Observing Global Surface Water Flood Dynamics

Flood waves moving along river systems are both a key determinant of globally important biogeochemical and ecological processes and, at particular times and particular places, a major environmental hazard. In developed countries, sophisticated observing networks and ancillary data, such as channel bathymetry and floodplain terrain, exist with which to understand and model floods. However, at global scales, satellite data currently provide the only means of undertaking such studies. At present, there is no satellite mission dedicated to observing surface water dynamics and, therefore, surface water scientists make use of a range of sensors developed for other purposes that are distinctly sub-optimal for the task in hand. Nevertheless, by careful combination of the data available from topographic mapping, oceanographic, cryospheric and geodetic satellites, progress in understanding some of the world’s major river, floodplain and wetland systems can be made. This paper reviews the surface water data sets available to hydrologists on a global scale and the recent progress made in the field. Further, the paper looks forward to the proposed NASA/CNES Surface Water Ocean Topography satellite mission that may for the first time provide an instrument that meets the needs of the hydrology community.     

Toward a ground-motion logic tree for probabilistic seismic hazard assessment in Europe

The Seismic Hazard Harmonization in Europe (SHARE) project, which began in June 2009, aims at establishing new standards for probabilistic seismic hazard assessment in the Euro-Mediterranean region. In this context, a logic tree for ground-motion prediction in Europe has been constructed. Ground-motion prediction equations (GMPEs) and weights have been determined so that the logic tree captures epistemic uncertainty in ground-motion prediction for six different tectonic regimes in Europe. Here we present the strategy that we adopted to build such a logic tree. This strategy has the particularity of combining two complementary and independent approaches: expert judgment and data testing. A set of six experts was asked to weight pre-selected GMPEs while the ability of these GMPEs to predict available data was evaluated with the method of Scherbaum et al. (Bull Seismol Soc Am 99:3234?C3247, 2009). Results of both approaches were taken into account to commonly select the smallest set of GMPEs to capture the uncertainty in ground-motion prediction in Europe. For stable continental regions, two models, both from eastern North America, have been selected for shields, and three GMPEs from active shallow crustal regions have been added for continental crust. For subduction zones, four models, all non-European, have been chosen. Finally, for active shallow crustal regions, we selected four models, each of them from a different host region but only two of them were kept for long periods. In most cases, a common agreement has been also reached for the weights. In case of divergence, a sensitivity analysis of the weights on the seismic hazard has been conducted, showing that once the GMPEs have been selected, the associated set of weights has a smaller influence on the hazard.     

Mass Discharge in a Tracer Plume: Evaluation of the Theissen Polygon Method

A tracer plume was created within a thin aquifer by injection for 299 d of two adjacent “sub‐plumes” to represent one type of plume heterogeneity encountered in practice. The plume was monitored by snapshot sampling of transects of fully screened wells. The mass injection rate and total mass injected were known. Using all wells in each transect (0.77 m well spacing, 1.4 points/m² sampling density), the Theissen Polygon Method (TPM) yielded apparently accurate mass discharge (M_d) estimates at three transects for 12 snapshots. When applied to hypothetical sparser transects using subsets of the wells with average spacing and sampling density from 1.55 to 5.39 m and 0.70 to 0.20 points/m², respectively, the TPM accuracy depended on well spacing and location of the wells in the hypothesized transect with respect to the sub‐plumes. Potential error was relatively low when the well spacing was less than the widths of the sub‐plumes (>0.35 points/m²). Potential error increased for well spacing similar to or greater than the sub‐plume widths, or when less than 1% of the plume area was sampled. For low density sampling of laterally heterogeneous plumes, small changes in groundwater flow direction can lead to wide fluctuations in M_d estimates by the TPM. However, sampling conducted when flow is known or likely to be in a preferred direction can potentially allow more useful comparisons of M_d over multiyear time frames, such as required for performance evaluation of natural attenuation or engineered remediation systems.     

Operational earthquake forecasting in Europe: progress,despite challenges

Is Europe-wide operational earthquake forecasting (OEF) possible? We discuss the myriad problems that prevent it today, many of which relate to heterogeneities in earthquake recording, processing, and reporting. We contemplate the difficulty of building models that cross political boundaries, and we consider the prospect of European OEF in light of recent efforts to harmonize long-term seismic hazard assessment among several nations. Emphasizing the Strategies and Tools for Real-time Earthquake Risk Reduction (REAKT) project, we report achievements related to short-term seismicity forecasting in Iceland and Italy that could apply elsewhere in Europe. In Iceland, collaboration fostered by REAKT resulted in a revised earthquake catalog and a prototype OEF system. We report results from an experiment conducted with this prototype; these results suggest ensemble models provide an information gain, updating models more frequently improves their forecast skill, and that OEF is computationally feasible. In Italy, REAKT supported the creation of an ensemble model that now issues weekly hazard forecasts. We present examples of these forecasts, highlighting the problem that OEF often yields low probabilities, which are difficult to interpret and convert into actionable decisions. Motivated by such low hazard probabilities, we highlight Europe’s pioneering efforts in operational earthquake loss forecasting and mention solutions to problems that currently prevent OEF at the European scale.     

