A conceptual model for meander initiation in bedload‐dominated streams

A simple analytic model is presented relating local sediment transport capacity to variance in the transverse shear stress distribution in a stream channel. The model is used to develop a physically based conceptual model for the initiation of meandering in straight, bedload‐dominated streams as a result of a feedback mechanism. The feedback maximizes the cross‐sectional shear stress variance and – in order to achieve stability – ultimately minimizes the energy slope at repeated locations along the channel, subject to steady‐state mass flux and the stability of the channel boundary. These locations develop into pools in a fully developed meandering channel; they represent attractor states wherein sediment continuity is satisfied using the least possible energy expenditure per unit length of channel. However, since the cross‐sectional geometry of a pool (and the adjacent bar) is asymmetric, these attractor states are only conditionally stable, requiring strong, curvature‐induced secondary circulation to maintain their asymmetry. Between two successive pools, a stream occupies a metastable, higher energy state (corresponding to a riffle) that requires greater energy expenditure per unit length of channel to transport the same volume of sediment. The model we present links processes at the scale of a channel width to adjustments of the channel sinuosity and slope at the scale of a channel reach. We argue that the reach‐scale extremal hypotheses employed by rational regime models are mathematical formalisms that permit a one‐dimensional theory to describe the three‐dimensional dynamics producing stream morphology. Our model is consistent with the results from stream table experiments, with respect to both the rate of development of meandering and the characteristics of the equilibrium channel morphology. Copyright © 2006 John Wiley & Sons, Ltd.     

Extreme variability of water table dynamics in temperate calcareous fens: Implications for biodiversity

Calcareous fens are minerotrophic peatlands with very high species diversity, and maintenance of the water table is assumed to be a key contributor to this diversity. However, this assumption is based on limited study of fen water table dynamics. Here we monitor water table fluctuation in distributed locations across three calcareous fens differing in hydrogeomorphic setting for three growing seasons. Water table position was extremely variable with absolute ranges of 89, > 100, and > 118 cm in the Riparian, Trough and Basin Fens, respectively, and was controlled by landscape position and weather variability. Areas adjacent to a second‐order stream experienced the least water table fluctuation, while the Basin Fen, at > 75 m from a surface water connection, was very prone to year‐to‐year precipitation differences. Mean and median water table values were found to be poor indicators of biologically relevant hydroperiods. We introduce the term ‘duration of initial growing season saturation’ as a potentially more useful statistic to relate to plant species distribution. Across the studied fens, this duration ranged from 1 to 14 weeks from the start of the growing season. The water table resided below the ground surface for between 0 and 22 weeks of the growing season across the calcareous fens and study period. These findings impart great differences in the development of oxidized rooting depths. Our results demonstrate that there is much more variation in calcareous fen hydrology than previously reported, and this variability has important implications for fen vegetation patterning and management. Copyright © 2011 John Wiley & Sons, Ltd.     

Doing NGO Work: the politics of being ‘civil society’ and promoting ‘good governance’ in Cambodia

Two fundamental pillars of what it is to be a contemporary NGO and ‘do’ development work are being a part of and promoting civil society, and extending ‘good governance’. This article will provide insights into how supposedly universally applicable concepts and models for NGOs to ‘improve’ post-conflict ‘developing countries’ are incorporated into different social and cultural landscapes. Ethnographic fieldwork highlights some of the particularities of governance, as well as everyday challenges of doing NGO work in a remote province of north-eastern Cambodia. In the context of an authoritarian government, where kinship relations cross-cut all aspects of society, NGOs can be understood as embedded in neo-patrimonial networks rather than as a separate and autonomous realm of society as classic liberal theory understands ‘civil society’. Fear also plays a significant role in configuring peoples’ attitude towards authority and is rarely acknowledged in the project of ‘good governance’ which simplistically tends to see post-colonial governments situated on a linear trajectory which will eventually, with significant NGO interventions, reach advanced liberal democracy status.     

