Reports from the Smithsonian's Global Volcanism Network,April 2009

Information included in this summary is based on more detailed reports published in the Bulletin of the Global Volcanism Network, vol. 34, no. 3, March 2009 (on the Internet at ). Edited by scientists at the Smithsonian, this bulletin includes reports provided by a worldwide network of correspondents. The reports contain the names and contact information for all sources. Please note that these reports are preliminary and subject to change as events are studied in more detail. The Global Volcanism Program welcomes further reports of current volcanism, seismic unrest, monitoring data, and field observations.     

Reports from the Smithsonian’s Global Volcanism Network,June 2009

Information included in this summary is based on more detailed reports published in the Bulletin of the Global Volcanism Network, vol. 34, no. 5, May 2009 (on the Internet at ). Edited by scientists at the Smithsonian, this bulletin includes reports provided by a worldwide network of correspondents. The reports contain the names and contact information for all sources. Please note that these reports are preliminary and subject to change as events are studied in more detail. The Global Volcanism Program welcomes further reports of current volcanism, seismic unrest, monitoring data, and field observations.     

A continental shelf scale examination of the Leeuwin Current off Western Australia during the austral autumn-winter

A continental shelf scale survey from 22°S to 34°S along the Western Australia coast provides the first detailed synoptic examination of the structure, circulation and modification of the southward flowing Leeuwin Current (LC) during the late austral autumn-early winter (May-June 2007). At lower latitudes (22°S-25°S), the LC was masked within a broad expanse of warm ambient surface water, which extended across the shelf and offshore before becoming constrained at the shelf break and attaining its maximum velocity of ∼1.0 m s⁻¹ at 28°S. The temperature and salinity signature of the LC experienced substantial modification as it flowed poleward; surface temperature of the LC decreased by ∼5.25 °C while surface salinity increased by ∼0.72, consistent with climatology estimates and smaller (larger) for temperature (salinity) than those found during summer. Subsequently, LC water was denser by ∼2σ_T in the south compared to the north, and the surface mixed layer of the LC revealed only a small deepening trend along its poleward trajectory. Modification of the LC resulted from a combination of mixing due to geostrophic inflow and entrainment of cooler, more saline surrounding subtropical waters, and convective mixing driven by large heat loss to the atmosphere. Air-sea heat fluxes accounted for 50% of the heat lost from the LC in the south, whilst only accounting for 25% in the north, where large geostrophic inflow occurred and the LC displayed its maximum flow. The onshore transport was characterised by distinct jet-like structures, enhanced in the upper 200 m of the water column, and the presence of eddies in the vicinity of the shelf break generated offshore transport.     

A process-based approach to attribution of historical streamflow decline in a data-scarce and human-dominated watershed

Human activities have resulted in rapid hydrological change around the world, in many cases producing shifts in the dominant hydrological processes, confounding predictions, and complicating effective management and planning. Identifying and characterizing such changes in hydrological processes is therefore a globally relevant problem, one that is particularly challenging in sparsely monitored environments. We develop a novel, process-based approach for attribution of hydrological change in such scenarios and apply the approach to the TG Halli watershed outside Bangalore, India, where streamflow has declined considerably over the last 50 years. The approach consists of (a) employing a range of field instrumentation and experiments to identify contemporary streamflow generation mechanisms, (b) using these observations to constrain our understanding and generate hypotheses pertaining to historical changes, and (c) evaluating these hypotheses with a range of evidence including proxies for historical hydrological processes. The body of evidence in the TG Halli watershed indicates the historical presence and subsequent loss of a shallow groundwater table that previously discharged to the stream, meaning that groundwater depletion is the most likely driver of streamflow decline. These findings present a viable path towards improved predictions of future water resources and sustainable water management within the watershed. Our process-based approach to attribution has the potential to improve understanding of human-driven hydrological change in regions with poor monitoring of hydrological systems.     

Analytical solutions to runoff on hillslopes with curvature: numerical and laboratory verification

Predicting the behavior of overland flow with analytical solutions to the kinematic wave equation is appealing due to its relative ease of implementation. Such simple solutions, however, have largely been constrained to applications on simple planar hillslopes. This study presents analytical solutions to the kinematic wave equation for hillslopes with modest topographic curvature that causes divergence or convergence of runoff flowpaths. The solution averages flow depths along changing hillslope contours whose lengths vary according hillslope width function, and results in a one-dimensional approximation to the two-dimensional flow field. The solutions are tested against both two-dimensional numerical solutions to the kinematic wave equation (in ParFlow) and against experiments that use rainfall simulation on machined hillslopes with defined curvature properties. Excellent agreement between numerical, experimental and analytical solutions is found for hillslopes with mild to moderate curvature. The solutions show that curvature drives large changes in maximum flow rate q_peak and time of concentration t_c , predictions frequently used in engineering hydrologic design and analysis.     

