Unstable Terrain Research Articles

Tropical forest dynamics in unstable terrain: a case study from New Guinea

Abstract:Long-term forest dynamics plots in the tropics tend to be situated on stable terrain. This study investigated forest dynamics on the north coast of New Guinea where active subduction zones are uplifting lowland basins and exposing relatively young sediments to rapid weathering. We examined forest dynamics in relation to disturbance history, topography and soil nutrients based on partial re-census of the 50-ha Wanang Forest Dynamics Plot in Papua New Guinea. The plot is relatively high in cations and phosphorus but low in nitrogen. Soil nutrients and topography accounted for 29% of variation in species composition but only 4% of variation in basal area. There were few areas of high biomass and most of the forest was comprised of small-diameter stems. Approximately 18% of the forest was less than 30 y old and the annual tree mortality rate of nearly 4% was higher than in other tropical forests in South-East Asia and the neotropics. These results support the reputation of New Guinea's forests as highly dynamic, with frequent natural disturbance. Empirical documentation of this hypothesis expands our understanding of tropical forest dynamics and suggests that geomorphology might be incorporated in models of global carbon storage especially in regions of unstable terrain.

Effects of episodic sediment supply on bedload transport rate in mountain rivers. Detecting debris flow activity using continuous monitoring

Monitoring of sediment transport from hillslopes to channel networks as a consequence of floods with suspended and bedload transport, hyperconcentrated flows, debris and mud flows is essential not only for scientific issues, but also for prevention and mitigation of natural disasters, i.e. for hazard assessment, land use planning and design of torrent control interventions. In steep, potentially unstable terrains, ground-based continuous monitoring of hillslope and hydrological processes is still highly localized and expensive, especially in terms of manpower. In recent years, new seismic and acoustic methods have been developed for continuous bedload monitoring in mountain rivers. Since downstream bedload transport rate is controlled by upstream sediment supply from tributary channels and bed-external sources, continuous bedload monitoring might be an effective tool for detecting the sediments mobilized by debris flow processes in the upper catchment and thus represent an indirect method to monitor slope instability processes at the catchment scale. However, there is poor information about the effects of episodic sediment supply from upstream bed-external sources on downstream bedload transport rate at a single flood time scale. We have examined the effects of sediment supply due to upstream debris flow events on downstream bedload transport rate along the Yotagiri River, central Japan. To do this, we have conducted continuous bedload observations using a hydrophone (Japanese pipe microphone) located 6.4 km downstream the lower end of a tributary affected by debris flows. Two debris flows occurred during the two-years-long observation period. As expected, bedload transport rate for a given flow depth showed to be larger after storms triggering debris flows. That is, although the magnitude of sediment supply from debris flows is not large, their effect on bedload is propagating >6 km downstream at a single flood time scale. This indicates that continuous bedload observations could be effective for detecting sediment supply as a consequence of debris flow events.

'O Chão é a Grande Pregunta': Non-Human Temporalities in Nuno Ramos

This essay argues that Nuno Ramos’s installations and writings represent a decisive intervention in current aesthetic debates on temporality and the non-human. Working on the unstable terrain between the living and the dead, the organic and the inorganic, the fossilized and the spectral, Ramos’s work sheds light on scales and frames that challenge the anthropocentric sensorium of our critical habits. By focusing on writings included in Cujo (1993) and Essaio Geral (2007), the essay analyzes how the configuration of such critical perspective in Ramos’s work interrogates and reformulates the ways in which memory and the political interface there where the very notion of life—the bios that articulates the biopolitical—is at stake.

Measuring displacements of the Thompson River valley landslides, south of Ashcroft, BC, Canada, using satellite InSAR

The Thompson River valley, south of Ashcroft in British Columbia, Canada, has experienced several landslides since the mid-1800s. The national railways that run along the valley cross a number of these landslides. All the landslides occur in glacial deposits, typically sliding on weak clay layers. Some have failed rapidly to very rapidly and are currently inactive or showing deformation rates from a few millimeters to centimeters per year. An evaluation of satellite InSAR (Interferometric Synthetic Aperture Radar) using RADARSAT-2 images between September 2013 and November 2015 provides new insight into landslide displacements in the Thompson River valley. This information enhances the ongoing hazard management of unstable terrain. This paper presents the comparison of the InSAR measurements with other instrumentation (GPS and ShapeAccelArrays™ -SAA) installed at one moving landslide and then addresses the extent and magnitude of the slope movements observed. InSAR was found to provide similar displacement values to those measured otherwise. The stable location showed displacements of +/−1 mm with an average near zero during the whole monitoring period. Six areas of slope movement were identified within the study area, all within or adjacent to the footprints of past landslides. The average line of sight (LOS) displacement rates range between 11 and 39 mm/year. Most of the landslides exhibited seasonal variations in velocity that corresponds to changes in river elevation in the valley.

