Eric Servat

Selection of calibration objective fonctions in the context of rainfall-ronoff modelling in a Sudanese savannah area

Abstract In the context of rainfall-runoff modelling carried out on the Sudanese savannah area in the northwest of the Ivory Coast, attempts are being made to reconstitute the flow at the outlets of catchments in 10 day time steps. By using algorithms with automatic fitting procedures for the parameters, it appeared necessary to make a choice concerning the calibration objective functions to be used. The paper presents the algorithms, data and objective functions that have been used. The results obtained from the calibrations made have been analysed. That analysis was done principally with the help of a comparative evaluation modulus which takes into account elements other than the value of the objective function alone and which enables the quality of the results to be picked out from a hydrological point of view. At the conclusion of the analysis, the objective function defined by Nash seems to stand out quite clearly in relation to the other formulae examined.

Analyse de séries pluviométriques de longue durée en Afrique de l'Ouest et Centrale non sahélienne dans un contexte de variabilité climatique

In the arid and semiarid areas of West and Central Africa, the scarcity of the water resources is an old and well-known notion. However, the drought, which has been observed in the tropical regions of Africa for the last two decades, shows notable harshness, persistence and extension. Annual rainfall time series registered over long duration for a set of stations all over non-Sahelian West and Central Africa have allowed the study of spatial and temporal rainfall evolution in this region. Results confirm the existence of an alternation of dry and wet periods since the beginning of the twentieth century. However, the current drought has no equivalent, neither regarding duration nor intensity, throughout the studied period. Nevertheless the strong spatial heterogeneity of the phenomenon is underlined by both a statistical study and some mapping.

Rainfall-runoff modelling and water resources assessment in northwestern Ivory Coast. Tentative extension to ungauged catchments

Most development projects in northern Ivory Coast require good water resource management. Rainfall-runoff modelling was performed in 20 catchments (100–4500 km2) to provide reliable tools for the assessment of water supply in the sudanese savannah area. After the calibration of two conceptual models (GR3 and CREC), the parameters were characterized so that these models could be applied to ungauged catchments. Predetermination equations were established using variables related to the catchment land use and the rainfall distribution over the year. The GR3 model produced good results although further work is needed to improve the reliability of the equations before operational use.

Relations eaux de surface–eaux souterraines d'une rivière tropicale au Mali

Abstract The annual average of the groundwater levels in 27 wells is connected to the annual climate impact on the groundwater resources in the Bani river basin (Douna, 101 600 square kilometers). The groundwater level variations are well correlated to rainfall variations, and for the lowest levels of groundwater, the runoff stops in the dry season at Douna. The humid year of 1994 caused a rise in the groundwater level to the same level as 1981, together with a reduction of the depletion coefficient. But the runoff is still weak, showing that the rainfall–runoff relationships are modified over the long term.

The concept of rainfall and streamflow normals in West and Central Africa in a context of climatic variability

Abstract This contribution to the debate on the revision of rainfall and streamflow normals concerns three countries of West Africa: Burkina Faso, Ivory Coast and Mali. The rainfall deficit (15–20% on average in the study area) and the river flow deficit (about 30–50% and sometimes more) observed over the past 30 years in West and Central Africa raise the problem of the reference period that must be considered in estimates of hydrological characteristics. The hypothesis of the concept of “normal” supporting these estimates is a stability of the climate. However, the effect of taking or not taking into account the data of recent years on the estimates of these characteristics can lead to very different results. This would have serious consequences for the development and management of water resources systems, as seen in two examples of dams.

Current state of Mediterranean water resources and future trends under climatic and anthropogenic changes

Abstract The Mediterranean basin has been identified as one of the worlds regions most vulnerable to climatic and anthropogenic changes. A methodology accounting for the basin specific conditions is developed to assess the impacts of these changes on water resources. Based on global climate projections and water-use scenarios inspired by national reports, the current water stress state is addressed first and then it is explored for the medium-term. Currently, the southern and eastern rims are experiencing high to severe water stress. By the 2050 horizon, this stress could increase over the whole Mediterranean basin, notably because of a 30–50% decline in freshwater resources as a result of climate change. In addition, under a business-as-usual water-use scenario, total water withdrawals are projected to double on the southern and eastern rims. These worrying trends indicate the need to develop mitigation scenarios. In accord with the Mediterranean Strategy for Sustainable Development, an alternative water-use scenario based on improvements in the efficiency of water distribution networks and of irrigated agriculture is investigated. Such progress would stabilize total water withdrawals over the Mediterranean basin and even make them decrease (10–40%) in many northern catchments. Water stress could thus be tempered in some eastern catchments and remain low on the northern rim. This study highlights the importance of developing sustainable development strategies to cope with climatic and anthropogenic changes in order to explore their impacts at regional scales. It supports the need to focus on the most vulnerable areas within the Mediterranean basin. Editor Z.W. Kundzewicz Citation Milano, M., Ruelland, D., Fernandez, S., Dezetter, A., Fabre, J., Servat, E., Fritsch, J.-M., Ardoin-Bardin, S., and Thivet, G., 2013. Current state of Mediterranean water resources and future trends under climatic and anthropogenic changes. Hydrological Sciences Journal, 58 (3), 498–518.

