Runoff curve numbers for steep hillslopes with natural vegetation in semi‐arid tropical highlands,northern Ethiopia

Daily runoff from 27 plots (5 m × 2 m) recorded during two rainy seasons in the Tigray highlands (Ethiopia) were analysed together with daily rainfall to calculate runoff curve numbers for hillslopes covered by semi‐natural vegetation in varying stages of vegetation restoration. Curve number model parameters were derived using a least squares fitting procedure on the collected rainfall–runoff datasets. Curve numbers varied from 29 to 97. Land use type was an important explanatory factor for the variation in curve numbers, whereas hydrologic soil group was not. Curve numbers were negatively correlated with vegetation cover. Taking into account antecedent soil moisture conditions did not improve runoff prediction using the curve number method. As runoff prediction was less accurate in areas with low curve numbers, two separate regression functions relating curve numbers with vegetation cover were proposed for different land use types. Copyright © 2008 John Wiley & Sons, Ltd.     

Effective precipitation in southern Spain ( 266 to 46 ka) based on a speleothem stable carbon isotope record

We present the longest-duration directly dated terrestrial palaeoclimate record from the western Mediterranean region: a flowstone speleothem from Gitana Cave, southeast Spain. The main phase of growth was 274 to 58 ka, dated by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) U-series methods. Effective precipitation, which we consider primarily responsible for flowstone calcite δ¹³C variations, measured at 300 μm resolution, was higher during interglacials associated with marine oxygen isotope stages (MIS) 7 and 5, and lower during glacial MIS 6. There is a close correspondence between speleothem δ¹³C and sea surface temperature (SST) estimates from adjacent Atlantic Ocean cores during MIS 6, which implies that oceanic conditions are critical in controlling the western Mediterranean terrestrial moisture balance during glacial periods. Other features of our record, such as the sequence of termination II warming/moistening between approximately 133 and 127 ka, including a “pause” around 130–128 ka, and the lagged termination of MIS 5 warm intervals (5e, 5c and 5a) are similar to other terrestrial records within the Mediterranean basin, indicating climate synchroneity along the northern Mediterranean coast. The Gitana cave region also may have been a refugium for temperate species during short-lived cold/arid periods during MIS 5.     

An Approach for the Classification of Urban Building Structures Based on Discriminant Analysis Techniques

Recognition of urban structures is of interest in cartography and urban modelling. While a broad range of typologies of urban patterns have been published in the last century, relatively little research on the automated recognition of such structures exists. This work presents a sample‐based approach for the recognition of five types of urban structures: (1) inner city areas, (2) industrial and commercial areas, (3) urban areas, (4) suburban areas and (5) rural areas. The classification approach is based only on the characterisation of building geometries with morphological measures derived from perceptual principles of Gestalt psychology. Thereby, size, shape and density of buildings are evaluated. After defining the research questions we develop the classification methodology and evaluate the approach with respect to several aspects. The experiments focus on the impact of different classification algorithms, correlations and contributions of measures, parameterisation of buffer‐based indices, and mode filtering. In addition to that, we investigate the influence of scale and regional factors. The results show that the chosen approach is generally successful. It turns out that scale, algorithm parameterisation, and regional heterogeneity of building structures substantially influence the classification performance.     

Monitoring recovery after earthquakes through the integration of remote sensing,GIS, and ground observations: the case of L’Aquila (Italy)

The usefulness of remote sensing (RS), geographical information systems, and ground observations for monitoring changes in urban areas has been demonstrated through many examples over the last two decades. Research has generally focused on the relief phase following a disaster, but we have instead investigated the subsequent phases involving early recovery, recovery, and development. Our aim was to determine to what extent integration of the available tools, techniques, and methods can be used to efficiently monitor the progress of recovery following an earthquake. Changes in buildings within the Italian city of L’Aquila following the 2009 earthquake were identified from Earth observation data and are used as indicators of progress in the recovery process. These changes were identified through (1) visual analysis, (2) automated change detection using a set of decision rules formulated within an object-based image analysis framework, and (3) validation based on a combination of visual and semiautomated interpretations. An accuracy assessment of the automated analysis showed a producer accuracy of 81% (error of omission: 19%) and a user accuracy of 55% (error of commission: 45%). The use of RS made it possible for the identification of changes to be spatially exhaustive, and also to increase the number of categories used for a recovery index. In addition, using RS allowed the area requiring extensive fieldwork (to monitor the progress of the recovery process) to be reduced.     

