Solar UV radiation and its absorption in the mesosphere and stratosphere

Solar radiation of >175 nm and of Lyman-alpha at 121.6 nm is absorbed in the mesosphere and stratosphere by molecular oxygen (<242 nm) and also by ozone molecules at >200 nm. This paper describes the photodissociation processes resulting from absorption in the Schumann-Runge bands and Herzberg continuum of molecular oxygen and also in the Hartley, Huggins and Chappuis bands of ozone. Special consideration is given to differences between the stratospheric and mesospheric problems.     

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A red algal bloom in the aftermath of the Marinoan Snowball Earth

Organic matter from the cap dolostones overlying the Marinoan‐age glacigenic diamictites of the Araras Group, Amazon craton, has been studied to reconstruct the post‐glacial ecosystem. Molecular fossils indicate that the post‐Marinoan ecosystem was marked by an apparent decline in marine algal diversity. The proliferation of red algae may be explained by environmental changes, such as a massive nutrient input accompanying continental weathering after the ice thaw and a dimer light penetrating sea water due to the drowning of the platform. In addition, the presence of green sulphur bacteria indicates that sea water was stratified with an anoxic (possibly euxinic, i.e. sulphidic) layer at the water–sediment interface. Sulphur cycling probably occurred at the redox boundary as suggested by the recognition of active sulphate reduction. This observation supports a microbially induced model for the formation of the cap dolostones.     

Evaluation of the carbon isotopic effects of NDIR and CRDS analyzers on atmospheric CO2 measurements

Non-dispersive infrared(NDIR) and cavity ring-down spectroscopy(CRDS) CO_2 analyzers use 12CO_2 isotopologue absorption lines and are insensitive to all or part of other CO_2-related isotopologues. This may produce biases in CO_2 mole fraction measurements of a sample if its carbon isotopic composition deviates from that of the standard gases being used. To evaluate and compare the effects of carbon isotopic composition on NDIR and CRDS CO_2 analyzers, we prepared three test sample air cylinders with varying carbon isotopic abundances and calibrated them against five standard cylinders with ambient carbon isotopic composition using CRDS and NDIR systems. We found that the CO_2 mole fractions of the sample cylinders measured by G1301(CRDS) were in good agreement with those measured by Lo Flo(NDIR). The CO_2 values measured by both instruments were higher than that of a CO_2 isotope measured by G2201i(CRDS) analyzer for a test cylinder with depleted carbon isotopic composition δ~(13)C =-36.828‰, whereas no obvious difference was found for other two test cylinders with δ~(13)C=-8.630‰ and δ~(13)C=-15.380‰, respectively. According to the theoretical and experimental results, we concluded that the total CO_2 mole fractions of samples with depleted isotopic compositions can be corrected on the basis of their 12CO_2 values calibrated by standard gases using Lo Flo and G1301 if the δ~(13)C and δ18O values are known.     

Self-thinning, biodeposit production, and organic matter input to the bottom in mussel suspension culture

Future mussel suspension-culture methods may increasingly bypass standard population density adjustment on culture gear. This likely to exacerbate mussel fall-off. The potential extent of mussel fall-off is unknown. Here I use Dynamic Energy Budget theory and self-thinning theory to model the contribution of mussel fall-off to the total input of organic matter to the bottom. I focus on mussel populations grown on collector ropes and undergoing space-regulated self-thinning. Over a complete growth cycle, fall-off of mussel biomass was three times as high as the quantity harvested and accounted for 59% of total input of organic matter to the bottom. Biomass fall-off with collectors was 6-30 fold higher than with sleeving methods, as estimated from the literature. The relative contribution of mussel fall-off increased in a step-like fashion with initial population density, indicating that either mussels or biodeposits may dominate the organic input to the bottom, unless year-to-year variability in spat abundance is very high. The relative contribution of mussel fall-off did not increase appreciably with the intensity of second-set, but total mussel fall-off did. Assessment of each input type to the bottom is required for forecasting the consequences of a shift in culture methods, as they have different, spatially structured, effects on benthic environments.     

