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Heavy rainfall events often occur in southern French Mediterranean regions during the autumn, leading to catastrophic flood events. A non-stationary peaks-over-threshold (POT) model with climatic covariates for these heavy rainfall events is developed herein. A regional sample of events exceeding the threshold of 100 mm/d is built using daily precipitation data recorded at 44 stations over the period 1958–2008. The POT model combines a Poisson distribution for the occurrence and a generalized Pareto distribution for the magnitude of the heavy rainfall events. The selected covariates are the seasonal occurrence of southern circulation patterns for the Poisson distribution parameter, and monthly air temperature for the generalized Pareto distribution scale parameter. According to the deviance test, the non-stationary model provides a better fit to the data than a classical stationary model. Such a model incorporating climatic covariates instead of time allows one to re-evaluate the risk of extreme precipitation on a monthly and seasonal basis, and can also be used with climate model outputs to produce future scenarios. Existing scenarios of the future changes projected for the covariates included in the model are tested to evaluate the possible future changes on extreme precipitation quantiles in the study area. Editor Z.W. Kundzewicz; Associate editor K. Hamed Citation Tramblay, Y., Neppel, L., Carreau, J., and Najib, K., 2013. Non-stationary frequency analysis of heavy rainfall events in southern France. Hydrological Sciences Journal, 58 (2), 280–294. [ABSTRACT FROM AUTHOR]

Flood frequency analysis based on a set of systematic data and a set of historical floods is applied to several Mediterranean catchments. After identification and collection of data on historical floods, several hydraulic models were constructed to account for geomorphological changes. Recent and historical rating curves were constructed and applied to reconstruct flood discharge series, together with their uncertainty. This uncertainty stems from two types of error: (a) random errors related to the water-level readings; and (b) systematic errors related to over- or under-estimation of the rating curve. A Bayesian frequency analysis is performed to take both sources of uncertainty into account. It is shown that the uncertainty affecting discharges should be carefully evaluated and taken into account in the flood frequency analysis, as it can increase the quantiles confidence interval. The quantiles are found to be consistent with those obtained with empirical methods, for two out of four of the catchments. Citation Neppel, L., Renard, B., Lang, M., Ayral, P.-A., Coeur, D., Gaume, E., Jacob, N., Payrastre, O., Pobanz, K. & Vinet, F. (2010) Flood frequency analysis using historical data: accounting for random and systematic errors. Hydrol. Sci. J.55(2), 192-208. [ABSTRACT FROM AUTHOR]

Abstract: This study is focused on the western part of the French Mediterranean area, namely the Pyrénées-Orientales and Aude administrative departments. The water resources (surface and groundwater) in the region are sensitive to climate change. The study addresses the question of whether any trend in the annual and monthly series of temperature, rainfall and potential evapotranspiration (PET) already appears at the scale of this region. Two data sources have been used: (a) direct local measurements using the meteorological network; and (b) spatially interpolated data from the French weather service model SAFRAN for the period 1970–2006. The non-parametric Mann–Kendall test was applied to identify significant trends at the local scale and, because of the natural spatial variability of the Mediterranean climate, regional interpretation was also performed. The trends observed in the 13 catchments of interest are consistent with those observed at a larger scale. An increase in annual mean temperature and annual PET was observed throughout the study area, whereas annual precipitation has not exhibited any trend. The monthly scale has revealed strong seasonal variability in trend. The trend for an increase in monthly PET has been observed mainly in the spring, and has not been seen in the coastal areas. A trend for an increase in monthly temperature has been observed in June and in the spring throughout the entire area. Monthly rainfall has been found to decrease in June and increase in November throughout the area. The significant trends observed in rainfall and temperature seem to be consistent between the different data sources. [Copyright &y& Elsevier]

Abstract: This study compares different methods used to estimate extreme rainfall quantiles in an area with a Mediterranean climate. The first method (M1) is based on a stochastic model of hourly rainfall, which provides hourly rainfall series. The cumulative distribution functions (cdf) of extreme rainfall over various durations are deduced from the simulations. The second method (M2) is a regional approach based on the construction of a regional cdf of maximum annual daily rainfall. In the third method (M3), an exponential cdf is fitted to observed rainfall at a given gauging station. The three methods are compared with their parameters calibrated locally using the information observed at a given gauge, and using regionalised parameters. Comparison of M1 and M2 showed good agreement, with only relatively small differences between the rarest quantiles, less than 20% for the 1000-year return period. These differences can be explained by the regionalisation of the parameters. With both methods, a heavy tail of the daily maxima annual rainfall cdf deviates from the exponential cdf. Moreover, the regionalisation of M1 and M2 is more robust than with M3, which is much more influenced by sampling uncertainties. [Copyright &y& Elsevier]