Paisajes de sobrevida

Algunas de las investigaciones contemporáneas más significativas en la literatura y en el arte de Brasil y del Cono Sur giran en torno a la cuestión de la memoria, el umbral entre lo vivo y lo muerto y las formas en las que temporalidades en conflicto y anacronismos articulan una crítica del presente. La memoria, evidentemente, se ha vuelto un terreno saturado políticamente en las democracias de América del Sir, donde las “políticas de la memoria”, conjugadas entre el testimonio y las pedagogías colectivas- definen en gran parte la articulación entre cultura y política. En este contexto, las instalaciones y la escritura de Nuno Ramos, trabajando en el terreno inestable entre lo vivo y lo muerto, lo orgánico y lo inorgánico, lo fosilizado y lo espectral, representan una intervención decisiva. En el trabajo de Ramos, la memoria nunca es enteramente humana; es inseparable de cuerpos y materias no humanas. Este ensayo se focaliza en los “paisajes de sobrevida” en los que la configuración de esta perspectiva crítica del trabajo de Ramos interroga y reformula las maneras en las que la memoria y lo política se entrecruzan allí donde la noción misma de vida –el bios ­que articula lo biopolítico- está en juego.

INTEGRATED SAR TECHNOLOGIES FOR MONITORING THE STABILITY OF MINE SITES: APPLICATION USING TERRASAR-X AND RADARSAT-2 IMAGES

Abstract. The last three decades have seen significant mining development in the northern regions of Canada, where the freeze and thaw cycle of permafrost and corresponding surface subsidence and heave represent a significant challenge at all mining stages, from the design of infrastructures to the monitoring of restored areas. Over the past ten years, SAR interferometry has been widely used to monitor ground surface deformation. With this technique, changes in phase between two SAR acquisitions are used to detect centimetre to millimetre surface displacements over a large area with high spatial resolution. This paper presents the results of a project that aims to develop a SAR solution to provide useful information for environmental monitoring and assessing the stability of mining sites. RADARSAT-2 and TerraSAR-X images acquired during the summer of 2014 were used to measure the displacements of ground surface, infrastructures and stockpiles caused by seasonal changes in permafrost extent. The study area is an open-pit mine located in Nunavut, northern Canada, in the continuous permafrost zone. Results shown that surface displacements calculated from RADARSAT-2 and TerraSAR-X are very similar and in agreement with scientific and terrain knowledge. Significant displacements were observed in loose soil areas while none was detected in bedrock and rock outcrop areas. The areas most affected by active layer changes showed surface subsidence during the thaw settlement period. Thus, InSAR can be used as a tool to guide the siting and design of new infrastructure as well as highlighting risks in areas of unstable terrain.

Forest carbon in lowland Papua New Guinea: Local variation and the importance of small trees.

Efforts to incentivize the reduction of carbon emissions from deforestation and forest degradation require accurate carbon accounting. The extensive tropical forest of Papua New Guinea (PNG) is a target for such efforts and yet local carbon estimates are few. Previous estimates, based on models of neotropical vegetation applied to PNG forest plots, did not consider such factors as the unique species composition of New Guinea vegetation, local variation in forest biomass, or the contribution of small trees. We analysed all trees >1 cm in diameter at breast height (DBH) in Melanesia's largest forest plot (Wanang) to assess local spatial variation and the role of small trees in carbon storage. Above-ground living biomass (AGLB) of trees averaged 210.72 Mg ha−1 at Wanang. Carbon storage at Wanang was somewhat lower than in other lowland tropical forests, whereas local variation among 1-ha subplots and the contribution of small trees to total AGLB were substantially higher. We speculate that these differences may be attributed to the dynamics of Wanang forest where erosion of a recently uplifted and unstable terrain appears to be a major source of natural disturbance. These findings emphasize the need for locally calibrated forest carbon estimates if accurate landscape level valuation and monetization of carbon is to be achieved. Such estimates aim to situate PNG forests in the global carbon context and provide baseline information needed to improve the accuracy of PNG carbon monitoring schemes.

