Spatial variability and interpolation of daily precipitation amount

Daily precipitation amounts show spatial variation over sub-continential regions. Point measurements, represntative for regions of land, have to be interpolated towards unobserved locations. In this study four days in 1984 were selected to investigate the spatial variability of daily precipitation amount in north-western Europe in relation to the meteorological conditions. Data were interpolated using kriging. Crossvalidation was used to compare interpolated values with measured values. Large differences in the spatial structure of daily precipitation amount are observed as a result of different meteorological conditions. Stratification of the study area into a coast, a mountain and an interior stratum proved to be successful, reducing the Mean Squared Error of Prediction with up to 55%.This article was inadvertently printed in SHH 6(3) 1992 without figures and figure legends. The article is being reprinted in this issue in complete form. The editor apologizes for this error in publication.     

Reconstructing ancient topography through erosion modelling

One of the main aims of geomorphology is to understand how geomorphic processes change topography over long time scales. Over the last decades several landscape evolution models have been developed in order to study this question. However, evaluation of such models has often been very limited due to the lack of necessary field data. In this study we present a topography based hillslope erosion and deposition model that is based on the WATEM/SEDEM model structure and works on a millennial time scale. Soil erosion, transport and deposition are calculated using slope and unit contributing area. The topography is iteratively rejuvenated by taking into account modelled erosion and deposition rates, thereby simulating topographic development backwards in time. A first attempt has been made to spatially evaluate the model, using detailed estimates for historical soil erosion and sediment deposition volumes, obtained from an augering campaign in a small catchment in the Belgian Loess Belt. The results show that the model can simulate realistic soil redistribution patterns. However, further research is necessary in order to deal with artificial flaws that cause routing problems and significantly influence results. Common problems and issues related to this type of backward modelling are also discussed.     

Microséismes et Séismes précédant l’éruption du Nyamuragira-Kitzimbanyi (Afrique Centrale) 1958

An analysis is made of microseismic and seismic activity for a period of about 3 months prior to the 1958 eruption in the Virunga Volcanoes, Western Rift Valley of Central Africa. It is believed that it would have been possible to predict the eruption some 30 h before, on the basis of two series of small earthquakes preceeding the 2 outbrakes by some 50 h, and presenting an exponential decay of the total number of shocks.     

Proposed methodology for building-specific earthquake loss assessment including column residual axial shortening

This paper proposes methodological developments for quantifying the impact of residual axial shortening of first-story steel columns on earthquake loss estimations in steel moment-resisting frame (MRF) buildings. A new formulation is proposed that accounts for the likelihood of having to demolish a steel MRF building due to column residual axial deformations in addition to residual story-drift ratios. The formulation is informed by means of data from a comprehensive survey conducted worldwide to assess the likelihood of steel column repairability due to residual axial shortening. A practical method for quantifying column axial-shortening in parameterized system-level numerical simulations is presented. The proposed approach is illustrated by conducting economic seismic loss estimations in two case-study steel MRF buildings designed in urban California according to the current seismic design practice. It is found that when the ground-motion duration is appreciable, the examined steel MRFs are more prone to column axial-shortening than residual story-drifts at moderate to high seismic intensities. The results suggest that economic losses due to demolition may be underestimated if column residual axial-shortening is neglected from loss estimations. Limitations as well as directions for future research are discussed.     

In-situ assessment of the spatial arrangement of step–pool units on eroded rills

Rills are generated on homogeneous hillslopes by the action of different discharges and evolve morphologically over short timescales due to a strong interaction between the flow and bed morphology. Such an interaction generates a reconfiguration of the bed geometry. Previous works suggest that bed geometry is often characterized by alternation between pools and flat reaches (steps). Each step–pool unit may contribute to hydraulic resistance and affects flow behaviour. The objectives of this work are (i) to assess different (innovative) techniques for the in-situ assessment of rill bed geometry, (ii) to use these techniques to assess the geometry of eroded rills in situ in order to determine the spatial arrangement in the bed macro-scale roughness and (iii) finally to analyse the role of slope and discharge as driving factors associated with the development of these macroforms. Roughly rectilinear, long rills were formed in the field as a result of combining different slope and discharges. Photogrammetry provided detailed digital elevation models (DEMs) before and after the experiments. The rills were morphologically characterized from the DEMs. In each rill, the presence of step–pools was identified from long profiles according mainly to morphological criteria published elsewhere, but with ad hoc critical threshold values more appropriate for small eroded channels. The minimum slope required for the development of step–pool units seems to be somewhere between 5 and 15%. Discharge seems to affect pool size or roughness amplitude. There does not seem to be a clear step–pool periodicity. However, external factors could have affected the normal growth and alternation of these structures. Identification of steps and pools from longitudinal elevation profiles can be objectively accomplished using a series of geometric rules originally proposed for rivers and large channels, and adapted to rills. © 2019 John Wiley & Sons, Ltd.     

