До 85-річчя Ісаака Бенціоновича Клеймана

У 2006 р. виповнилося 85 років одному з найшановніших археологів Одеси, відомому досліднику стародавньої Тіри Ісааку Бенціоновичу Клейману

Variabilité hydroclimatique et impacts sur les ressources en eau de grands bassins hydrographiques en zone soudano-sahélienne

Modelling the impact of the expected climate variability on water resources is fundamental for the planning of facilities, mainly in Western Africa subjected to an unprecedented drought since around thirty years. The purpose of that work is to be able to use rainfall and potential evapotranspiration data from climatic scenarios as input to hydrological models; these models being previously calibrated on large catchments from western Africa (Senegal, Gambia, Sassandra) and central Africa (Logone-Chari). Thus, the main goal is to study the likely variability of water resources during the 21st century. That approach is based on: - The knowledge of the variability of rainfall and hydrological regimes. From a general point of view, the 1970 – 1998 period is in deficit regarding rainfall and the 1990 decade demonstrates that the drought is going on. Those deficits can be explained by a modification of the area influenced by the monsoon which takes part in a more global dynamic regarding the atmospheric circulation. - The assessment of tools to model hydrological regimes of large rivers. Two hydrological models, based on balance equations (GR2M and WBM) have been selected. They work at a monthly time step and with data grids established at a resolution of a square half degree. The sensitivity analysis of the models underlines the significance of rainfalls, land cover and land use regarding hydrological modelling. That assessment also emphasizes the quality of the results and the adaptability of the GR2M model. - The elaboration and the use of climatic scenarios from data provided by GCM. Outputs from climatic models are not really reliable but the foreseen variations can be combined with observed data to generate climatic scenarios. From rainfall and temperature data provided by the HadCM3-A2 model, two likely climatic scenarios could be elaborated. Used with GR2M and WBM they lead to the reduction of the Senegal and Gambia flow, to an increase followed by a decrease of the Sassandra flow and to an increase of the Logone-Chari flow.Pour les pays du sous-continent ouest africain soumis depuis une trentaine d'années à un changement climatique sans précédent au XXème siècle, la modélisation de l'impact de la variabilité prévisible du climat sur les ressources en eau est fondamentale pour la planification des aménagements. L'objectif de ce travail est l'utilisation de simulation de pluies et d'évapotranspiration potentielle issues de scénarii climatiques en entrée de modèles hydrologiques, calés pour les grandes rivières d'Afrique de l'Ouest (Sénégal, Gambie, Sassandra) et Centrale (Logone-Chari), afin d'étudier la variabilité probable des ressources en eau au cours du XXIème siècle. Cette approche se fonde sur : - la connaissance de la variabilité des régimes pluviométriques et hydrologiques. Globalement, la période 1970-1998 est déficitaire et la décennie 90 témoignent de la persistance de la sécheresse. Ces déficits peuvent s'expliquer par une variation de l'espace couvert par la mousson s'inscrivant dans une dynamique à l'échelle de la circulation atmosphérique générale ; - l'évaluation d'outils de modélisation des régimes hydrologiques des grands cours d'eau régionaux. Deux modèles de bilans hydrologiques (GR2M et WBM), fonctionnant au pas de temps mensuel sur des grilles à la résolution du demi degré carré, ont été retenus. L'analyse de sensibilité modèles montre l'importance des précipitations et des états de surface dans la modélisation hydrologique. Cette évaluation souligne également la performance et la robustesse du modèle GR2M ; - le développement et l'application de scénarii climatique à partir des données issues des Modèles de Circulation Générale. Les sorties de modèles climatiques ne sont pas vraiment fiables mais les variations prévues peuvent être combinées aux observations pour générer des scénarii climatiques. L'utilisation des données de précipitations et de température du modèle HadCM3-A2, via deux scénarii climatiques, dans les modèles hydrologiques GR2M et WBM conduisent à la diminution des débits du Sénégal et de la Gambie, une augmentation suivie d'une diminution des débits du Sassandra et une augmentation des débits du Logone-Chari

Variabilité hydroclimatique et impacts sur les ressources en eau des grands bassins hydrographiques en zone soudano-sahélienne

MONTPELLIER-BU Sciences (341722106) / SudocFONTAINEBLEAU-MINES ParisTech (771862302) / SudocSudocFranceF

Sensitivity of a lumped and semi-distributed hydrological model to several methods of rainfall interpolation on a large basin in West Africa