GIS-based bivariate statistical modelling for earthquake-triggered landslides susceptibility mapping related to the 2008 Wenchuan earthquake, China

The main purpose of this research is to evaluate the modelling capability and predictive power of a bivariate statistical method for earthquake-triggered landslide susceptibility mapping. A weight index ( W i ) model was developed for the 2008 Wenchuan earthquake region in Sichuan Province, China, using a wide range of optical remote sensing data, and carried out on the basis of a geographic information system (GIS) platform. The 2008 Wenchuan earthquake triggered 196007 landslides, with a total area of 1150.43 km 2 , in an approximately oblong area around the Yingxiu–Beichuan coseismic surface fault-rupture (the Yingxiu–Beichuan fault). The landslides of the study area were mapped using visual interpretation of high-resolution satellite images and aerial photographs, both pre- and post-earthquake, and checked in the field at various locations. As a consequence, a nearly complete inventory of landslides triggered by the Wenchuan earthquake was constructed. Topographic and geological data and earthquake-related information were collected, processed and constructed into a spatial database using GIS and image processing technologies. A total of 10 controlling parameters associated with the earthquake-triggered landslides were selected, including elevation, slope angle, slope aspect, slope curvature, slope position, lithology, seismic intensity, peak ground acceleration (PGA), distance from the Yingxiu–Beichuan fault, and distance along this fault. To assist with the development of the model, the complete dataset of 196007 landslides was randomly partitioned into two subsets; a training dataset, which contains 70% of the data (137204 landslides, with a total area of 809.96 km 2 ), and a testing dataset accounting for 30% of the data (58803 landslides, with a total area of 340.47 km 2 ). A landslide susceptibility index map was generated using the training dataset, the 10 impact factors, and the W i model. In addition, for a conditionally dependent factor analysis, seven other factor-combination cases were also used to construct landslide susceptibility index maps. Finally, these eight landslide susceptibility maps were compared with the training data and testing data to obtain model capability (success rate) and predictive power (predictive rate) information. The validation results show that the success and predictive rates of the W i modelling exceeded 90% for the approaches that include the use of seismic factors. The final landslide susceptibility map can be used to identify and delineate unstable susceptibility-prone areas, and help planners to choose favourable locations for development schemes, such as infrastructure, construction and environmental protection schemes. The generic component of this research would allow application in other regions affected by high-intensity earthquakes and unstable terrain covering very large areas.

SEDIMENT YIELD ESTIMATION AND PRIORITIZATION OF WATERSHED USING REMOTE SENSING AND GIS

Abstract. Soil erosion is the greatest destroyer of land resources in Indravati catchment. It carries the highest amount of sediment compared to other catchment in India. This catchment spreading an area of 41,285 square km is drained by river Indravati, which is one of the northern tributaries of the river Godavari in its lower reach. In the present study, USLE is used to estimate sediment yield at the outlet of river Indravati catchment. Both magnitude and spatial distribution of potential soil erosion in the catchment is determined. From the model output predictions, it is found that average erosion rate predicted is 18.00 tons/ha/year and sediment yield at the out let of the catchment is 22.31 Million tons per year. The predicted sediment yield verified with the observed data. The Indravathi basin is divided into 424 sub-watersheds and prioritization of all 424 sub-watersheds is carried out according to soil loss intensity for soil conservation purpose. Generated soil loss map will be useful to soil conservationist and decision makers for watershed management. Overall 19.71 % of the area is undergoing high erosion rates which are a major contributor to the sediment yield (78.04 %) in the catchment. This area represents high-priority area for management in order to reduce soil losses, which are mostly found in upstream of the catchment. It is indicated that the areas of high soil erosion can be accounted for in terms of steep unstable terrain, and the occurrence of highly erodible soils and low vegetation cover.

Rethinking the Idea/Ideal of Pedagogical Control: Assemblages of De/Stabilisation

Inspired by a Deleuzian focus on a-personal machineries, this discussion focuses on a challenge to conventional pedagogical relationships and presents the possibility to think about pedagogical control differently. The work of Deleuze also underpins the methodological approach. Drawing on an extract constructed from the author's doctoral project and a painting by the Norwegian artist Lars Elling, the article features assemblages of control and silence that fold and unfold in the resonance of each other, working through destabilising and stabilising elements. The movement between these assemblages challenges the idea/ideal of teachers as the ones in control and command, as it opens to a shifting in between the subjects involved, regardless of their age or status. The article concludes by presenting some of the ethical implications involved when pedagogy is pulled into uncertain and unstable terrains.

