined using two quite separate techniques within each experimental plot. A traditional Darcy-approach (i.e. use of the Darcy-Buckingham equation) was used in combination with an artificial tracer approach, to determine if sound flow predictions can be made from local measurements of hydraulic conductivity and capillary potential.
Analysis of the results indicates that the flow pathways within the two soils contrasted markedly. Tracer evidence suggests that soil water movement within the Ferric Acrisol (Jungle Falls catchment) was predominantly vertical. In the Haplic Alisol (W8S5 catchment), the dominant flow pathway of the tracer was laterally though A and B1 horizons. Often rapid tracer breakthrough velocities were observed, which exceeded the measured soil saturated hydraulic conductivity by several orders of magnitude. This infers the macropores are an important flow route within both the Ferri Acrisol and the Haplic Alisol. In general, the Darcy-based approach failed to predict the velocity and dominant pattern of the tracer flows. This resulted from problems associated with the measurement of the soil parameters needed to solve the Darcy-Buckingham equation. Uncertainty analysis of the measurement and empirical techniques used to derive the Darcy-Buckingham equation parameters indicated that there is serious potential for error, particularly in the derivation of saturated hydraulic conductivity.
Sherlock, M.D. 1997. Unpublished PhD thesis, Lancaster University, UK
Identification of hillslope-scale preferential flow from pulse-wave tracing
The prediction of distributed water flows on hillslopes requires the parameterisation of the flow processes at appropriate scales. A methodology is presented for the field parameter-isation of whole hillslopes using pulse-wave experiments with tagged and untagged waters. These tests allow the characterisation of the advective flow and wave-speed distribution. The approach is illustrated using experimental and numerical results from two catchments, one underlain by temperate gley soils, the other tropical acrisols. Whole-slope parameters are compared with the distribution of parameters derived from point-scale measurements and catchment scale inversion of an effective transmissivity value.
Chappell, N.A., Lancaster, J.W., and Beven, K.J. 1996. Proceedings of the 1996 Annual Meeting of the Geological Society of America, Denver, Colorado, 28-31 October 1996.
Tracer and Darcy-based identification of subsurface flow, Bukit Timah forest, Singapore
The spatial distribution of soil hydrological properties and associated flow pathways within a single 0.01 to 10 km2 catchment are usually poorly defined by typical programmes of point-scale measurements. This is due in part to 'measurement uncertainty' caused by instrument error and inadequate spatial sampling, but also to 'conceptual uncertainty' resulting from a deficiency in the Darcy-Buckingham-Richards description of subsurface flow. This study examines these two sources of uncertainty in the identification of subsurface flow beneath the Bukit Timah forest, Singapore. The two distinct approaches of water-tracing and Darcy-Buckingham calculations are compared.
Flow estimates resulting from quasi-steady Darcy-Buckingham calculations were found to be very sensitive to the magnitude and pattern of the saturated hydraulic conductivity, which itself was sensitive to the permeametry technique used. The use of a Hydro-Physics ring permeameter in the estimation of subsurface flows gives rates that are far greater than the mean propagation rates of tagged water, though the approach identifies the relative difference between the behaviour of the two slopes studied.
Whilst the lumped differences between the two tropical slopes may be identified with the Darcy method, the poor correlation with the absolute velocity and direction of the tracer plume means that catchment or even slope scale simulation using parameters of the Darcy-Buckingham-Richards equation will be highly uncertain.
Sherlock, M.D., Chappell, N.A., and Greer, T. 1995. S. Jnl. of Tropical Geography, 16, 2, 197-215.
Danum Valley hydrology programme
There are many popular misconceptions about the hydrological behaviour of rainforests and the impact of forestry activities on these aquatic processes. In part, this is because large natural variability (in space and time) makes it difficult to either extrapolate local observations to the landscape-scale, or identify purely anthropogenic change. To overcome these difficulties, work within tropical rainforests requires application of the best field and modelling technologies, not those that are easiest to resource.
One site where these fundamental issues are being addressed is the Danum Valley region of the Malaysian State of Sabah. Here, on Borneo Island, a group of Malaysian and UK scientists (notably Waidi Sinun, Ian Douglas and Tony Greer) established hydrological monitoring in 1986, as part of a joint enterprise between the Sabah Foundation and the Royal Society of London. The first projects were focused on the potential impacts of the first phase of selective, commercial forestry on the hydrology of lowland rainforest (Douglas et al., 1992 Phil. Trans. Roy. Soc. Lond. B.). This study and subsequent ecohydrological projects established a series of experimental catchments, ranging in size from the 44 ha Baru catchment to the 721 km sq. Segama catchment.
Analysis of the most recent Danum Valley data has shown that the local rainfall regime conforms to that expected of Equatorial region with relatively little annual seasonality and a dominance of short-duration, convective storms that generate flashy river behaviour. Over distances of only a few kilometers, the rainfall exhibits a very high degree of spatial variability, being strongly moderated by the combined effects of monsoonal wind direction and topography (Bidin, 2001 PhD thesis). Additionally, El Niño Southern Oscillation (ENSO) phenomena give rise to cycles in the rainfall (Chappell et al., 2001 Plant Ecology) that become magnified in the river sediment records. Magnified partly by the changing incidence of extreme events (Douglas et al., 1999 Phil. Trans. Roy. Soc. Lond. B.; Chappell et al., 2002 CUP-UNESCO).
Less than 5% of the incoming rainfall generates surface-flow away from stream channels, with almost all riverflow being generated only after rainfall has entered the ground. This flow is strongly related to the preferential pathways of natural soil pipes, percoline zones and rock fractures. Such routes are poorly characterised by traditional methods, and their presence in the Danum catchments has lead to the development of new whole-hillslope, hydraulic tests (Chappell et al., 1998 Hydrological Processes). The role of such preferential or localised phenomena is also seen within the erosion / sediment system. The nested catchment structure utlised within the Baru, clearly demonstrated that over the long-term, a few landslides and smaller collapses were the key source of soil particles flowing down river channels (Chappell et al., 1999 Phil. Trans. Roy. Soc. Lond. B.). While soil-slope instabilities were observable within the undisturbed forest of the Danum Valley Conservation Area (Balamurgan, 1997 PhD thesis; Chappell et al., 1999b Soil Till. Res.; Tangki, 2001 MPhil) the incidence of collapses along timber-lorry, haulage roads seemed particularly high. The greater role of landslides (triggered by changing subsurface-water conditions) relative to canopy disturbance and surficial erosion along skidder-vehicle trails may have important implications for the revision of current, sustainable forestry guidelines. Canopy disturbance did, however, impact on the evaporative transfers to the atmosphere. Surprisingly, greater rainfall reached the ground below undisturbed remnants of rainforest, relative to areas with a high degree of canopy change (Bidin 2001 PhD thesis). Such changes may be offset by reduced transpiration losses following disturbance and this is the focus of a series of new projects.
The role of extreme events, and the changing patterns of evaporation and erosion with forest recovery, underline the importance of continued monitoring at one of the few reference sites for lowland tropical rainforest.
Chappell, N.A., and Bidin, K. 2001. ETFRN News, 33, 5-6.
