Alan D. Ziegler

The Rubber Juggernaut

The demise of swidden cultivation in Southeast Asia may have devastating environmental consequences. Rubber plantations are expanding rapidly throughout montane mainland Southeast Asia (1–3). More than 500,000 ha may have been converted already in the uplands of China, Laos, Thailand, Vietnam, Cambodia, and Myanmar (see the figure, panel A). By 2050, the area of land dedicated to rubber and other diversified farming systems could more than double or triple, largely by replacing lands now occupied by evergreen broadleaf trees and swidden-related secondary vegetation (2). What are the environmental consequences of this conversion of vast landscapes to rubber?

Importance of rural roads as source areas for runoff in mountainous areas of northern Thailand

Abstract Unpaved road surfaces have extremely low infiltration rates compared with other watershed land surfaces and, therefore, are significant source areas for erosion-producing Horton overland flow. The hydrologic role of roads is an important issue in mountainous areas of the tropics where erosion control efforts are predominately focused on deforestation and agricultural practices. We report on an investigation of soil physical properties that control excess rainfall (rainfall intensity in excess of infiltration capacity) on rural roads and surrounding lands in a mountainous watershed in northern Thailand. The results of our disk permeameter measurements indicate that saturated hydraulic conductivity on unpaved roads is about one order of magnitude lower than on any other land-surface type. Median saturated hydraulic conductivities were not exceeded by measured rainfall intensity on any land use except road surfaces and roadside margins. By simulating excess rainfall, we found that in contrast with other areas of the watershed, the road surface tends to generate excess rainfall early in a rain event, and on nearly all of its area. Despite the relatively small areal extent of road-related surfaces (

Detection of Intensification in Global- and Continental-Scale Hydrological Cycles: Temporal Scale of Evaluation

Diagnostic studies of offline, global-scale Variable Infiltration Capacity (VIC) model simulations of terrestrial water budgets and simulations of the climate of the twenty-first century using the parallel climate model (PCM) are used to estimate the time required to detect plausible changes in precipitation ( P), evaporation (E), and discharge (Q) if the global water cycle intensifies in response to global warming. Given the annual variability in these continental hydrological cycle components, several decades to perhaps more than a century of observations are needed to detect water cycle changes on the order of magnitude predicted by many global climate model studies simulating global warming scenarios. Global increases in precipitation, evaporation, and runoff of 0.6, 0.4, and 0.2 mm yr21 require approximately 30‐45, 25‐35, and 50‐60 yr, respectively, to detect with high confidence. These conservative detection time estimates are based on statistical error criteria (a 5 0.05, b 5 0.10) that are associated with high statistical confidence, 1 2 a (accept hypothesis of intensification when true, i.e., intensification is occurring), and high statistical power, 1 2 b (reject hypothesis of intensification when false, i.e., intensification is not occurring). If one is willing to accept a higher degree of risk in making a statistical error, the detection time estimates can be reduced substantially. Owing in part to greater variability, detection time of changes in continental P, E, and Q are longer than those for the globe. Similar calculations performed for three Global Energy and Water Experiment (GEWEX) basins reveal that minimum detection time for some of these basins may be longer than that for the corresponding continent as a whole, thereby calling into question the appropriateness of using continental-scale basins alone for rapid detection of changes in continental water cycles. A case is made for implementing networks of small-scale indicator basins, which collectively mimic the variability in continental P, E, and Q, to detect acceleration in the global water cycle.

Carbon outcomes of major land‐cover transitions in SE Asia: great uncertainties and REDD+ policy implications

Policy makers across the tropics propose that carbon finance could provide incentives for forest frontier communities to transition away from swidden agriculture (slash-and-burn or shifting cultivation) to other systems that potentially reduce emissions and/or increase carbon sequestration. However, there is little certainty regarding the carbon outcomes of many key land-use transitions at the center of current policy debates. Our meta-analysis of over 250 studies reporting above- and below-ground carbon estimates for different land-use types indicates great uncertainty in the net total ecosystem carbon changes that can be expected from many transitions, including the replacement of various types of swidden agriculture with oil palm, rubber, or some other types of agroforestry systems. These transitions are underway throughout Southeast Asia, and are at the heart of REDD+ debates. Exceptions of unambiguous carbon outcomes are the abandonment of any type of agriculture to allow forest regeneration (a certain positive carbon outcome) and expansion of agriculture into mature forest (a certain negative carbon outcome). With respect to swiddening, our meta-analysis supports a reassessment of policies that encourage land-cover conversion away from these [especially long-fallow] systems to other more cash-crop-oriented systems producing ambiguous carbon stock changes - including oil palm and rubber. In some instances, lengthening fallow periods of an existing swidden system may produce substantial carbon benefits, as would conversion from intensely cultivated lands to high-biomass plantations and some other types of agroforestry. More field studies are needed to provide better data of above- and below-ground carbon stocks before informed recommendations or policy decisions can be made regarding which land-use regimes optimize or increase carbon sequestration. As some transitions may negatively impact other ecosystem services, food security, and local livelihoods, the entire carbon and noncarbon benefit stream should also be taken into account before prescribing transitions with ambiguous carbon benefits.

