Nick A. Chappell

Correlation of physicochemical properties and sub-erosional landforms with aggregate stability variations in a tropical Ultisol disturbed by forestry operations

The stability of soil aggregates against water erosion is a largely unstudied factor in research on the spatially-distributed impacts of tropical forestry. Soil strata with particularly weak aggregates are likely to be the focus for erosional activity whether buried or exposed by either natural processes or forestry activities. Understanding the location and likely cause of such intrinsic instability would, therefore, allow better spatially-distributed parameterisation of erosion models. This study has aimed to identify the range in aggregate stability within a region of Bornean Ultisol disturbed by forestry operations, and to identify the bulk soil properties associated with such stability variations. The sites sampled were subject to a range of denudational processes including piping, rilling and landslide-triggered erosion. Soil profiles with rates of erosion in excess of 10 mm a−1 were shown to have less aggregate stability (as characterised by the rainfall simulation survival index (RSSI)) than those with no visible signs of erosion. Further, large differences in aggregate stability between soil horizons of the same profile were observed and seen to be statistically correlated with the bulk soil properties of organic carbon and clay content (i.e. stabilising agents) and the dispersing agent of exchangeable sodium percentage (ESP) at sites undergoing erosion. Organic carbon appeared to be the most important governing factor, accounting for 56% of the variance in the aggregate stability. The presence of strongly and weakly expanding 2 : 1 clays within the soils may have been an additional de-stabilising factor. Reduced porosity was also observed at eroding sites with weak soil aggregates. The properties of EC25 and sodium adsorption ratio (SAR) were not correlated with the RSSI or the presence of soil erosion. Identification of the role of organic carbon, clay and ESP in the stability of these tropical forest aggregates is important in focusing future, more intensive research on the spatial parameterisation of models to simulate forestry impacts on erosion.

Multi‐scale permeability estimation for 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 up-scaled. 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 parameterizing or validating distributed models, rather than to 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 rainforest. Combined field measurement and modelling programmes are rare within such environments.

Forest Environments in the Mekong River Basin.

Until now, there have been few research works on Cambodian forests because of the long period of civil war, which restricted forest researchers and surveyors in the area. This book presents many new topics of research in forests such as those of Cambodia which were unavailable until now. One of the most attractive features of the volume is that it fills the gaps in data about the world’s forests. The book consists of three parts: forest hydrology, forest management, and forest ecology, designed to provide an understanding of continental river basins. The latest data are presented here, as derived from advanced observation systems for atmospheric flux, ground water level, soil water movement, and stable isotope variation as well as remote sensing, which are used for continuous measurements of forest environments. These research results provide a bounty of fresh scientific information, creating a valuable resource not only for researchers and university students but also for forest administrators. Table of contents Part 1: Forest Hydrology Part II: Forest Management Part III: Forest Ecology

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.

Preliminary analysis of water and solute movement beneath a coniferous hillslope in Mid-Wales, U.K.

Abstract Streams draining coniferous forests are often loaded with solutes such as hydrogen ion, sulphate, nitrate and aluminium. As a result, fish populations can be reduced and water quality may fall below recommended potable standards. The transport of ions into water-courses is governed by the movement of water. Within most temperate and tropical areas the stream discharge and chemistry, during periods of rapid runoff, is dominated by the exfiltration of water and solutes from stream-side soils. The movement of water to stream-side or ‘riparian’ areas remains, however, an enigma. This paper attempts to explain how the riparian area might be rapidly recharged during storm events. Two analytical techniques, the free-surface method and tangent-continuity method, are applied to hydrological properties monitored on a steep coniferous hillslope, during a selected storm event. Comparison of the ionic concentrations of waters within each component of the hydrological system, is used to verify the hydrological analysis. Perched water-tables developed within the basal zones of the O/Ah and Eag soil horizons of the steep podzolic hillslope, during all major storm events. Most of the rapid response within the riparian zone could be explained by lateral flow in these near-surface soil horizons, particularly in the saturated basal zones. This pathway is corroborated by the similarity of riparian zone and near-surface (or topsoil) chemistries. Relatively low concentrations of monomeric aluminium and relatively high concentrations of chloride, sodium and hydrogen ion were observed within these zones, compared with the subsoil (Bsl and B/C) horizons.

Spatially-significant effects of selective tropical forestry on water, nutrient and sediment flows: a modelling-supported review

Selective forestry is a set of commercial forestry practices that involves the selective removal of particular trees within an ‘annual logging coupe’ of forest (Conway, 1982). Selective harvesting within ‘natural forests’ (i.e. those forests that have not be clearfelled for non-forest uses or converted to plantation or agroforestry) covers a very wide range of practices, including highlead and tractor yarding, harvesting of only large, commercial trees, protection of riparian vegetation along rivers, and protection of forest on very steep hills. As a consequence, the intensities of the impacts on the water environment (i.e. water, nutrient and sediment systems) are expected to be very varied. Some of these impacts can be profound. One of the most significant environmental impacts of all types of forestry operations within the humid tropics is accelerated soil erosion (Bruijnzeel, 1992). The resultant input of sediments into rivers leads to damage to fish populations (Martin-Smith, 1998), reduced quality of water supplies, reductions in channel capacity which affects flood risk and boat traffic (Sheffield et al., 1995), and the inundation of offshore corals (MacDonald et al., 2001). Development of selective harvesting techniques when applied to natural forests in the tropics are currently being focused on socalled ‘Reduced-Impact-Logging’ (RIL) or ‘closely supervised’ methods which aim to improve the ‘sustainability’ of timber production and reduce wider environmental damage. These RIL procedures include optimising skid trail networks, given a knowledge of the exact location of each tree to be felled, minimising stream crossings, minimising ‘skid-trail’ earthworks, maintaining canopy-cover over skid trails, construction of water-bars on unused haulage roads, and critically, careful supervision of all forestry operations (Abdul Rahim et al., 1997; Pinard et al., 1995; van der Hout, 1999). Additionally, where natural forests have been logged and regeneration has been poor, then ‘enrichment planting’ of commercial (and non-commercial) trees is beginning to be used as part of selective forestry management (Adjers et al., 1995; Kobayashi et al., 2001). Research into the water-related impacts of natural forest management is important throughout tropical regions given (i) the economic importance of such forestry, (ii) the desire to identify the least damaging forestry practices, and (iii) the large areal extent of natural forest. For example, within South and South East Asia, natural forests (managed and undisturbed) currently cover 25% of the land area while within the continents of South America and Africa they occupy 47 and 12%, respectively (Iremonger et al., 1997).

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.

Mobilization of optically invisible dissolved organic matter in response to rainstorm events in a tropical forest headwater river

This study emphasizes the importance of rainstorm events in mobilizing carbon at the soil-stream interface from tropical rainforests. Half-hourly geochemical/isotopic records over a 13.5 h period from a 20 km2 tropical rainforest headwater in Guyana show an order of magnitude increase in dissolved organic carbon (DOC) concentration in less than 30 mins (10.6–114 mg/L). The composition of DOC varies significantly and includes optically invisible dissolved organic matter (iDOM) that accounts for a large proportion (4–89%) of the total DOC, quantified using size exclusion chromatography (SEC). SEC suggests that iDOM is comprised of low molecular weight organic moieties, which are likely sourced from fresh leaf litter and/or topsoil, as shown in soils from the surrounding environment. Although poorly constrained at present, the presence of iDOM further downstream during the wet season suggests that this organic matter fraction may represent an unquantified source of riverine CO2 outgassing in tropical headwaters, requiring further consideration.

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.