Mineralogical sources of the buffer capacity in a granite catchment determined by strontium isotopes

The role of different minerals in base cation release and thus the increase of buffering capacity of groundwater against acid deposition is controversially discussed in the literature. The ⁸⁷Sr/⁸⁶Sr ratios and base cation concentration were investigated in whole rock leachates, mineral separates, precipitation, soil solution, groundwater and stream water samples in the Lehstenbach catchment (Germany) to identify the weathering sequence of the granite bedrock. Three different approaches were followed in parallel. It was assumed that the contribution of different minerals to base cation supply of the groundwater with increasing weathering intensity would be observed by investigating (1) unweathered rock leachates, deep groundwater and shallow groundwater, (2) groundwater samples from new groundwater wells, reflecting the initial weathering of the drilled bedrock, and groundwater from wells that were drilled in 1988, (3) stream water during baseflow, dominated by deep groundwater, and stream water during high flow, being predominantly shallow groundwater. Whereas the first approach yielded consistent patterns, there was some evidence that groundwater from the new wells initially reflected contamination by the filter gravel rather than cation release in an initial stage of weathering. Time series samples of stream water and groundwater solute concentrations and isotope ratios turned out to reflect varying fractions of soil water and precipitation water at baseflow and high flow conditions rather than varying contributions of different minerals that prevail at different stages of granite weathering.     

Further developments of a transient Poisson-cluster model for rainfall

Stochastic point processes for rainfall are known to be able to preserve the temporal variability of rainfall on several levels of aggregation (e.g. hourly, daily), especially when the cluster approach is used. One major assumption in most of the applications todate is the stationarity of the rainfall properties in time, which must be reconsidered under a climate change hypothesis. Here, we propose new theoretical developments of a Poisson-based model with cluster, namely the Neyman–Scott Rectangular Pulses Model, which treats storm frequency as a nonstationary function. In this paper, storm frequency is modelled as a linear function of time in order to reproduce an increase (or decrease) of the mean annual precipitation. The basic theory is reconsidered and the moments are derived up to the third order. Then, a calibration method based on the generalized method of moments is proposed and discussed. An application to a rainfall time series from Uccle illustrates how this model can reproduce a trend for the average rainfall. This work opens new avenues for future developments on transient stochastic rainfall models and highlights the major challenges linked to this approach.     

Stochastic Post-Processing of CFSR Daily Precipitation across Canada

Reanalyses, based on numerical weather prediction methods assimilating past observations, provide continuous precipitation datasets and represent interesting options for assessing the climatology of regions with sparse station networks (e.g., northern Canada). However, reanalysis series cannot be used directly because of possible biases and mismatch between their spatial and temporal resolutions with that needed for local applications. To address these issues, a Stochastic Model Output Statistics (SMOS) approach was selected to post-process precipitation series simulated by the Climate Forecast System Reanalysis (CFSR) across Canada. This approach uses CFSR precipitation as a covariate and is based on two regression models: the first one is a logistic regression that deals with precipitation occurrence, and the second is a vector generalized linear model for precipitation intensity. At-site post-processed daily precipitation series are randomly generated using the SMOS approach, and selected climate indicators from the Expert Team on Climate Change Detection and Indices, which is jointly sponsored by the Commission for Climatology of the World Meteorological Organization's (WMO) World Climate Data and Monitoring Programme, the Climate Variability and Predictability Programme of the World Climate Research Programme, and the Joint WMO-IOC Technical Commission for Oceanography and Marine Meteorology (CCI/CLIVAR/JCOMM) are estimated and compared with corresponding observed and CFSR values. The two models in the SMOS approach, in addition to adequately correcting systematic biases, produced better predictions than the climatology of the wet and dry and intensity sequences. Additionally, the SMOS generally yields consistent climate indices when compared with those from CFSR without post-processing, though there is still room for improvement for specific indices (e.g., annual maximum of cumulative wet days).     

A geographic approach of the global vulnerability in urban area: case of Manizales, Colombian Andes

Manizales, intermediate city in the Andes of Colombia, is exposed to three major natural phenomena: frequent landslides, earthquakes and, to a lesser extent, floods, all sometimes causing catastrophic disasters. The city was established in 1849 on a high ridge between two rivers (interfluve). This narrow plateau has steep slopes that have been occupied since the early 1940's. Since then, a lack of sites approved for construction has not stopped urban expansion, which continues affecting unstable areas, even when geotechnic works have been realized in those areas. The people most frequently affected by hazards live in `sub-integrated' districts (shantytowns) situated in the more sloping lands. Nevertheless, recent disasters have occurred in the new developments where the middle classes live, showing the role of the urban growth in the creation of a growth of vulnerability and risk. The investigation consists of perfecting a method of analysis of the relationships between the urban process and the installation of such a situation. Using statistical tools (PCA and Bertin matrices), we estimated the global vulnerability of the city and represented it, at the scale of the neighbourhood level, in an integrated map. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthetic design hydrographs for ungauged catchments: a comparison of regionalization methods