Runoff generation and sediment production on unpaved roads, footpaths and agricultural land surfaces in northern Thailand

Rainfall simulation was used to examine runoff generation and sediment transport on roads, paths and three types of agricultural fields in Pang Khum Experimental Watershed (PKEW), in mountainous northern Thailand. Because interception of subsurface flow by the road prism is rare in PKEW, work focused on Horton overland flow (HOF). Under dry antecedent soil moisture conditions, roads generated HOF in c. 1 min and have event runoff coefficients (ROCs) of 80 per cent, during 45 min, c. 105 mm h−1 simulations. Runoff generation on agricultural fields required greater rainfall depths to initiate HOF; these surfaces had total ROCs ranging from 0 to 20 per cent. Footpaths are capable of generating erosion-producing overland flow within agricultural surfaces where HOF generation is otherwise rare. Paths had saturated hydraulic conductivity (Ks) values 80–120 mm h−1 lower than those of adjacent agricultural surfaces. Sediment production on roads exceeded that of footpaths and agricultural lands by more than eight times (1·23 versus < 0·15 g J−1). Typically, high road runoff volumes (owing to low Ks, c. 15 mm h−1) transported relatively high sediment loads. Initial road sediment concentrations exceeded 100 g l−1, but decayed with time as loose surface material was removed. Compared with the loose surface layer, the compacted, underlying road surface was resistant to detachment forces. Sediment concentration values for the road simulations were slightly higher than data obtained from a 165 m road section during a comparable natural event. Initial simulation concentration values were substantially higher, but were nearly equivalent to those of the natural event after 20 min simulation time. Higher sediment concentration in the simulations was related to differences in the availability of loose surface material, which was more abundant during the dry-season simulations than during the rainy season natural event. Sediment production on PKEW roads is sensitive to surface preparation processes affecting the supply of surface sediment, including vehicle detachment, maintenance activities, and mass wasting. The simulation data represent a foundation from which to begin parameterizing a physically based runoff/erosion model to study erosional impacts of roads in the study area. Copyright

Transpiration in a small tropical forest patch

A field study was conducted of microclimate and transpiration within a 12 ha patch of advanced secondary forest surrounded by active or recently abandoned swidden fields. Differences in microclimate among stations located within and near the patch, give evidence of the effects of the adjacent clearing on the environment in the patch. Volumetric soil moisture content at the end of the dry season was lowest at the two edge sites, suggesting greater cumulative dry season evapotranspiration (ET) there than at swidden and forest interior sites. Total evaporation, based on energy balance methods, was also higher at the two edge sites than at the swidden or forest interior sites. Spatial differences in evaporation decreased as conditions became wetter. Measurements of sap flow in nine trees near the southwestern edge of the patch and nine trees in the patch interior indicate considerable variability in transpiration among the three monitored tree species, Vernicia montana, Alphonsea tonkinensis, and Garcinia planchonii. Dry-period transpiration averaged about 39 and 43% of total evaporation for edge and interior trees, respectively, increasing to 60 and 68% after the start of rains. Transpiration in both zones was well-correlated with micrometeorological conditions in the adjacent clearing, implying that transpiration edge effect is greatest when conditions are favorable for high positive heat advection from the clearing to the forest edge. Transpiration rates of well-exposed trees were higher than poorly-exposed trees, and decreased with distance from the edge at a statistically significant rate of −0.0135 mm per day m −1 . Although the results on the strength of transpiration edge effect are somewhat equivocal due to variability within the small sample, there is clear evidence that ET within the patch is influenced by the surrounding clearings. If edges experience higher ET, greater fragmentation would result in higher regional evaporative flux, which would partly compensate for the reduction in regional ET due to deforestation.