Pedotransfer functions estimating soil hydraulic properties using different soil parameters

Estimates of soil hydraulic properties using pedotransfer functions (PTF) are useful in many studies such as hydrochemical modelling and soil mapping. The objective of this study was to calibrate and test parametric PTFs that predict soil water retention and unsaturated hydraulic conductivity parameters. The PTFs are based on neural networks and the Bootstrap method using different sets of predictors and predict the van Genuchten/Mualem parameters. A Danish soil data set (152 horizons) dominated by sandy and sandy loamy soils was used in the development of PTFs to predict the Mualem hydraulic conductivity parameters. A larger data set (1618 horizons) with a broader textural range was used in the development of PTFs to predict the van Genuchten parameters. The PTFs using either three or seven textural classes combined with soil organic mater and bulk density gave the most reliable predictions of the hydraulic properties of the studied soils. We found that introducing measured water content as a predictor generally gave lower errors for water retention predictions and higher errors for conductivity predictions. The best of the developed PTFs for predicting hydraulic conductivity was tested against PTFs from the literature using a subdata set of the data used in the calibration. The test showed that the developed PTFs gave better predictions (lower errors) than the PTFs from the literature. This is not surprising since the developed PTFs are based mainly on hydraulic conductivity data near saturation and sandier soils than the PTFs from the literature. Copyright © 2007 John Wiley & Sons, Ltd.     

Specifying the differentiated contribution of farmers to groundwater depletion in two irrigated areas in North Africa

Much attention has been paid to the issue of groundwater depletion linked to intensive groundwater-based agriculture in (semi-)arid areas. Often referred to as the “overexploitation” of aquifers, groundwater depletion is generally attributed to the entire agricultural sector without distinguishing between different uses and users. Although it expresses a general concern for future users, the ambiguous term of “overexploitation” does not acknowledge the contested nature of groundwater use and emerging inequalities. Also, the impact of inequality on groundwater depletion is rarely questioned. The aim of this article is to investigate how and by whom groundwater is depleted, and in turn, how unequal access to groundwater fuels the socioeconomic differentiation of farms and groundwater depletion. Based on a detailed analysis of groundwater use from a user perspective in two irrigated areas in North Africa (Morocco and Algeria), this study shows how the context of groundwater depletion exacerbates—and is exacerbated by—existing inequalities. The paper concludes that knowing how much is withdrawn, where, and by whom provides helpful information for more informed groundwater management by a better understanding of the response of users to declining groundwater conditions and the interests and incentives of different social categories of famers to contribute to groundwater management.     

Considering groundwater use to improve the assessment of groundwater pumping for irrigation in North Africa

Groundwater resources in semi-arid areas and especially in the Mediterranean face a growing demand for irrigated agriculture and, to a lesser extent, for domestic uses. Consequently, groundwater reserves are affected and water-table drops are widely observed. This leads to strong constraints on groundwater access for farmers, while managers worry about the future evolution of the water resources. A common problem for building proper groundwater management plans is the difficulty in assessing individual groundwater withdrawals at regional scale. Predicting future trends of these groundwater withdrawals is even more challenging. The basic question is how to assess the water budget variables and their evolution when they are deeply linked to human activities, themselves driven by countless factors (access to natural resources, public policies, market, etc.). This study provides some possible answers by focusing on the assessment of groundwater withdrawals for irrigated agriculture at three sites in North Africa (Morocco, Tunisia and Algeria). Efforts were made to understand the different features that influence irrigation practices, and an adaptive user-oriented methodology was used to monitor groundwater withdrawals. For each site, different key factors affecting the regional groundwater abstraction and its past evolution were identified by involving farmers’ knowledge. Factors such as farmer access to land and groundwater or development of public infrastructures (electrical distribution network) are crucial to decode the results of well inventories and assess the regional groundwater abstraction and its future trend. This leads one to look with caution at the number of wells cited in the literature, which could be oversimplified.