Abstract: A sample of 92 daily precipitation series was selected in the French Mediterranean region over the period 1945–2004, to study the stationarity of extreme events. The criticized series are used to define seven homogeneous climatic zones. For each one, regional series of daily peaks over threshold records are extracted from the raw data. A new adaptation of the Peak Over Threshold (POT) methodology is proposed for the search of nonstationarity. For that purpose, we suppose that the scale and occurrence parameters of the model are not constant, and linearly depend on time. This approach allows us to study the stationarity of both the occurrence and the intensity of extreme events, at the regional scale. The only statistically significant results concern an increase of the occurrence and intensity of extreme daily rains in the North of the Lozère ‘department’ (France) and the southern part of the Auvergne, as well as an increase of the intensity of extreme rainfalls in Languedoc-Roussillon. [Copyright &y& Elsevier]

Highlights • Simultaneous temporal disaggregation within a region based on the analog method. • Definition of regional daily features that convey information on the subdaily scale. • Spatial pattern of precipitation combines synoptic circulation and orography. • Spatial pattern features performed well for temporal disaggregation. • Weather types convey less information on the subdaily scale. Abstract The analog method relies on the subdaily distribution of analog days – also called fragments – to perform temporal disaggregation. The features that serve to define analog days are generally at the daily scale although they are assumed to convey information on the subdaily scale. In most cases, the only feature used is the daily total at the location of interest. Features pertaining to the synoptic circulation are also often used although with the purpose of increasing the spatial rather than the temporal resolution. Precipitation is conditioned not only on synoptic circulation but also on regional features such as orography. This combined effect is reflected on the spatial pattern of daily precipitation. We propose to rely on descriptions of the spatial pattern to select analog days in order to perform temporal disaggregation. Three analog method variants based on different sets of spatial pattern features, a variant that rely on synoptic circulation alone and a variant that combines synoptic circulation with spatial pattern features are evaluated and compared. All the features considered characterize a region therefore the same analog day is employed at all the locations within the region. Inter-site coherence is thus enforced. A leave-one-out procedure is carried out on hourly radar precipitation reanalyses covering a mountainous watershed in the French Mediterranean on a 1 km2 grid over the period 1997–2006. Uncertainty is taken into account by generating five realizations of each variant. Performance is measured in terms of return levels (1–20 years) at various temporal scales (1 h to 12 h) and at two spatial scales – grid box and watershed. The proposed regional approach to temporal disaggregation globally preserves inter-site coherence. This can be seen from the relatively good reproduction of the regional pattern of return levels of the study area by all variants. Synoptic circulation conveys less information on the subdaily scale than spatial pattern features. The variant that depends on synoptic circulation alone largely overestimates return levels especially at lower temporal scales (1 h and 3 h). Combining synoptic circulation with spatial pattern features yielded no significant improvement. [ABSTRACT FROM AUTHOR]

Summary Although vital for millions of inhabitants, Himalayan water resources remain currently poorly known, mainly because of uncertainties on hydro-meteorological measurements. In this study, the authors propose a new assessment of the water budget components of the Dudh Koshi River basin (3720 km 2 , Eastern Nepal), taking into account the associated uncertainties. The water budget is studied through a cross analysis of field observations with the result of a daily hydrological conceptual distributed snow model. Both observed datasets of spatialized precipitations, interpolated with a co-kriging method, and of discharge, provided by the hydrological agency of Nepal, are completed by reanalysis data (NCEP/NCAR) for air temperature and potential evapotranspiration, as well as satellite snow products (MOD10A2) giving the dynamics of the snow cover area. According to the observation, the water budget on the basin is significantly unbalanced; it is attributed to a large underestimation of precipitation, typical of high mountain areas. By contrast, the water budget simulated by the modeling approach is well balanced; it is due to an unrealistic overestimation of the glacier melt volume. A reversing method led to assess the precipitation underestimation at around 80% of the annual amount. After the correction of the daily precipitation by this ratio, the simulated fluxes of rainfall, icemelt, and snowmelt gave 63%, 29%, and 8% of the annual discharge, respectively. This basin-wide precipitation correction is likely to change in respect to topographic or geographic parameters, or in respect to seasons, but due to an insufficient knowledge of the precipitation spatial variability, this could not be investigated here, although this may significantly change the respective proportions for rain, ice or snow melt. [ABSTRACT FROM AUTHOR]

The glaciers in the Nepalese Himalayas are retreating due to rising temperatures. Lack of data and information on Nepal’s cryosphere has impeded scientific studies and field investigations in the Nepalese Himalayas. Therefore, IRD France and Ev-K2 CNR Italy have conducted the PAPRIKA (CryosPheric responses to Anthropogenic PRessures in the HIndu Kush-Himalaya regions: impacts on water resources and society adaptation in Nepal) project in Nepal with the financial support of the French and Italian scientific agencies. This project aims to address the current and future evolution of the cryosphere in response to overall environmental changes in South Asia, and its consequences for water resources in Nepal. Thus, two hydrological models, the GR4J lumped precipitation–runoff model and the snowmelt runoff model (SRM), were used in the Dudh Koshi basin. The GR4J model has been successfully applied in different parts of Europe. To obtain better results in such a harsh and rugged topography, modifications needed to be made, particularly in the snow module. The runoff pattern is analysed herein both for past years and, in a sensitivity analysis, for possible future climatic conditions (i.e. precipitation and temperature) using the SRM and GR4J modelling approaches. The results reveal a significant contribution of snow- and glacier-melt to runoff, and the SRM model shows better performance in Nepalese catchments than the GR4J model.Editor D. Koutsoyiannis; Associate editor D. GertenCitation Pokhrel, B.K., Chevallier, P., Andréassian, V., Tahir, A.A., Arnaud, Y., Neppel, L., Bajracharya, O.R., and Budhathoki, K.P., 2014. Comparison of two snowmelt modelling approaches in the Dudh Koshi basin (eastern Himalayas, Nepal). Hydrological Sciences Journal, 59 (8), 1507–1518. http://dx.doi.org/10.1080/02626667.2013.842282 [ABSTRACT FROM PUBLISHER]