Interstorm surface preparation and sediment detachment by vehicle traffic on unpaved mountain roads

Road survey and field rainfall simulation experiments have shown that the erodibility of a road surface is dynamic. In the absence of extreme runoff events, dynamic erodibility results from the generation and removal of easily entrained surface material by human road surface maintenance activities, vehicular detachment and overland flow events. Maintenance activities introduce easily transportable material to the road surface where it can be entrained by overland flow. Traffic in dry conditions detaches material that is quickly removed during subsequent overland flow events. The pre-storm erodibility of a road is therefore largely a function of maintenance and vehicle activity since the last overland flow event. During rainstorms, vehicle passes increase sediment production by detaching/redistributing surface material and creating efficient overland flow pathways for sediment transport. However, if incision of tracks by overland flow does not occur, post-pass sediment transport quickly returns to pre-pass rates. Field rainfall simulation data suggest that sediment transport resulting from during-storm vehicle passes is greatly influenced by the presence of existing loose material, which again is a function of prior road usage and maintenance activities. Incorporation of vehicular effects into physically based road erosion models may be possible by parameterizing both during-storm and inter-storm changes in the supply of loose surface material as changes in surface erodibility. Copyright

Road-deposited sediments in an urban environment: A first look at sequentially extracted element loads in grain size fractions

Sediments stored in urban drainage basins are important environmental archives for assessing contamination. Few studies have examined the geochemical fractionation of metals in individual grain size classes of solid environmental media. This is the first study of road sediments to quantify the mass loading of Al, Cu, Pb, and Zn in individual grain size classes (<63μm to 1000-2000μm) and partition contributions amongst four sequentially extracted fractions (acid extractable, reducible, oxidizable, and residual). The optimized BCR sequential extraction procedure was applied to road sediments from Palolo Valley, Oahu, Hawaii. Road sediments from this non-industrialized drainage basin exhibited significant enrichment in Cu, Pb, and Zn. Metal mass loading results indicate that the <63μm grain size class dominated almost all fraction loads for a given element. The residual fraction dominated the Al loading for this geogenic element. The reducible fraction, associated with Fe and Mn oxides, was the most important component for Cu, Pb, and Zn loading. These results have direct implications for environmental planners charged with reducing sediment-associated contaminant transport in urbanized drainage basins.

Acceleration of Horton overland flow and erosion by footpaths in an upland agricultural watershed in northern Thailand

Through field rainfall simulation experiments in an upland mountainous watershed of northern Thailand, we have identified two phenomena that increase the potential for Horton overland flow (HOF) generation on agricultural lands. First, there appears to be a transition period of 12–18 months, extending from the time of abandonment until the formation of a dense vegetation layer capable of intercepting rainfall and ponding surface water, during which HOF generation is accelerated. Simulation data indicate these recently abandoned fields may have runoff coefficients (ROCs) as high as 40% during large seasonal storms with wet antecedent soil moisture conditions. In comparison, actively cultivated lands and advanced (>16–18 months) fallow fields, the land surfaces existing before and after the threshold period, have ROCs≤4%. Secondly, compacted path surfaces initiate HOF within agricultural fields, which have saturated hydraulic conductivity (Ks) values that are 100–200 mm h−1 higher. In the study area, path/furrow networks, comprising 8–24% of field surface areas, are designed to provide walking access within fields and channel excess surface flow from the fields. Compared with hoed surfaces, path/furrows reduce the time to HOF generation by about 85% and have ROCs that are six times higher. Access paths have the lowest Ks values of all watershed surfaces, but conveyance efficiency of HOF generated on these surfaces is low. Even recently created field paths are capable of reducing runoff generation time by 40–90% and producing sixfold increases in ROCs. Collectively, the data suggest that agricultural erosion rates are accelerated during the 12–18-month threshold period following abandonment and during storms where path-generated HOF interacts with adjacent planting surfaces. Despite having periods of increased HOF generation, the total HOF contribution from agricultural fields to the basin stream hydrograph is similar in magnitude to that of unpaved roads, which occupy 95% less land area.

Partitioning total erosion on unpaved roads into splash and hydraulic components: The roles of interstorm surface preparation and dynamic erodibility

Field rainfall simulation experiments at two sites are used to partition sediment transport on unpaved roads into splash and hydraulic erosion components. Rain splash processes contributed 38 - 45% of total sediment output, with instantaneous contributions being variable throughout 60-min high-energy events. For low- and medium-magnitude rainstorms, splash erosion on roads is initially controlled by the removal of easily erodible material, followed by a dramatic reduction in sediment output associated with limited detachment from the resistant, highly compacted road surface. A conceptual model explaining temporal variations in splash and hydraulic erosion as functions of prestorm surface preparation (via traffic, maintenance, and mass wasting processes) is presented. For situations where loose sediment is readily available, rain splash energy is less important to sediment detachment. If the loose layer is diminished (e.g., following an overland flow event) or protected by a surface crust, splash energy is needed to detach material from the road surface. Equations in most physically based erosion models do not predict temporal variations in road sediment transport that result from the removal of a loose surface layer of finite depth. A strategy that successfully treats this removal as changes in road erodibility is introduced.