Gas migration through Opouawe Bank at the Hikurangi margin offshore New Zealand

This study presents 2D seismic reflection data, seismic velocity analysis, as well as geochemical and isotopic porewater compositions from Opouawe Bank on New Zealand’s Hikurangi subduction margin, providing evidence for essentially pure methane gas seepage. The combination of geochemical information and seismic reflection images is an effective way to investigate the nature of gas migration beneath the seafloor, and to distinguish between water advection and gas ascent. The maximum source depth of the methane that migrates to the seep sites on Opouawe Bank is 1,500–2,100 m below seafloor, generated by low-temperature degradation of organic matter via microbial CO₂ reduction. Seismic velocity analysis enabled identifying a zone of gas accumulation underneath the base of gas hydrate stability (BGHS) below the bank. Besides structurally controlled gas migration along conduits, gas migration also takes place along dipping strata across the BGHS. Gas migration on Opouawe Bank is influenced by anticlinal focusing and by several focusing levels within the gas hydrate stability zone.     

The usefulness of outcrop analogue air permeameter measurements for analyzing aquifer heterogeneity: quantifying outcrop hydraulic conductivity and its spatial variability

Saturated hydraulic conductivity (K) is one of the most important parameters determining groundwater flow and contaminant transport in both unsaturated and saturated porous media. Although several well‐established laboratory methods exist for determining K, in situ measurements of this parameter remain very complex and scale dependent. Often, the limited accessibility of subsurface sediments for sampling means an additional impediment to our ability to quantify subsurface K heterogeneity. One potential solution is the use of outcrops as analogues for subsurface sediments. This paper investigates the use of air permeameter measurements on outcrops of unconsolidated sediments to quantify K and its spatial heterogeneity on a broad range of sediment types. The Neogene aquifer in northern Belgium is used as a case study for this purpose. To characterize the variability in K, 511 small‐scale air permeability measurements were performed on outcrop sediments representative over five of the aquifer's lithostratigraphic units. From these measurements, outcrop‐scale equivalent K tensors were calculated using numerical upscaling techniques. Validation of the air permeameter‐based K values by comparison with laboratory constant head K measurements reveals a correlation of 0.93. Overall, the results indicate that hand‐held air permeameters are very efficient and accurate tools to characterize saturated K, as well as its small‐scale variability and anisotropy on a broad range of unconsolidated sediments. The studied outcrops further provided a qualitative understanding of aquifer hydrostratigraphy and quantitative estimates about K variability at the centimetre‐scale to metre‐scale. Copyright © 2013 John Wiley & Sons, Ltd.     

The effect of dock length on harbour siltation

Density-driven exchange flows between estuaries and harbour docks are influenced by the length of the dock. As a result, increasing dock size through its lengthening, not necessarily results in an increase in sedimentation rates. The propagation of a low-salinity surface patch into the dock is blocked at the head of a relatively short dock, resulting in a reversal of density-driven flows, and a reduction of the hydrostatic pressure gradients in the entrance of the dock. A reduced hydrostatic pressure in the dock, in turn, promotes near-bed inflow. When this increased near-bed inflow coincides with a high sediment supply on the adjacent river, the sediment transport into the dock increases. This has been tested with an extensively validated high-resolution numerical model developed for the Deurganckdok in the Port of Antwerp. In the Deurganckdok, siltation rates are expected to decrease when the dock is fully excavated compared to the present half-opened dock.Whether exchange flows between estuaries and harbour docks are influenced by the length of the dock, depends on the tidal variation in salinity. For small tidal density variations (around 0.5 kg/m³), the dock length is expected to influence exchange flows in a short dock (approximately 1 km), whereas the dock should be much longer (4 km) when the tidal density variation is higher (around 5 kg/m³). Whether these changing exchange flow result in a lowering or increase of sediment import, depends on the phase difference between sediment concentration peaks on the adjacent river/estuary and the salinity variation, and on the vertical distribution of sediment.     

Late Quaternary behavior of the East African monsoon and the importance of the Congo Air Boundary

Both Atlantic and Indian Ocean climate dynamics exert influence over tropical African hydroclimate, producing complex patterns of convergence and precipitation. To isolate the Indian Ocean influence on African paleohydrology, we analyzed the deuterium/hydrogen ratio of higher plant leaf waxes (δD_wax) in a 25 000-year sediment record from Lake Challa (3° S, 38° E) in the easternmost area of the African tropics. Whereas both the seismic record of inferred lake level fluctuations and the Branched and Isoprenoidal Tetraether (BIT) index proxy record changes in hydrology within the Challa basin, δD_wax, as a proxy for the isotopic composition of precipitation (δD_P) is interpreted as a tracer of large-scale atmospheric circulation that integrates the history of the moisture transported to the Lake Challa area. Specifically, based on modern-day isotope–rainfall relationships, we argue that Lake Challa δD_wax reflects the intensity of East African monsoon circulation. The three hydrological proxy records show generally similar trends for the last 25 000 years, but there are important differences between them, primarily during the middle Holocene. We interpret this deviation of δD_wax from local hydrological history as a decoupling of East African monsoon intensity – which heavily influences the isotopes of precipitation in East Africa today – from rainfall amount in the Challa basin. In combination, the hydrological proxy data from Lake Challa singularly highlight zonal gradients in tropical African climate that occur over a variety of timescales, suggesting that the Congo Air Boundary plays a fundamental role in controlling hydroclimate in the African tropics.