Design flood estimates for a given return period are required in both gauged and ungauged catchments for hydraulic design and risk assessments. Contrary to classical design estimates, synthetic design hydrographs provide not only information on the peak magnitude of events but also on the corresponding hydrograph volumes together with the hydrograph shapes. In this study, we tested different regionalization approaches to transfer parameters of synthetic design hydrographs from gauged to ungauged catchments. These approaches include classical regionalization methods such as linear regression techniques, spatial methods, and methods based on the formation of homogeneous regions. In addition to these classical approaches, we tested nonlinear regression models not commonly used in hydrological regionalization studies, such as random forest, bagging, and boosting. We found that parameters related to the magnitude of the design event can be regionalized well using both linear and nonlinear regression techniques using catchment area, length of the main channel, maximum precipitation intensity, and relief energy as explanatory variables. The hydrograph shape, however, was found to be more difficult to regionalize due to its high variability within a catchment. Such variability might be better represented by looking at flood-type specific synthetic design hydrographs.     

Acidification processes and soil leaching influenced by agricultural practices revealed by strontium isotopic ratios

In natural river systems, the chemical and isotopic composition of stream- and ground waters are mainly controlled by the geology and water-rock interactions. The leaching of major cations from soils has been recognized as a possible consequence of acidic deposition from atmosphere for over 30 years. Moreover, in agricultural areas, the application of physiological acid fertilizers and nitrogen fertilizers in the ammonia form may enhance the cation leaching through the soil profile into ground- and surface waters. This origin of leached cations has been studied on two small and adjacent agricultural catchments in Brittany, western France. The study catchments are drained by two first-order streams, and mainly covered with cambisoils, issued from the alteration and weathering of a granodiorite basement. Precipitations, soil water- and NH₄ acetate-leachates, separated minerals, and stream waters have been investigated. Chemical element ratios, such as Ba/Sr, Na/Sr and Ca/Sr ratios, as well as Sr isotopic ratios are used to constrain the relative contribution from potential sources of stream water elements.Based on Sr isotopic ratio and element concentration, soil water- and NH₄ acetate leaching indicates (1) a dominant manure/slurry contribution in the top soil, representing a cation concentrated pool, with low ⁸⁷Sr/⁸⁶Sr ratios; (2) in subsoils, mineral dissolution is enhanced by fertilizer application, becoming the unique source of cations in the saprolite. The relatively high weathering rates encountered implies significant sources of cations which are not accessory minerals, but rather plagioclase and biotite dissolution.Stream water has a very different isotopic and chemical composition compared to soil water leaching suggesting that stream water chemistry is dominated by elements issued from mineral and rock weathering. Agriculture, by applications of chemical and organic fertilizers, can influence the export of major base cations, such as Na⁺. Plagioclase dissolution, rather than anthropogenically controlled soil water, seems to be the dominant source of Na⁺ in streams. However, Ca²⁺ in streams is mostly derived from slurries and manures deposited on top soils, and transferred into the soil ion-exchange pool and stream waters. Less than 10% of Na⁺, 5-40% of Sr²⁺ and 20-100% of Ca²⁺ found in streams can be directly derived from the application of organic fertilizers.     

Behavior of Re and Os during low-temperature alteration: Results from Himalayan soils and altered black shales

Re-Os analyses were performed on five soil profiles developed on silicate lithologies in the Narayani drainage basin of the Himalayas, as well as on several altered black shales and associated alteration phases. The results indicate that all of these samples have lost large amounts of Re. This Re loss approached 100% in the black shale-derived soils, which also most probably lost substantial quantities of Os. The near constancy of Os and Re concentrations with depth indicates that this loss occurred quite early in the alteration process, rather than progressively during the course of soil development. A slight decrease in ¹⁸⁷Os/¹⁸⁸Os was observed toward the surface of the two black shale soil profiles, possibly indicating a minor preferential mobility of radiogenic ¹⁸⁷Os for this lithology. In the black shale-derived saprolite and soils, the loss of Re and Os was related to the loss of organic carbon. The importance of this phase is underscored by the fact that organic material separated from a black shale sample accounted for nearly all of the Os contained in the whole rock. Analysis of alteration phases from the surface of a weathered black shale showed that Os liberated during alteration was partially reprecipitated in Fe oxides, with Os isotopic compositions very similar to that of the original rock, whereas Re was removed with the fluid phase.These results demonstrate the disproportionately large role that black shale weathering may play in determining the Re and Os contents of continental runoff and, thus, ultimately of seawater, but they also emphasize the possible complexity of this process. The data suggest that a large fraction of the Os contained in black shales is released during chemical alteration, which is apparently not the case for most other silicate rocks. Therefore, the potential contribution of black shales to the dissolved Os budget of runoff and river water may be even greater than that expected from the high Os concentrations of these rocks. The extent to which this Os will affect the seawater Os composition depends on how efficiently it is recaptured by secondary phases such as Fe-Mn oxides in situ or during transport. These results also underscore the high mobility of Re with respect to Os at every stage in the erosional process, thus explaining the order of magnitude increase in Re/Os ratio from the black shale source rocks to seawater.