Freeze–thaw processes degrade post-fire water repellency in wet soils

Wildfires are a cause of soil water repellency (hydrophobicity), which reduces infiltration whilst increasing erosion and flooding from post-fire rainfall. Post-fire soil water repellency degrades over time, often in response to repeated wetting and drying of the soil. However, in mountainous fire-prone forests such as those in the Western USA, the fire season often terminates in a cold and wet winter, during which soils not only wet and dry, but also freeze and thaw. Little is known about the effect of repeated freezing and thawing of soil on the breakdown of post-fire hydrophobicity. This study characterized the changes in hydrophobicity of Sierra Nevada mountain soils exposed to different combinations of wet–dry and freeze–thaw cycling. Following each cycle, hydrophobicity was measured using the Molarity of Ethanol test. Hydrophobicity declined similarly across all experiments that included a wetting cycle. Repeated freezing and thawing of dry soil did not degrade soil water repellency, but freeze–thaw cycles decreased hydrophobicity in wet soils. Total soil organic matter content was not different between soils of contrasting hydrophobicity. Macroscopic changes such as fissures and cracks were observed to form as soil hydrophobicity decayed. Microscopic changes revealed by scanning electron microscope imagery suggest different levels of soil aggregation occurred in samples with distinct hydrophobicities, although the size of aggregates was not clearly correlated to the change in water repellency due to wet–dry and freeze–thaw cycling. A 9-year climate and soil moisture record from Providence Critical Zone Observatory was combined with the laboratory results to estimate that hydrophobicity would persist an average of 144 days post-fire at this well-characterized, typical mid-elevation Sierra Nevada site. Most of the breakdown in soil water repellency (79%) under these climate conditions would be attributable to freeze–thaw cycling, underscoring the importance of this process in soil recovery from fire in the Sierra Nevada.     

Coastal flooding in the Maldives: an assessment of historic events and their implications

With many inhabited islands only at about 1 m above mean sea level, the Maldives is among the nations most threatened by coastal flooding and sea level rise. However, the understanding of recent coastal flood events in the Maldives is limited and is important to understanding future flood threats. This paper assesses (1) the sea level and wave climate of the Maldives, (2) the sea level and wave conditions during recent coastal flood events, and (3) the implications for flood management and future research. The analysis uses observed still water levels (1987–2015) and hindcast wave conditions (1979–2015). Two significant flood events on 10–13 April 1987 and 15–17 May 2007 are examined in detail. This shows that coastal flooding in the Maldives occurs due to multiple interacting sources. These include long-period (up to 20 s) energetic waves generated in the Southern Ocean combined with spring tides. Wave run-up (mainly wave set-up) appears an essential mechanism for a flood, but is currently poorly quantified. However, as sea levels continue to rise the conditions that produce a flood will occur more frequently, suggesting that flooding will become common in the Maldives. This analysis is a starting point for future research and highlights the need to continue research on flood sources, pathways and receptors, and plan adaptation measures. Priorities include monitoring of waves, sea levels and flood events, and a better understanding of set-up (and other shallow water processes over reefs).     

The ability of general circulation models to simulate tropical cyclones and their precursors over the North Atlantic main development region

The ability of General Circulation Models (GCMs) to generate Tropical Cyclones (TCs) over the North Atlantic Main Development Region (MDR; 10–20°N, 20–80°W; Goldenberg and Shapiro in J Clim 9:1169–1187, 1996) is examined through a subset of ocean–atmosphere coupled simulations from the World Climate Research Programme (WCRP) Coupled Model Intercomparison Project phase 3 (CMIP3) multimodel data set and a high-resolution (0.5°) Sea Surface Temperature (SST)-forced simulation from the Australian Conformal-Cubic Atmospheric Model GCM. The results are compared with National Center for Environmental Prediction (NCEP-2) and European Center for Medium Range Weather Forecasts Re-Analysis (ERA-40) reanalyses over a common period from 1980 to 1998. Important biases in the representation of the TC activity are encountered over the MDR. This study emphasizes the strong link in the GCMs between African Easterly Waves (AEWs) and TC activity in this region. However, the generation of AEWs is not a sufficient condition alone for the models to produce TCs. Precipitation over the Sahel, especially rainfall over the Fouta Djallon highlands (cf. Fig.?1), is playing a role in the generation of TCs over the MDR. The influence of large-scale fields such as SST, vertical wind shear and tropospheric humidity on TC genesis is also examined. The ability of TC genesis indices, such as the Genesis Potential Index and the Convective Yearly Genesis Potential, to represent TC activity over the MDR in simulations at low to high spatial resolutions is analysed. These indices are found to be a reasonable method for comparing cyclogenesis in different models, even though other factors such as AEW activity should also be considered.     

Shifting spatialities of power: The case of Australasian aviation

This paper explores how different modalities, spatialities and scales of power operate in a geopolitical context. By tracing the dynamic geographies of state and firm power in the events leading up to the collapse of a major Australian firm, Ansett Airlines, it reveals the difference that place and position make to the creation and use of power. The paper stresses agents’ relational positioning, their ‘places’ in multiple networks of association and the ways in which their past actions and visions of the future condition their strategic options. The paper contextualises the workings of power and explores how power relationships are re-configured in specific contested events. It concludes that power cannot be separated from the spatial and temporal dimensions of actual contexts, from actor’s positions in contexts, or from their strategic objectives.