Changes in water quality following tidal inundation of coastal lowland acid sulfate soil landscapes

This study examines the remediation of surface water quality in a severely degraded coastal acid sulfate soil landscape. The remediation strategy consisted of partial restoration of marine tidal exchange within estuarine creeks and incremental tidal inundation of acidified soils, plus strategic liming of drainage waters. Time-series water quality and climatic data collected over 5 years were analysed to assess changes in water quality due to this remediation strategy. A time-weighted rainfall function (TWR) was generated from daily rainfall data to integrate the effects of antecedent rainfall on shallow groundwater levels in a way that was relevant to acid export dynamics. Significant increases in mean pH were evident over time at multiple monitoring sites. Regression analysis at multiple sites revealed a temporal progression of change in significant relationships between mean daily electrical conductivity (EC) vs. mean daily pH, and TWR vs. mean daily pH. These data demonstrate a substantial decrease over time in the magnitude of creek acidification per given quantity of antecedent rainfall. Data also show considerable increase in soil pH (2–3 units) in formerly acidified areas subject to tidal inundation. This coincides with a decrease in soil pe, indicating stronger reducing conditions. These observations suggest a fundamental shift has occurred in sediment geochemistry in favour of proton-consuming reductive processes. Combined, these data highlight the potential effectiveness of marine tidal inundation as a landscape-scale acid sulfate soil remediation strategy.     

Episodic riverine influence on surface DIC in the coastal Gulf of Maine

Anomalously high precipitation and river discharge during the spring of 2005 caused considerable freshening and depletion of dissolved inorganic carbon (DIC) in surface waters along the coastal Gulf of Maine. Surface pCO₂ and total alkalinity (TA) were monitored by repeated underway sampling of a cross-shelf transect in the western Gulf of Maine (GOM) during 2004–05 to examine how riverine fluxes, mixing, and subsequent biological activity exert control on surface DIC in this region. Most of the variability in surface DIC concentration was attributable to mixing of low DIC river water with higher DIC, saline GOM waters, but net biological uptake of DIC was significant especially during the spring and summer seasons. The extent and persistence of the coastal freshwater intrusion exerted considerable influence on net carbon dynamics. Integrated over the 10-m surface layer of our study region (∼5 × 10⁴ km²), net biological DIC uptake was 0.48 × 10⁸ mol C during April–July of 2004 compared to 1.33 × 10⁸ mol C during April–July of 2005. We found the temporal signature and magnitude of DIC cycling to be different in adjacent plume-influenced and non-plume regions. Extreme events such as the freshwater anomaly observed in 2005 will affect mean estimates of coastal carbon fluxes, thus budgets based on short time series of observations may be skewed and should be viewed with caution.     

Experimentally determined Mg/Ca and Sr/Ca ratios in juvenile bivalve calcite for Mytilus edulis: implications for paleotemperature reconstructions

To further evaluate the potential use of Mg/Ca and Sr/Ca ratios as a paleothermometer in the shell carbonate of the blue mussel Mytilus edulis, we grew juvenile mussels (～15 mm shell height; <2 years old) collected from Maine, USA, in controlled environments for 4 months. The four-by-three factorial design consisted of four circulating temperature baths (7, 11, 15 and 19°C), and three salinity ranges (23, 28, and 32). During the experiment, water Mg/Ca and Sr/Ca molar ratios were monitored weekly, and showed little variation across all salinity and temperature ranges. Data from sampled shells including all salinity treatments yielded relatively poor relationships between shell elemental chemistry and water temperatures. However, if only the low salinity treatment data (23) are used, the relationships between shell elemental chemistry and water temperature improve moderately. Based on the data presented here, it may be possible to use Mg/Ca and Sr/Ca ratios from the shell carbonate of juvenile M. edulis to reconstruct paleotemperatures in estuarine settings (salinity below 24) with a corresponding RMSE (root mean squared error; 95% confidence interval) of ±2.4°C and ±2.8°C, respectively. In order for this methodology to be statistically meaningful, water temperature changes must be rather large, as the errors associated with using Mg/Ca and Sr/Ca ratios from the shell material of M. edulis are substantial. Further work is required to determine if the findings presented here can be duplicated, and if the potential salinity effect is pervasive.     

Effects of diel period and tow duration on estuarine fauna sampled with a beam trawl over bare sediment: Consequences for designing more reliable and efficient surveys

The effects of diel period and tow duration (5, 10 and 20 min) on samples of estuarine fauna in a beam trawl, were tested over bare sediment in Tuggerah Lake (New South Wales, Australia). Mean catch rates (numbers of fish caught 5 min⁻¹) were significantly larger at night for the total numbers of individuals and abundant, economically important species of fish and invertebrates (e.g. Gerres subfasciatus, Metapenaeus macleayi, Penaeus plebejus). Greater proportions of larger fish were also caught at night for some species (e.g. G. subfasciatus, Acanthopagrus australis, Rhabdosargus sarba), but not across all tow durations. Multivariate analyses detected dissimilarities in the composition and structure of assemblages between diel periods, which were driven by species caught predominately, or in larger proportions, at night. Short tows (5 min) were more efficient than longer tows (10 or 20 min) for sampling the diversity of species (i.e. most species were caught in the first 5 min of a tow). There were, however, no clear or consistent patterns relating to the effect of tow duration on the catch rates of other variables, the size ranges of abundant species, or the structure and composition of assemblages. Our data confirm that at night, bare sediment is an important habitat for a wide size- and species-range of estuarine fish and invertebrates. In future, more cost-effective and reliable information concerning these taxa would be achieved by sampling with the beam trawl at night, using tow durations of 5 min. We also highlight a problem inherent in the design of many studies of diel variation of fauna (i.e. the potential non-independence of data among day and night periods) and discuss its solution.