This paper examines the sensitivity of a hydrological model to several methods of spatial interpolation of rainfall data. The question is investigated in a context of scarcity of data over a large West African catchment (100,000 km(2)) subject to a drastic trend of rain deficit since the 1970s. Thirteen widely scattered rainfall stations and their daily time series were used to interpolate gridded rainfall surfaces over the 1950-1992 period via various methods: Thiessen polygons, inverse distance weighted (IDW) method, thin smooth plate splines (spline), and ordinary kriging. The accuracy of these interpolated datasets was evaluated using two complementary approaches. First, a point-by-point assessment was conducted, involving comparison of the interpolated values by reference to observed point data. Second, a conceptual rainfall-runoff model (Hydrostrahler) was used in order to assess whether and to what extent the alternative sets of interpolated rainfall impacted on the hydrological simulations. A lumped modelling exercise over a tong period (1952-1992) and a semi-distributed exercise over a short period (1971-1976) were performed, using calibrations aimed at optimizing a Nash-Sutcliffe criterion. The results were evaluated for each interpolated forcing dataset using statistical analysis and visual inspection of the simulated and observed hydrographs and the parameters obtained from calibration. Assessment of the interpolation methods by reference to point data indicates that interpolations using the IDW and kriging methods are more efficient than the simple Thiessen technique, and, to a lesser extent, than spline. The use of these data in a daily lumped modelling application shows a different ranking of the various interpolation methods with regard to various hydrological assessments. The model is particularly sensitive to the differences in the rainfall input volume produced by each interpolation method: the IDW dataset yields the highest hydrological efficiency white the spline dataset gets the poorest results. Although the calibration procedure makes it possible to partly compensate for the differences (or errors) between rain input datasets, the semi-distributed hydrological model remains sensitive to volumetric and spatial differences. Then, assessment of these combined differences through the sensitivity of the semi-distributed model provides us with more complete discrimination between the interpolated data inputs. The output results at the basin outlet do not decrease between the lumped and semi-distributed modelling exercises with the IDW and kriging datasets, in contrast to the Thiessen and spline datasets, which tends to indicate the superiority of the former two interpolated inputs. In this hydrological application, the IDW dataset is still shown to provide the most realistic results. Moreover, despite the scarcity of rainfall data, coherent semi-distributed values of the model parameters are obtained by calibration over a large gradient of climate conditions. Finally, it is observed that although the model reproduced the rainfall-runoff relationship before 1970 very well, regardless of which interpolated datasets were used, it was not able to satisfactorily simulate the basin behaviour after the change in rainfall regime. This inability needs further investigation and is the subject of ongoing research

How could hydro-climatic conditions evolve in the long term in West Africa? The case study of the Bani River catchment

International audienceThis paper assesses the future variability of water resources in the long term over a large Sudano- Sahelian catchment in West Africa. Flow simulations were performed with a daily conceptual model. The climate models HadCM3 and MPI-M (based on SRES-A2) were used to provide future climate scenarios over the catchment. Outputs from these models were used to generate daily rainfall and temperature series for the 21st century according to: (i) application of the unbias and delta methods, and (ii) spatial and temporal downscaling. A temperature-based formula was used to calculate present and future potential evapotranspiration (PE). The daily rainfall and PE series were introduced into the calibrated and validated hydrological model to simulate future discharge. The model correctly reproduces the observed discharge at the basin outlet with the Nash-Sutcliffe efficiency criterion over 0.89, and the volume error close to null over 1952-2000. With regard to future climate, the results show clear trends of reduced rainfall with a continuing increase in PE over the catchment. This suggests that the catchment discharge could fall in the long term to the same levels as those observed during the severe drought of the 1980s

Assessment of climate change impacts on the hydrology of the Peruvian Amazon-Andes basin

In this article, we propose an investigation of the modifications of the hydrological response of two Peruvian Amazonas-Andes basins in relationship with the modifications of the precipitation and evapotranspiration rates inferred by the IPCC. These two basins integrate around 10% of the total area of the Amazonian basin. These estimations are based on the application of two monthly hydrological models, GR2M and MWB3, and the climatic projections come from BCM2, CSMK3 and MIHR models for A1B and B1 emission scenarios (SCE A1B and SCE B1). Projections are approximated by two simple scenarios (anomalies and horizon) and annual rainfall rates, evapotranspiration rates and discharge were estimated for the 2020s (2008-2040), 2050s (2041-2070) and 2080s (2071-2099). Annual discharge shows increasing trend over Requena basin (Ucayali river), Puerto Inca basin (Pachitea river), Tambo basin (Tambo river) and Mejorada basin (Mantaro river) while discharge shows decreasing trend over the Chazuta basin (Huallaga river), the Maldonadillo basin (Urubamba river) and the Pisac basin (Vilcanota river). Monthly discharge at the outlet of Puerto Inca, Tambo and Mejorada basins shows increasing trends for all seasons. Trends to decrease are estimated in autumn discharge over the Requena basin and spring discharge over Pisac basin as well as summer and autumn discharges over both the Chazuta and the Maldonadillo basins