Comparison of soil moisture fields estimated by catchment modelling and remote sensing: a case study in South Africa

International audienceThe paper compares two independent approaches to estimate soil moisture at the regional scale over a 4625 km2 catchment (Liebenbergsvlei, South Africa). The first estimate is derived from a physically-based hydrological model (TOPKAPI). The second estimate is derived from the scatterometer on board on the European Remote Sensing satellite (ERS). Results show a very good correspondence between the modelled and remotely sensed soil moisture, illustrated over two selected seasons of 8 months by regression R2 coefficients lying between 0.78 and 0.92. Such a close similarity between these two different, independent approaches is very promising for (i) remote sensing in general (ii) the use of hydrological models to back-calculate and disaggregate the satellite soil moisture estimate and (iii) for hydrological models to assimilate the remotely sensed soil moisture

Climate and water cycle in the tropical zone : a problem of scale

The climate of the tropics is characterized by a strong interaction between the land surface conditions and the monsoon dynamics controlling the formation of precipitation. The water cycle of these regions is thus both a controlling factor and a depending factor of the climate dynamics. West Africa, which suffered the most extensive and severe drought worldwide in the 20th century, may be viewed as a paradigm for studying this interaction. One major issue when trying to relate climate to the water cycle is the scale gap between the models commonly used for climate simulations and the models used for hydrological studies. (C) 2004 Academie des sciences. Publie par Elsevier SAS. Tous droits reserves

Climat et cycle de l'eau en zone tropicale : un problème d'échelle

En zone tropicale, les conditions de surface continentales sont un facteur important pour l'établissement du régime de mousson qui génère les pluies tropicales. Il y a donc une forte interaction entre dynamique du climat et cycle de l'eau. L'exemple de l'Afrique de l'Ouest, encore sous l'influence de la plus forte sécheresse observée au XXe sur le globe, est ici traité plus en détail pour illustrer cette interaction et les questions, notamment d'échelle, qui se posent lorsque l'on cherche à établir des scénarios hydroclimatiques pour le futur

Recent trends in the regime of extreme rainfall in the Central Sahel

Ongoing global warming raises the hypothesis of an intensification of the hydrological cycle, extreme rainfall events becoming more frequent. However, the strong time-space variability of extreme rainfall makes it difficult to detect meaningful trends in the regime of their occurrence for recent years. Using an integrated regional approach, it is shown that over the last 10 years, the Sahelian rainfall regime is characterized by a lasting deficit of the number of rainy days, while at the same time the extreme rainfall occurrence is on the rise. As a consequence, the proportion of annual rainfall associated with extreme rainfall has increased from 17% in 1970-1990 to 19% in 1991-2000 and to 21% in 2001-2010. This tends to support the idea that a more extreme climate has been observed over 2001-2010: this climate is drier in the sense of a persisting deficit of rainfall occurrence compared to 1950-1969, while at the same time there is an increased probability of extreme daily rainfall

Conditional simulation schemes of rain fields and their application to rainfall-runoff modeling studies in the Sahel

In regions characterized by a great inter-annual variability or by decadal-scale changes of the rainfall regime, the simulation of long series of rainfall events is an efficient way to explore the runoff fluctuations or modifications resulting from this rainfall variability. In a context of great uncertainty regarding the Sahelian rainfall regime in a changing climate, a coherent stochastic framework is presented here to produce high spatial resolution rain fields in order to force a rainfall-runoff model and to perform sensitivity analyses. The focus of the paper is on the comparison of various conditioning methods reflecting the various types of information available for the study of past situations (data from rain gage and satellite) as well as of future scenarios (outputs of atmospheric models). Various types of rainfall simulations are performed over a 13 year period, using four levels of conditioning information obtained from a 15 gauge network covering a 60 x 60 km(2) region. These simulations are then used as inputs to a Hortonian rainfall-runoff model. The simulation relevance is first assessed by studying the simulated rain field series (event time-step mean characteristics, seasonal cycle and inter-annual variability) in comparison with reference rain fields estimated by kriging. This shows that the conditioning of the simulations, even by a minimal information provided by a unique station, is of great relevance for constraining the stochastic dispersion and thus to retrieve the rainfall variability at the considered scales. Significant differences are reported between runoff obtained by the different types of created rain fields, one of the most noticeable being that runoff obtained from kriging is 25% lower than runoff obtained from point conditional simulations. The results confirm the sensitivity of Hortonian hydrological systems to rainfall intensity and particularly point out the importance of representing realistic spatial rainfall patterns to force hydrological models

Reconstitution of a continuous climatic and rainfall series for the central Sahel (1950-2012): methodology and application.

International audienc

A boundary forcing sensitivity analysis of the West African monsoon simulated by the modele atmospherique regional

The rainfall regime of West Africa is highly variable over a large range of space and time scales. With rainfall agriculture being predominent in the region, the local population is extremely vulnerable to intraseasonal dry spells and multi-year droughts as well as to intense rainfall over small time steps. Were this variability to increase, it might render the area close from becoming unhabitable. Anticipating any change is thus crucial from both a societal and a scientific perspective. Despite continuous efforts in Global Climate Model (GCM) development, there is still no agreement on the sign of the future rainfall regime change in the region. Regional Climate Models (RCMs) are used for more accurate projections of future changes as well as end-user-oriented impact studies. In this study, the sensitivity of the Modele Atmospherique Regional (MAR) to homogeneous perturbations in boundary forcing air temperature and/or SST is assessed with the aim to better understand (i) the thermodynamical imprint of the recent rainfall regime changes and (ii) the impact of errors in driving data on the West African rainfall regime simulated by an RCM. After an evaluation step where the model is proved to satisfactorily simulate the West African Monsoon (WAM), sensitivity experiments display contrasted, sizable and robust responses of the simulated rainfall regime. The rainfall responses to the boundary forcing perturbations compare in magnitude with the intrinsic model bias, giving support for such an analysis. A physical interpretation of the rainfall anomalies provides confidence in the model response consistency and shows the potential of such an experimental protocol for future climate change downscalling over this region