Global Change Abstracts The Swiss Contribution - SCNAT

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146 Global Change Abstracts – The Swiss Contribution | Oceans and Fresh Water Systems the number of individuals, while the zooplanktivores made up more than 80%. The patterns of decline and recovery indicate that, though fishery played a role locally, predation by Nile perch and eutrophication were the main factors determining the fate of the haplochromines. However, it has so far been impossible to establish the causal relationship between the two, and the relative impact of each of these phenomena separately. The potential effects of the changed trophic dominance, and the importance of the haplochromines for the ecosystem and a sustainable fishery, are discussed. Aquatic Ecosystem Health Management, 2007, V10, N4, pp 416-433. 08.1-285 Oxygen isotope biogeochemistry of pore water sulfate in the deep biosphere: Dominance of isotope exchange reactions with ambient water during microbial sulfate reduction (ODP Site 1130) Wortmann U G, Chernyavsky B, Bernasconi S M, Brunner B, Böttcher M E, Swart P K Canada, Switzerland, USA, Germany Geology , Modelling , Geochemistry & Geophysics , Microbiology Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Experiments and field data show that the delta O-18(SO 4) 2- composition is also modified in the presence of sulfate-reducing microorganisms. This has been attributed either to a kinetic isotope effect during the reduction of sulfate to sulfite, cell-internal exchange reactions between enzymatically-activated sulfate (APS), and /or sulfite with cytoplasmic water. The isotopic fingerprint of these processes may be further modified by the cellexternal reoxidation of sulfide to elemental sulfur, and the subsequent disproportionation to sulfide and sulfate or by the oxidation of sulfite to sulfate. Here we report delta O-18(SO 4) 2- values from interstitial water samples of ODP Leg 182 (Site 1130) and provide the mathematical framework to describe the oxygen isotope fractionation of sulfate during microbial sulfate reduction. We show that a purely kinetic model is unable to explain our delta(OSO 4 2-)- O-18 data, and that the data are well explained by a model using oxygen isotope exchange reactions. We propose that the oxygen isotope exchange occurs between APS and cytoplasmic water, and/or between sulfite and adenosine monophosphate (AMP) during APS formation. Model calculations show that cell external reoxidation of reduced sulfur species would require up to 3000 mol/m(3) of an oxidant at ODP Site 1130, which is incompatible with the sediment geochemical data. In addition, we show that the volumetric fluxes required to explain the observed delta O-18(SO 4) 2- data are on average 14 times higher than the volumetric sulfate reduction rates (SRR) obtained from inverse modeling of the porewater data. The ratio between the gross sulfate flux into the microbes and the net sulfate flux through the microbes is depth invariant, and independent of sulfide concentrations. This suggests that both fluxes are controlled by cell density and that cell-specific sulfate reduction rates remain constant with depth. Geochimica et Cosmochimica Acta, 2007, V71, N17, SEP 1, pp 4221-4232. 08.1-286 Virtual water: An unfolding concept in integrated water resources management Yang H, Zehnder A Switzerland Political Sciences , Economics , Water Resources In its broadest sense, virtual water refers to the water required for the production of food commodities. Issues relating to virtual water have drawn much attention in scientific communities and the political sphere since the mid 1990s. This paper provides a critical review of major research issues and results in the virtual water literature and pinpoints the remaining questions and the direction of research in future virtual water studies. We conclude that virtual water studies have helped to raise the awareness of water scarcity and its impact on food security and to improve the understanding of the role of food trade in compensating for water deficit. However, the studies so far have been overwhelmingly concerned with the international food trade, and many solely quantified virtual water flows associated with food trade. There is a general lack of direct policy relevance to the solutions to water scarcity and food insecurity, which are often local, regional, and river basin issues. The obscurity in the conceptual basis of virtual water also entails some confusion. The methodologies and databases of the studies are often crude, affecting the robustness and reliability of the results. Looking ahead, future virtual water studies need to enhance the policy relevance by strengthening their linkages with national and regional water resources management. Meanwhile, integrated approaches taking into consideration the spatial and temporal variations of blue and green water resources availability and the complexity of natural, socioeconomic, and political conditions are necessary in assessing the trade-offs of the virtual water strategy in dealing with water scarcity. To this end, interdisciplinary efforts and quantitative methods supported by improved data availability are greatly important. Water Resources Research, 2007, V43, N12, DEC 5 ARTN: W12301.