Pattern Studies of Strain Distributions for Detecting Pipe Wrinkling

As both onshore and offshore pipeline constructions push further into higher risk terrains, such as geologically unstable terrain and the Arctic region, the risk of local buckling failure (wrinkling) for these buried pipelines has been increasing gradually. However, current methods used to prevent buried pipelines from buckling failure are expensive, time consuming, and unreliable. Therefore, to overcome these problems, a reliable method of predicting pipeline wrinkling is proposed. The method can provide active warning for pipeline wrinkling through a decision-making system (DMS). The DMS has been designed to identify strain distribution patterns and their development on critical pipe segments and detect the onset of pipe wrinkling. To create a reliable DMS, studies of the strain distribution patterns of line-pipes during pipe buckling are very important. In this paper, the strain distribution patterns of various line-pipes are presented. These line-pipes have different material and geometric properties, loading conditions, and manufacturing conditions. A total of 32 sets of experimental results and 72 sets of finite element analyses (FEA) along with parametric studies were included in the study. The study revealed significant behavioral characteristics of the strain distribution patterns during pipe buckling and important parameters affecting these strain patterns. For practical application, three thresholds of the strain distribution patterns are proposed. Furthermore, the optimal positions and spacing of the strain measurements for early detecting pipelines wrinkling are discussed as well.

Terrain stability mapping on British Columbia forest lands: an historical perspective

Land management associated with forest practices in British Columbia (BC) over the last three decades has led to the development of terrain stability hazard mapping. Terrain stability mapping (TSM) in BC originated in the early 1970s, when forest harvesting was progressing from valley bottoms onto steep, unstable terrain, which led to an increase in harvesting- and road-related landslides. Since then TSM methods have been evolved. Beginning in the early 1970s, terrain hazards were incorporated into the forest inventory classification system to delineate environmentally sensitive areas for land-use planning. By 1974, operational terrain stability maps were introduced by the MacMillan Bloedel forest company on the Queen Charlotte Islands. In the 1980s, this method was adopted by other forest companies and government agencies along the BC coast and then extended to the BC interior in the 1990s. The system was refined over time, based on new knowledge and on the introduction of mapping standards, including standards for capture and presentation of digital maps. In 1995, reconnaissance terrain stability mapping and detailed terrain stability mapping were formalized with three and five hazard classes, respectively. More recently, qualitative and semi-quantitative approaches to predict landslide occurrence based on terrain and landslide inventories have been incorporated into the techniques for TSM.

Replication of a terrain stability mapping using an Artificial Neural Network

Subjective geomorphic mapping is a method commonly used for landslide hazard zonation. This method relies heavily on the skills and experience of the mapper, and therefore, its major drawbacks are the high costs and lack of consistency between products generated by different terrain mappers. In this study a method for cost-effective and consistent replication of subjective geomorphic mappings is demonstrated, by using a type of Artificial Neural Network named Learning Vector Quantization. This paper presents a study conducted in the Canadian province of British Columbia employing a high-quality data set. By utilizing Learning Vector Quantization, stable and unstable terrains were delineated with a similarity of approximately 91%, compared to the mapping produced by terrain specialists. Also, in this process, slope, elevation, aspect, and existing geomorphic processes were identified as the terrain attributes that contributed most to the quality of the mapping.

Application of decision analysis to forest road deactivation in unstable terrain.

Resource managers require objective methodologies to optimize decisions related to forest road deactivation and other aspects of road management, especially in steep terrain, where road-related slope failures inflict extensive environmental damage. Decision analysis represents a systematic framework that clearly identifies real options and critical decision points. This framework links current decisions with expected future outcomes and provides advantages such as a common currency to systematically explore the liability consequences of limited budget expenditures to road deactivation and other road-related activities. Furthermore, the decision framework prevents the analysis from becoming hopelessly entangled by the vast number of possibilities generated by the alternative occurrences, magnitudes, and consequences of landslide/debris flow events and provides the information required for the first step of an adaptive management process. Here, a structured analysis of potential environmental risks for a road deactivation project in coastal British Columbia, Canada is presented. The application of decision analysis generates a ranking of the expected benefits of proposed deactivation activities on various road sections. The ranking distinguishes between road sections that offer high expected benefit from those that offer moderate to low expected benefit. Seventeen of 171, 100-m road segments accounted for 18% of the cumulative cost and 98% of the cumulative expected net benefits from road deactivation. Furthermore, the cost of deactivating a section of road is related to the expected benefit from such deactivation, thus providing the basis for more effective resource allocation and budgeting decisions.