Using general circulation model outputs to assess impacts of climate change on runoff for large hydrological catchments in West Africa

Abstract The problem of using data outputs from general circulation models (GCMs) to assess the impacts of climate change on runoff in West Africa is addressed. The annual and monthly precipitation data from four GCMs used in the Third Assessment Report of the IPCC were studied over the 1950–1998 period: the CSIRO-Mk2, ECHAM4, HadCM3 and NCAR-PCM models. Two weaknesses common to all these models are their inability to reproduce rainfall volumes in the Sahelian zone, and their difficulty in simulating the seasonal dynamics of rainfall in the Guinean zone. Two climate scenarios were then developed based on the changes predicted by the HadCM3-A2 model, which aimed to generate time series for rainfall and potential evapotranspiration up to the end of the 21st century, in order to simulate probable future climatic conditions. These scenarios were used as input to the hydrological model GR2M to assess the impacts of climate change on the discharge of four main rivers: the Senegal, the Gambia, the Sassandra and the Chari. The results show that the possible future changes in runoff are highly dependent on rainfall and, hence, on the quality of the output of a given GCM.

Analysis of the temporal variability of runoff in Ivory Coast: statistical approach and phenomena characterization

Abstract The runoffs at four Ivory Coast hydrometric stations (monitoring flows from an area covering between 5930 and 66500 km2) were analysed with a set of statistical methods for the detection of breaks in the time series. The variables studied were the annual mean discharge and some characteristic discharges. From a quantitative standpoint, the existence of a clear break in the series of annual mean discharges at the beginning of the decade from 1970, the date from which the runoffs decrease significantly, was noted. A more qualitative study of the results showed that low flows were more affected than high flows by this variability of the regime. This fluctuation appears to be in accord with the drought phenomena observed during the same period in the Sahel, to the north of Ivory Coast.

De la diminution des écoulements en Afrique de l'Ouest et centrale

An important climatic fluctuation has now been observed for 25 years all over Western and Central Africa. This fluctuation is translated by a notable decrease in annual rainfall since the end of 1960s and the beginning of 1970s. This reduction of rainfall has serious consequences on flow regimes within this region. This study has allowed the characterization of these changes and has interpreted them on a regional level. It has also emphasized the very important decrease of the volume of flow in the rivers, which has economical and environmental results.

Assessing the long-term impact of climatic variability and human activities on the water resources of a meso-scale Mediterranean catchment

Abstract This article addresses the critical need for a better quantitative understanding of how water resources from the Hérault River catchment in France have been influenced by climate variability and the increasing pressure of human activity over the last 50 years. A method is proposed for assessing the relative impacts of climate and growing water demand on the decrease in discharge observed at various gauging stations in the periods 1961–1980 and 1981–2010. An annual water balance at the basin scale was calculated first, taking into account precipitation, actual evapotranspiration, water withdrawals and water discharge. Next, the evolution of the seasonal variability in hydroclimatic conditions and water withdrawals was studied. The catchment was then divided into zones according to the main geographical characteristics to investigate the heterogeneity of the climatic and human dynamics. This delimitation took into account the distribution of climate, topography, lithology, land cover and water uses, as well as the availability of discharge series. At the area scale, annual water balances were calculated to understand the internal changes that occurred in the catchment between both past periods. The decrease in runoff can be explained by the decrease in winter precipitation in the upstream areas and by the increase during summer in both water withdrawals and evapotranspiration in the downstream areas, mainly due to the increase in temperature. Thus, water stress increased in summer by 35%. This work is the first step of a larger research project to assess possible future changes in the capacity to satisfy water demand in the Hérault River catchment, using a model that combines hydrological processes and water demand. Editor Z.W. Kundzewicz Citation Collet, L., Ruelland, D., Borrell-Estupina, V., and Servat, E., 2014. Assessing the long-term impact of climatic variability and human activities on the water resources of a meso-scale Mediterranean catchment. Hydrological Sciences Journal, 59 (8), 1457–1469. http://dx.doi.org/10.1080/02626667.2013.842073