Impacts of large-scale climatic disturbances on the terrestrial carbon cycle

Background

The amount of carbon dioxide in the atmosphere steadily increases as a consequence of anthropogenic emissions but with large interannual variability caused by the terrestrial biosphere. These variations in the CO₂ growth rate are caused by large-scale climate anomalies but the relative contributions of vegetation growth and soil decomposition is uncertain. We use a biogeochemical model of the terrestrial biosphere to differentiate the effects of temperature and precipitation on net primary production (NPP) and heterotrophic respiration (Rh) during the two largest anomalies in atmospheric CO₂ increase during the last 25 years. One of these, the smallest atmospheric year-to-year increase (largest land carbon uptake) in that period, was caused by global cooling in 1992/93 after the Pinatubo volcanic eruption. The other, the largest atmospheric increase on record (largest land carbon release), was caused by the strong El Niño event of 1997/98.

Results

We find that the LPJ model correctly simulates the magnitude of terrestrial modulation of atmospheric carbon anomalies for these two extreme disturbances. The response of soil respiration to changes in temperature and precipitation explains most of the modelled anomalous CO₂ flux.

Conclusion

Observed and modelled NEE anomalies are in good agreement, therefore we suggest that the temporal variability of heterotrophic respiration produced by our model is reasonably realistic. We therefore conclude that during the last 25 years the two largest disturbances of the global carbon cycle were strongly controlled by soil processes rather then the response of vegetation to these large-scale climatic events.     

Effects of heterogeneous land use/cover types on river channel morphology in the Solo River catchment,Eastern Uganda

In the tropics, unmonitored land use/cover types cause significant effects on the narrowing and widening of river channels which affects the integrity of water resources. River channel planform extent was characterized using Landsat images, while water and bedload samples were collected and analysed for a period of one year. The results revealed that in 1986, the channel planform covered 3.7 sq km in length than in 2013 where it increased to 4.2 sq km. Wetland (537.1mgl^?1) and bushland (186.3mgl^?1) cover types had the highest concentration of suspended sediments. Fine sand (0.25 mm), silty sand (1 mm) and silty clay (0.125 mm) bedload particle types dominated the riverbed along the channel from the sampled land use/cover types. The high concentration of sediments, bedload materials, bank instability, and streamflow were significant contributors to the narrowing and widening of the channel (p < 0.05). Agricultural land use was the major contributor of channel aggradation (0.8 m) and degradation (0.25 m) compared to tree plantations, bushlands, forest and wetland cover types.     

Identifying hazards associated with lava deltas

Lava deltas, formed where lava enters the ocean and builds a shelf of new land extending from the coastline, represent a significant local hazard, especially on populated ocean island volcanoes. Such structures are unstable and prone to collapse—events that are often accompanied by small explosions that can deposit boulders and cobbles hundreds of meters inland. Explosions that coincide with collapses of the East Lae ‘Apuki lava delta at Kīlauea Volcano, Hawai‘i, during 2005–2007 followed an evolutionary progression mirroring that of the delta itself. A collapse that occurred when the lava–ocean entry was active was associated with a blast of lithic blocks and dispersal of spatter and fine, glassy tephra. Shortly after delta growth ceased, a collapse exposed hot rock to cold ocean water, resulting in an explosion composed entirely of lithic blocks and lapilli. Further collapse of the delta after several months of inactivity, by which time it had cooled significantly, resulted in no recognizable explosion deposit. Seaward displacement and subsidence of the coastline immediately inland of the delta was measured by both satellite and ground-based sensors and occurred at rates of several centimeters per month even after the lava–ocean entry had ceased. The anomalous deformation ended only after complete collapse of the delta. Monitoring of ground deformation may therefore provide an indication of the potential for delta collapse, while the hazard associated with collapse can be inferred from the level of activity, or the time since the last activity, on the delta.     

From community-based to co-management: Improvement or deterioration in fisheries governance in the Cherai Poyil fishery in the Cochin Estuary,Kerala, India?

During the last 50 years, several different forms of fisheries governance have been tried and failed in the Cochin Estuary, Kerala, India. The latest shift has been from a community-based system to a co-management system, and this paper evaluates the current system in the light of the theoretical debate over the respective merits of community-based management and co-management. The paper is based on documentary material and data from interviews carried out during fieldwork in 2004, from which it concludes that provided the co-management system incorporates community principles within it, it will be an effective form of fisheries governance.