Integration of Comprehensive Environmental and Social Measures into Low-Volume Road Design for the Hills of Nepal

Road construction through Nepal’s hilly and mountainous terrain has historically had a significant impact on the local environment and land-owners, primarily because of the steep and often unstable terrain traversed, monsoonal climate, necessity for low-cost roads, and no landowner compensation for compulsorily acquired land. The design phase of the Road Maintenance and Development Project sought to incorporate comprehensive environmental and social measures into the design of low-cost roads in the western hills of Nepal to achieve more stable and lower-impact roads. The project involved the design of 196 km of new roads and 253 km of upgrading to existing roads by integrating engineering, environmental, social, and economic factors throughout the design process. The project began with the screening of candidate project roads on the basis of economic, environmental, and social factors by using a weighted scoring system to select those roads that would be most advantageous in relation to passenger and goods transport and associated economic development, while minimizing adverse environmental and social impacts. Proposed new road alignments were refined primarily through a walkover survey by a multidisciplinary team, ensuring that environmental and social issues were incorporated into this key stage of road design, when significant impacts can be avoided. Detailed road design incorporated comprehensive environmental management and social planning through integrated planning. Project outcomes include road designs that would increase road stability and have fewer construction impacts and the first comprehensive land and property compensation program for road construction in Nepal.

Dynamic responses of tibial campaniform sensilla studied by substrate displacement in freely moving cockroaches.

Responses of the tibial campaniform sensilla, receptors that encode strains in the exoskeleton, were characterized by recording sensory activities during perturbations in freely standing cockroaches. The substrate upon which the animal stood was displaced horizontally using ramp and hold stimuli at varied rates. The sensilla showed short latency responses that were initiated in the first 30 ms of platform movement. Responses of individual receptors depended upon the direction of displacement and the orientation of their cuticular cap. Proximal receptors, whose caps are perpendicular to the long axis of the tibia, responded to displacements directed from the contralateral side of the body and from the head toward the abdomen. The distal sensilla, oriented parallel to the tibia, discharged at longer latency to displacements in opposite directions. Plots of receptor activity versus displacement direction showed that proximal and distal sensilla are activated in non-overlapping ranges of movement direction. Afferent responses also increased as the platform was displaced more rapidly. These results are consistent with a model in which displacements produce forces that result in bending of the tibia. This information could be utilized to detect the direction and rate of forces that occur during leg slipping or in walking on unstable terrains.

Paleo‐equilibrium line altitude estimates from late Quaternary glacial features in the Inland Kaikoura Range, South Island, New Zealand

A reconnaissance investigation of glacial features in the Inland Kaikoura Range (IKR), South Island, New Zealand, confirms that over 30 km of the length of the range, with a maximum elevation of 2885 m, has been extensively glaciated in the past, in contrast to previous reports. Identified glacial features include cirques, lateral moraines, moraine fragments and associated features, and rock glaciers. Up to five glacial advances have been identified in any one catchment in the range, with the most complete sequence found in Branch Stream, which drains the steep southeastern flank of the highest portion of the range. The lateral moraines of Branch Stream are impressive; they range in elevation from c. 1700 to 1200 m, and, because of their large size and relatively good state of preservation, they are interpreted to be of late Otiran age. Glacial features that are less well preserved and more fragmentary are found at lower elevations in other catchments (e.g., Dart, Muzzle, and Bluff Streams), and are here inferred to be of early‐mid Otiran age. The lack of preservation of older glacial landforms is attributed to the steep, unstable terrain of the IKR generated by the relatively high uplift rate (c. 5 mm/ yr), and consequent high erosion rate in the range, which is located in the hanging wall of the active Clarence Fault. A sequence of paleo‐equilibrium line altitudes (paleo‐ELAs) for the past glaciers in the IKR have been derived using cirque floor elevations, accumulation‐area ratios (AARs) and interpolated estimates, and give snowline depressions below the present (2530 m) of 830 m for the early‐mid Otiran, 740 m for late Otiran, 575 m for final Otiran, 450 m for Late Glacial, 270 m for Neoglacial, and 150 m for the Little Ice Age. Because glacial features in the IKR have yet to be directly dated, these age estimates should be regarded as interpretive in nature and are based solely on relative size, elevation, sequence position, and degree of preservation. The paleo‐ELAs have been normalised to a southerly aspect using a correction established in the northern Southern Alps that has yet to be tested in the IKR. Correcting for accrued uplift over the past c. 18 000 yr conceivably provides an additional c. 90 m of depression, suggesting a total ELA depression of as much as c. 830 m during the late Otiran compared to modern climate. The paleo‐ELA depression estimates for the IKR appear to be similar to estimates calculated in other areas in the South and North Islands. Evidence of glacial features in the IKR, and the apparent similarity of ELA depressions in the range with that elsewhere in New Zealand, suggests that past climate fluctuations in the IKR have been, to at least some degree, “in harmony” with those throughout the remainder of the country.