Abstract: Morocco is a North African country highly vulnerable to extreme precipitation events. In the present study, past trends in extreme precipitation and future projections using an ensemble of regional climate models (RCM) are evaluated. The extreme precipitation distributions during the extended winter season (October to April) in 10 stations are fitted with Generalized Extreme Value models (GEV). The dependence of the GEV parameters with time, winter North Atlantic Oscillation (NAO) and Mediterranean Oscillation (MO) indexes have been tested. Results indicate no significant trends in extreme precipitation during the observation period 1961–2007. However, dependences between precipitation extremes and NAO or MO indexes are detected, in particular for the Atlantic stations. Then 15 RCM simulations provided by the ENSEMBLES European project ran with the A1B scenario are considered to provide future projections. The Cramér–von Mises (CM) statistic is introduced as a measure of adequacy between the observed extreme precipitation distributions at the different stations and the distributions simulated by the RCMs. The CM statistic can thus provide weights to build a multi-model ensemble of future projections based on model performance in the present climate. Even if some models exhibit good skills, there is a great variability in the RCM performances to reproduce the seasonal cycle and the extreme precipitation distributions at the different stations. The projected changes on extreme precipitation at the stations are evaluated with quantiles computed for different return periods, ranging from 2 to 40years, during the control period 1961–2007 and two projection periods, 2020–2050 and 2070–2099. The climate change scaling factors on extreme quantiles provided by the different RCMs are averaged with equal weights, or with weights obtained from the inverse of the CM statistic. The climate change signal in the RCM simulations indicate a decrease in extreme precipitation quantiles, −12% in average for the projection period 2070–2099 but a great variability and lower convergence between models is found for the projection period 2020–2050. Overall, there is a good model convergence towards a decrease for the Atlantic stations. For the Mediterranean stations, the projected changes are difficult to assess due to the great variability. The two weighting schemes tested for model outputs provide similar results. [Copyright &y& Elsevier]

Summary: A major proportion of flow in the Indus River is contributed by its snow and glacier-fed river catchments situated in the Karakoram Range. It is therefore essential to estimate the snowmelt runoff from these catchments (with no or scarce precipitation records) for water resources management. The snowmelt runoff model (SRM) integrated with MODIS remote-sensing snow cover products was selected to simulate the daily discharges and to study the climate change impact on these discharges in the Hunza River basin (the snow- and glacier-fed sub-catchment of the Indus River). The results obtained suggest that the SRM can be used efficiently in the snow- and glacier-fed sub-catchments of the Upper Indus River Basin (UIB). The application of the SRM under future climate (mean temperature, precipitation and snow cover) change scenarios indicates a doubling of summer runoff until the middle of this century. This analysis suggests that new reservoirs will be necessary for summer flow storage to meet with the needs of irrigation supply, increasing power generation demand, flood control and water supply. [Copyright &y& Elsevier]

The Cévennes–Vivarais Mediterranean Hydrometeorological Observatory (OHM-CV) is a research initiative aimed at improving the understanding and modeling of the Mediterranean intense rain events that frequently result in devastating flash floods in southern France. A primary objective is to bring together the skills of meteorologists and hydrologists, modelers and instrumentalists, researchers and practitioners, to cope with these rather unpredictable events. In line with previously published flash-flood monographs, the present paper aims at documenting the 8–9 September 2002 catastrophic event, which resulted in 24 casualties and an economic damage evaluated at 1.2 billion euros (i.e., about 1 billion U.S. dollars) in the Gard region, France. A description of the synoptic meteorological situation is first given and shows that no particular precursor indicated the imminence of such an extreme event. Then, radar and rain gauge analyses are used to assess the magnitude of the rain event, which was particularly remarkable for its spatial extent with rain amounts greater than 200 mm in 24 h over 5500 km2. The maximum values of 600–700 mm observed locally are among the highest daily records in the region. The preliminary results of the postevent hydrological investigation show that the hydrologic response of the upstream watersheds of the Gard and Vidourle Rivers is consistent with the marked space–time structure of the rain event. It is noteworthy that peak specific discharges were very high over most of the affected areas (5–10 m3 s-1 km-2) and reached locally extraordinary values of more than 20 m3 s-1 km-2. A preliminary analysis indicates contrasting hydrological behaviors that seem to be related to geomorphological factors, notably the influence of karst in part of the region. An overview of the ongoing meteorological and hydrological research projects devoted to this case study within the OHM-CV is finally presented. [ABSTRACT FROM AUTHOR]