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Each of the main land-masses of the world is largely divided piecemeal into catchment areas of drainage systems supplying major rivers, the example of Africa being illustrated in Fig. i. A river may be defined as a mixture of water, rock detritus, and other materials, flowing in a channel having pro-gressively lower levels. Strahler (1951) quotes from John Playfair’s Law (published in 1802): ‘Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together forming a system of valleys connecting with one another, and having such a nice adjustment of their declivities that none of them joins the principal valley either on too high or too low a level; a circumstance which would be infinitely improbable if each of these valleys were not the work of the stream which flows in it.’ This statement comprises three main points: (1) valleys are proportioned in size to streams flowing in them, (2) stream junctions are accordant in level, and (3) therefore, valleys are carved by streams flowing in them.

A complèter : pagination et WOS; International audience; Large wood (LW, or pieces of dead wood longer than 1 m and thicker than 10 cm in diameter) is a key element in forested streams, but its abundance has decreased worldwide as a result of snagging and clearing of riparian forests. Therefore, many restoration projects introduce LW into stream channels to enhance geomorphology, biotic communities, and ecosystem functioning. Because LW enhances the retention of organic matter and sediments, its restoration can reduce siltation in receiving reservoirs, although so far little information on this subject is available. We studied the effects of restoring the natural loading of LW in four streams in the Aiako Harria Natural Park (the Basque Country, Spain) in pool abundance, channel form, and storage of organic matter and sediments. In all reaches log jams induced the formation of new geomorphic features and changes in physical habitat, especially an increase in the number and size of pools and in the formation of gravel bars and organic deposits. The storage of organic matter increased 5- to 88-fold and streambed level rose 7 ± 4 to 21 ± 4 cm on average, resulting in the storage of 35.2 ± 19.7 to 711 ± 375 m3 (733–1400 m3 ha− 1 y− 1) of sediment per reach. Extrapolation of these results to the entire drainage basin suggests that basinwide restoration of LW loading would enhance the retention potential of stream channels by 66,817 ± 27,804 m3 (1075 m3 ha− 1 y− 1) of sediment and by 361 t (5.32 T ha− 1 y− 1) of organic matter, which represents 60% of the estimated annual inputs of sediments to the downstream Añarbe Reservoir and almost twice as much as the annual input of organic matter to the entire river network. Therefore, basinwide restoration of LW loading is a potentially important tool to manage catchments that feed reservoirs, where retention of sediments and organic matter can be considered important ecosystem services as they reduce reservoir siltation.

International audience; The aim of this study was to develop a conceptual model of saline intrusion within coastal karst aquifers by analyzing Rainfall–Discharge–Salinity data and to assess freshwater resources using a lumped modeling approach. In a first step, we analyzed 4 years of data (rainfall, discharge and salinity times series) ofthe Port-Miou brackish submarine spring in South France (400 km2). A conceptual model of the aquifer was then designed to differentiate a deep brackish reservoir and a shallower fresh one. Salinity variations at the spring are assumed to be controlled mainly by dilution originating from the fresh water in the shallower reservoir. In a second step, a lumped modeling approach was developed based on the conceptual model to simulate discharge as well as salinity over time. We proposed a reservoir-model to take into account slow and fast components in the shallower part of the aquifer and a saline intrusion in the deeper one. This Rainfall–Discharge–Salinity model was calibrated and validated for two periods of 1.5 years at a daily time step and was also tested to reproduce a multi-annual evolution of the available discharge and salinity time series. Good simulation results were obtained to reproduce water and mass budgets as well as discharge and salinity dynamics during several hydrological cycles. The simultaneous modeling of hydrodynamics and quality data showed the robustness of the model in addition to its easy implementation. Our results led us to propose a new type of seawater mixing mechanism for brackish springs: the dilution type, in addition to the well-known Ventury suction and Head balance types. The application of the lumped model on the Port-Miou brackish spring validated the hydrogeological processes deduced from experimental data, given an initial quantification of the freshwater resources available in such complex brackish karstic aquifers.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SYNERGIE [Axe_IRSTEA]TETIS-ATTOS; International audience; The Phased Array L-band Synthetic Aperture Radar (PALSAR-1) has provided very useful images dataset for several applications such as forestry. L-band radar measurements have been widely used but with somewhat contradictory conclusions on the potential of this radar wavelength to estimate the aboveground biomass. The first objective of this study was to analyze the L-band SAR backscatter sensitivity to forest biomass for Eucalyptus plantations. The results showed that the radar signal is highly dependent on biomass only for values lower than 50 t/ha, which corresponds to plantations of approximately three years of age. Next, Random Forest regressions were performed to evaluate the potential of PALSAR data to predict the Eucalyptus biomass. Regressions were constructed to link the biomass to both radar signal and age of plantations. Results showed that the age was the variable that best explained the biomass followed by the PALSAR HV polarized signal. For biomasses lower than 50 t/ha, HV signal and plantation age were found to have the same level of importance in predicting biomass. For biomasses higher than 50 t/ha, plantation age was the main variable in the random forest models. The use of PALSAR signal alone did not correctly predict the biomass of Eucalyptus plantations (R² lower than 0.5 and RMSE higher than 46.7 t/ha). The use of plantation age in addition to the PALSAR signal improved slightly the prediction results (R² increased from 0.88 to 0.92 and RMSE decreased from 22.7 to 18.9 t/ha). PALSAR imagery does not allow a direct estimation of the planting date of Eucalyptus stands but can follow efficiently the occurrence of clear-cuts if images are acquired sequentially, therefore allowing a rough estimate of the following plantation date because a stand of Eucalyptus is generally re-planted 2 to 4 months after cutting. With a time series of radar images, it could be therefore possible to estimate the plantation age, and therefore improving the estimates of plantation biomass.