The influence of environmental cues on the activity of silver eels (Anguilla dieffenbachii) in Lake Manapouri,New Zealand

Lake Manapouri in the South Island of New Zealand has a significant hydroelectric generation facility, which potentially has a negative impact on the successful emigration of longfin eels (Anguilla dieffenbachii) from the upper Waiau catchment. Consequently, a trap-and-transfer programme has been implemented to allow silver (migratory) eels access to the sea. The present study analysed data on the timing of captures and numbers of silver eels over four seasons, including the influence of possible migration cues. Silver eels were captured over an extended season of seven months (November–May), with peak numbers during December and January. Flow at the lake outflow was found to have a weak relationship with the number of eels caught, as did lake level, but moon phase and rainfall had no significant effect. These results highlight the variability in silver longfin eel behaviour both in terms of timing and response to potential environmental cues.     

Evolution of the western Barents Sea

Information from multichannel seismic reflection data complemented by seismic refraction, gravity and magnetics forms the basis for a regional structural and evolutionary model of the western Barents Sea during post-Caledonian times. The western Barents Sea contains a thick succession, locally > 10 km, of Upper Paleozoic to Cenozoic sedimentary rocks covering a basement of probably Caledonian origin. The area is divided into three regional geological provinces: (1) an east-west trending basinal province between 74°N and the coast of Norway; (2) an elevated platform area to the north towards Svalbard; and (3) the western continental margin. Several structural elements of different origin and age have been mapped within each of these provinces. The main stratigraphic sequence boundaries have been tentatively dated from available well information, correlation with the geology of adjacent areas, and correlation with the interregional unconformities caused by relative changes of sea level. The main structural elements were developed during three major post-Caledonian tectonic phases: the Svalbardian phase in Late Devonian to Early Carboniferous times, the Mid and Late Kimmerian phase in Mid Jurassic to Early Cretaceous times and Cenozoic tectonism related to the progressive northward opening of the Norwegian-Greenland Sea. The sediments are predicted to be of mainly clastic origin except for a thick sequence of Middle Carboniferous — Lower Permian carbonates and evaporites. Salt diapirs have developed in several sub-basins, especially in the Nordkapp Basin where they form continuous salt walls that have pierced through > 7 km of sediments.     

Model simulations of dense bottom currents in the Western Baltic Sea

Only recently, medium intensity inflow events into the Baltic Sea have gained more awareness because of their potential to ventilate intermediate layers in the Southern Baltic Sea basins. With the present high-resolution model study of the Western Baltic Sea a first attempt is made to obtain model based realistic estimates of turbulent mixing in this area where dense bottom currents resulting from medium intensity inflow events are weakened by turbulent entrainment. The numerical model simulation which is carried out using the General Estuarine Transport Model (GETM) during nine months in 2003 and 2004 is first validated by means of three automatic stations at the Drogden and Darss Sills and in the Arkona Sea. In order to obtain good agreement between observations and model results, the 0.5×0.5$0.5\times 0.5$ nautical mile bathymetry had to be adjusted in order to account for the fact that even at that scale many relevant topographic features are not resolved. Current velocity, salinity and turbulence observations during a medium intensity inflow event through the Øresund are then compared to the model results. Given the general problems of point to point comparisons between observations and model simulations, the agreement is fairly good with the characteristic features of the inflow event well represented by the model simulations. Two different bulk measures for mixing activity are then introduced, the vertically integrated decay of salinity variance, which is equal to the production of micro-scale salinity variance, and the vertically integrated turbulent salt flux, which is related to an increase of potential energy due to vertical mixing of stably stratified flow. Both measures give qualitatively similar results and identify the Drogden and Darss Sills as well as the Bornholm Channel as mixing hot spots. Further regions of strong mixing are the dense bottom current pathways from these sills into the Arkona Sea, areas around Kriegers Flak (a shoal in the western Arkona Sea) and north–west of the island of Rügen.     

Isotopic fingerprints on lacustrine organic matter from Laguna Potrok Aike (southern Patagonia, Argentina) reflect environmental changes during the last 16,000 years

A combination of carbon-to-nitrogen ratios (TOC/TN), Rock Eval-analyses, and stable isotope values of bulk nitrogen (δ¹⁵N) and organic carbon (δ¹³C_org) was used to characterize bulk organic matter (OM) of a piston core from the Patagonian maar lake Laguna Potrok Aike (Argentina) for the purpose of palaeoenvironmental reconstruction. Sedimentary data were compared with geochemical signatures of potential OM sources from Laguna Potrok Aike and its catchment area to identify the sources of sedimentary OM. Correlation patterns between isotopic data and TOC/TN ratios allowed differentiation of five distinct phases with different OM composition. Before 8470 calibrated ¹⁴C years before present (cal. yrs BP) and after 7400 cal. yrs BP, isotopic and organo-geochemical fingerprints indicate that the sediments of Laguna Potrok Aike consist predominantly of soil and diatom OM with varying admixtures of cyanobacterial and aquatic macrophyte OM. For a short phase of the early Holocene (ca. 8470–7400 cal. yrs BP), however, extremely high input of soil OM is implied by isotopic fingerprints. Previous seismic and geochronological results indicate a severe lake-level drop of 33 m below present-day shortly before 6590 cal. yrs BP. It is suggested that this lake level drop was accompanied by increased erosion of shore banks and channel incision enhancing soil OM deposition in the lake basin. Thus, isotopic data can be linked to hydrological variations at Laguna Potrok Aike and allow a more precise dating of this extremely low lake level. An isotopic mixing model was used including four different sources (soil, cyanobacteria, diatom and aquatic macrophyte OM) to model OM variations and the model results were compared with quantitative microfossil data.