Global Change Abstracts – The Swiss Contribution | Oceans and Fresh Water Systems 08.1-287 Hydrological modelling of the chaohe basin in china: Statistical model formulation and Bayesian inference Yang J, Reichert P, Abbaspour K C, Yang H Switzerland Hydrology , Modelling Calibration of hydrologic models is very difficult because of measurement errors in input and response, errors in model structure, and the large number of non-identifiable parameters of distributed models. The difficulties even increase in arid regions with high seasonal variation of precipitation, where the modelled residuals often exhibit high heteroscedasticity and autocorrelation. On the other hand, support of water management by hydrologic models is important in and regions, particularly if there is increasing water demand due to urbanization. The use and assessment of model results for this purpose require a careful calibration and uncertainty analysis. Extending earlier work in this field, we developed a procedure to overcome (i) the problem of non-identifiability of distributed parameters by introducing aggregate parameters and using Bayesian inference, (ii) the problem of heteroscedasticity of errors by combining a Box-Cox transformation of results and data with seasonally dependent error variances, (iii) the problems of autocorrelated errors, missing data and outlier omission with a continuous-time autoregressive error model, and (iv) the problem of the seasonal variation of error correlations with seasonally dependent characteristic correlation times. The technique was tested with the calibration of the hydrologic sub-model of the Soil and Water Assessment Toot (SWAT) in the Chaohe Basin in North China. The results demonstrated the good performance of this approach to uncertainty analysis, particularly with respect to the fulfilment of statistical assumptions of the error model. A comparison with an independent error model and with error models that only considered a subset of the suggested techniques clearly showed the superiority of the approach based on all the features (i)-(iv) mentioned above. Journal of Hydrology, 2007, V340, N3-4, JUL 15, pp 167-182. 08.1-288 Bayesian uncertainty analysis in distributed hydrologic modeling: A case study in the Thur River basin (Switzerland) Yang J, Reichert P, Abbaspour K C Switzerland Modelling , Hydrology , Instruments & Instrumentation 147 Calibration and uncertainty analysis in hydrologic modeling are affected by measurement errors in input and response and errors in model structure. Recently, extending similar approaches in discrete time, a continuous time autoregressive error model was proposed for statistical inference and uncertainty analysis in hydrologic modeling. The major advantages over discrete time formulation are the use of a continuous time error model for describing continuous processes, the possibility of accounting for seasonal variations of parameters in the error model, the easier treatment of missing data or omitted outliers, and the opportunity for continuous time predictions. The model was developed for the Chaohe Basin in China and had some features specific for this semiarid climatic region (in particular, the seasonal variation of parameters in the error model in response to seasonal variation in precipitation). This paper tests and extends this approach with an application to the Thur River basin in Switzerland, which is subject to completely different climatic conditions. This application corroborates the general applicability of the approach but also demonstrates the necessity of accounting for the heavy tails in the distributions of residuals and innovations. This is done by replacing the normal distribution of the innovations by a Student t distribution, the degrees of freedom of which are adapted to best represent the shape of the empirical distribution of the innovations. We conclude that with this extension, the continuous time autoregressive error model is applicable and flexible for hydrologic modeling under different climatic conditions. The major remaining conceptual disadvantage is that this class of approaches does not lead to a separate identification of model input and model structural errors. The major practical disadvantage is the high computational demand characteristic for all Markov chain Monte Carlo techniques. Water Resources Research, 2007, V43, N10, OCT 2 ARTN: W10401. 08.1-289 Salinization of groundwater in the Nefzawa oases region, Tunisia: results of a regionalscale hydrogeologic approach Zammouri M, Siegfried T, El Fahem T, Kriaa S, Kinzelbach W Tunisia, Switzerland, Germany, Nigeria Water Resources , Hydrology , Modelling Groundwater pumped from the semi-confined Complexe Terminal (CT) aquifer is an important production factor in irrigated oases agriculture in southern Tunisia. A rise in the groundwater salinity has been observed as a consequence of increas-
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