Contenu du sommaire

If the reduction of annual runoff due to forest cover is commonly accepted, the effective amount of this reduction is severely disputed. Two mains processes contribute to the explanation of this reduction. First, forest vegetation is generaly deeply rooted, an can use more of the soil water content, especialy during summer if pedologie and climatic conditions are favorable. Second, interception of incident rainfall allows a better use of advective heath and, consequently, a higher evaporation amount. The consequences of a forest cover on annual water balance depend mainly on the combination of both climatic and pedologie conditions. Recent processes-based researches contribute to a better understanding of the discrepancy observed beetween the estimated reduction of annuel runoff. But, it still remains widespread misunderstanding, especialy in the field of interception studies.

The effect of land use and vegetal cover changes on runoff regimes is taken as one of the most typical effects of mon activity on the hydrological cycle. A presentation of the available methods and tools used by the hydrologist to assess these effects (such as the paired watershed approach), shows that even if these impacts do exist, it might be very difficult to quantify them. The data collected on ten small watersheds during the Ecerex experiment in French Guyana is taken as a case study. According to the watershed and to the hydrological parameter considered (annual runoff, peak flow), the hydrological effects of land use changes were assessed with high accuracy and quantified with a very low statistical risk, but for some of the experiments, the uncertainty was much wider and the outcome is more a presumption than a scientific proof. The major issue for evaluating the effects of any treatment is the capacity of the hydrologist to predict the hydrological behaviour which would have been observed on a specifie treated watershed under the same climatic conditions, if the treatment would not have been applied. The Ecerex experiment started with a 2-year calibration period, which has shown a wide range of runoff under natural forest : for a similar amount of rainfall, the variation of runoff within the set of the ten watershed was in a range of 1 to 5, Le. stormflow volumes varied from 7 % to 54 % of the rainfall, according to soil characteristics. The initial treatment (logging followed by mechanised land clearing) has created bare soil conditions for seven of the watersheds. The increase of stormflow during the first year after the treatment was very high (+ 228 mm to + 714 mm, or + 66 % to + 199 % in relative terms). The highest relative increases were observed for the watersheds having the lowest flows in natural conditions. The treatments applied include natural regrowth of the forest, fast-growing tree plantation (pine, eucalyptus), fruit trees plantation, grazing, traditional slash-and-burn agriculture. The effects on annual stormflow and on peak discharges were quantified for the first years after the application of the treatments.

The research catchments studied since 1975 by the C.E.R.E.G. in the Vosges mountains have permitted to precise the spatial and temporal variability of the main components of the water cycle and the controlling parameters in a temperate middle mountain environment. The main results concern the precipitations, the evapotranspiration, the soil water properties and the streamflow. A synthesis of the information available is in progress, using spatiaily semi-distributed and physically-based models. The interdisciplinary, systemic, dynamic approach used makes the results and models transposable to other catchments, environments and scales.

The cropland of the Brie plateau, characterized by temporary water logged and silt-clay rich soils, has been drained as the results of intensive agriculture development. Floods have been monitored in order to assess the importance of the transfers of water and suspended matter discharged both from a collecting pipe of a cultivated field and front the upstream part of the river Vannetin (an agricultural catchment basin). The first data collected show that the output of water and suspended matter discharged by the drains in the river channel is low at the biginning of autumn and becomes high with the start of the intensive drainage period. Finally, the cumulated output represents a loss of fine material from the soil.

Water has always been a very important problem in the area corresponding to the " Pyrenees Orientales " district. On the slopes and valley floors, man-made equipments and development have induced heavy modifications in water and sediment discharge. The spatial variety of mountain areas divided in mountain upslopes, downslopes and coastal plain, has influenced the development programs in the district. Today the frequency of floods has clearly decreased. However, the risk of a high flood remains, which could damage urban areas recently developed downslope.

In France, and especially in the Region Centre (Val de Loire, Berry,...), the period 1977-1984 was noticeable with numerous floods. And, as often the case, several developments were made responsible for generating higher and more frequent floods than usual, whatever the extent of these developments in the watersheds concemed. An analysis of these developments enables us to take into account climatic factors and consequences of anthropic modifications on amplitude and frequency of these floods. In conclusion, it is possible to write that floods depend primarily on climatic factors while anthropic factors play a major role in the increase of flood consequences (human and economic) more than in flood magnitude.

Bazed on an examination of a river system with a reputation for stability, the Saône, the purpose of this study is to demonstrate the role played by development planning in the transformation of a waterway. The result of detailed analysis based on the last 26 kilometers of the Saône, allow us to demonstrate and quantify the morphological transformation of the river since the first work carried out during the nineteenth century. This area is representative of all kinds of efforts to prevent the flooding of an occupied and cultivated plain, as well as the developments which can be imposed on an old navigable waterway. In this deepered and standardized " new Saone ", the most important area in ecological terms remains the environment of the banks. The description of the different kind of banks in the area studed shows both an active recolonization of the old constructions and a marked decline in the typical natural bank. As a conclusion, after a theoretical classification of the banks' ecological value, only the insular areas, Le. islands, channels and the banks they protect, stand out to any decisive degree and, to a lesser extent, the " platis " of the Saone.