Geomorphological mapping

There has been a long tradition of geomorphological mapping to support land use planning, especially in Poland (e.g. Klimaszewski 1956, 1961; Galon 1962) and France (e.g. Tricart 1965; for a history of geomorphological mapping in Europe, see Verstappen 1983). The Geological Society Working Party Report on maps and plans (Anon. 1972) identified examples of geomorphological mapping that could be of use to engineers. However, the value of the technique was best highlighted by its application to road projects in unstable terrain in Nepal and South Wales during the early 1970s (e.g. Brunsden et al. 1975a, b; Doornkamp et al. 1979; Jones et al. 1983). The techniques have also been successfully applied to dryland problems (e.g. Brunsden et al. 1979; Bush et al. 1980, Doornkamp et al 1980; Cooke et al. 1982, 1985; Jones et al. 1986), soil erosion (e.g. Morgan 1995) and river management (e.g. Doornkamp 1982; Richards et al. 1987). Despite these applications, BS5930, Code of Practice for Site Investigations (British Standards Institution 1981), contains little reference to geomorphological maps (Griffiths & Marsh 1986) and the technique remains a marginal skill practised by a few experienced engineering geomorphologists. Examples of good mapping practice can be found within the references cited in this paper. The style and format of a geomorphological map needs to reflect the nature of the environment and the problems that need to be addressed. The following are some of the more common types of map.

Mapping landslide susceptibility in Travis County, Texas, USA.

A geographic information system (GIS) was used to construct a landslide hazard map for Travis County, Texas. The County is experiencing rapid growth, and development has encroached into unstable terrain that is vulnerable to landslides. Four layers of data were superimposed to create the landslide hazard map. Slope was given the most emphasis, followed by geology, vegetation, and proximity to faults. The final map shows areas of low, medium, and high landslide susceptibility. Areas of high susceptibility occupy stream and reservoir banks, rock escarpments, and agricultural land. The landslide hazard map can be a useful geologic criterion for land use planning. Planners can use the map to allocate appropriate land uses to unstable terrain, and to identify existing structures at risk from landslide activity. The methods presented in this paper can be adapted to other counties in the U.S. and elsewhere. Results of this study suggest that geographic information systems can effectively compile and overlay several data layers relevant to landslide hazards.

PROFILE: Management of Sedimentation in Tropical Watersheds.

/ The sedimentation of reservoirs is a serious problem throughout the tropics, yet most attempts to control sedimentation in large river basins have not been very successful. Reliable information on erosion rates and sources of sediments has been lacking. In regions where geologically unstable terrain combines with high rainfall, natural erosion rates might be so high that the effects of human activity are limited. Estimates of natural erosion in these situations often have been poor because of the episodic nature of most erosion during large storms and because mass-wasting may supply much of the sediment. The predominance of mass-wasting in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedimentation to "live" rather than "dead" storage within reservoirs. Furthermore, the inappropriate use of the Universal Soil Loss Equation to assess the effectiveness of erosion control measures has led to inaccurate estimates of the sediment reduction benefits that could accrue to watershed treatment efforts. Although reducing erosion from cultivated areas is desirable for other reasons, efforts aimed at reducing reservoir sedimentation by controlling agricultural sources of erosion may have limited benefits if the principal sources are of natural origin or are associated with construction of the dams and reservoirs and with rural roads and trails. Finally, the most appropriate locations for watershed rehabilitation depend on the magnitude of temporary storage of colluvium and alluvium within the river basin: Where storage volume is large and residence time of sediment very long, reducing agricultural erosion may have limited impacts on sedimentation within the expected life of a reservoir. Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resources towards reducing sedimentation. When sedimentation of reservoirs is the key issue, sediment budgets must focus especially on channel transport rates and sediment delivery from hillsides. Sediment budgets are especially critical for tropical areas where project funds and technical help are limited. Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective. KEY WORDS: Tropical watersheds; Sedimentation; Reservoirs; Erosion control