Modélisation pluie-débit et classification hydroclimatique

La recherche de similarité hydrologique est très importante pour l’estimation des débits aux bassins non jaugés. L’indice radiatif d’aridité, proposé dans le modèle de bilan hydrique de Budyko, en combinaison avec le bilan radiatif, représente un paramètre de contrôle de l’ETR (évapotranspiration réelle). Cet indice permet de définir des régions climatiques ou géobotaniques dans lesquelles s’inscrivent les modèles pluie-débit ajustés d’après des historiques d’observations hydroclimatiques. Le présent travail utilise le modèle HBV muni d’une routine d’optimisation à l’aide de l’algorithme SCE‑UA. Il propose une méthodologie de calage dans laquelle on tient explicitement compte de l’ETR établie à grande échelle, à partir de l’indice d’aridité. Cette méthode de calage adopte comme fonction objective la combinaison de trois critères : minimisation de l’écart quadratique sur les débits, minimisation de l’écart sur le bilan hydrique, minimisation de l’écart à l’ETR régionale. On montre qu’ainsi, on améliore la performance du modèle en période de validation.The research of hydrological similarity is very important for runoff estimation with respect to ungauged basins. The Budyko radiative dryness index may represent a control parameter for the estimation of actual evapotranspiration ETR, as output of rainfall-ruonff models. These models are generally adjusted according to hydro – climatic observations, whithout taking account for energy balance insights. Budyko index helps defining climatic or geobotanic regions, in which rainfall-runoff models may be enrolled. To develop these ideas, the HBV rainfall-runoff model is adopted, coupled to a SCE-UA optimisation tool. It is proposed to perform the model adjustment taking explicitly account for ETR regional estimation, as a constraint. The calibration method adopts an objective function combining three criteria: minimization of runoff root mean square error, minimization of water budget simulation error, minimization of the difference between mean annual simulated ETR and regional ETR. It is found that, by this way, model performances are enhanced, especially for the validation period

Trends in flow intermittence for European rivers

Intermittent rivers are prevalent in many countries across Europe, but little is known about the temporal evolution of intermittence and its relationship with climate variability. Trend analysis of the annual and seasonal number of zero-flow days, the maximum duration of dry spells and the mean date of the zero-flow events is performed on a database of 452 rivers with varying degrees of intermittence between 1970 and 2010. The relationships between flow intermittence and climate are investigated using the standardized precipitation evapotranspiration index (SPEI) and climate indices describing large-scale atmospheric circulation. The results indicate a strong spatial variability of the seasonal patterns of intermittence and the annual and seasonal number of zero-flow days, highlighting the controls exerted by local catchment properties. Most of the detected trends indicate an increasing number of zero-flow days, which also tend to occur earlier in the year, particularly in southern Europe. The SPEI is found to be strongly related to the annual and seasonal zero-flow day occurrence in more than half of the stations for different accumulation times between 12 and 24 months. Conversely, there is a weaker dependence of river intermittence with large-scale circulation indices. Overall, these results suggest increased water stress in intermittent rivers that may affect their biota and biochemistry and also reduce available water resources

Hydrological impacts of climate change in northern Tunisia

Tunisia is a water-stressed country, which derives most of its surface water from its northern regions. Given Northern Tunisia’s role as a water provider, this study investigated the hydrological impacts of climate change on five catchments located in this region. Three hydrological models are considered: HBV, GR4, andIHACRES. Climate projections were derived from eleven high-resolution EURO-CORDEX regional climate models (forced by general circulation models; GCM-RCMs). A quantile mapping (QM) bias correction method was applied to correct the climate simulations. Historical streamflow simulations (1970–2000), achieved by forcing the hydrological models with GCM-RCM precipitation and temperature, were first assessed in order to select the most realistic GCM-RCMs for future projections. The remaining bias corrected GCM-RCMs were then used to force the hydrological models in order to achieve projections of streamflow. The evaluation of the stream-flow projections was conducted over two time periods(i) mid-term: 2040–2070 and (ii) long-term: 2070–2100 to identify the magnitude of the projected change of streamflow under the climate scenarios RCP 4.5 and RCP8.5. The hydrological projections were analyzed according to several metrics commonly used by water managers

A general differential split-sample test to select sub-periods of discontinuous years gathering similar to different climate conditions

International audienceThis article introduces a Matlab© code to implement the General Differential Split Sample Test (GDSST) (Dakhlaoui et al. [5]). As an illustration, the GDSST is applied to five catchments in northern Tunisia over 30-year reference period and compared to three benchmark Split Sample Test (SST) methods. The techniques are compared as regards to the number of validation exercises and to the differences in temperature (ΔT) and precipitation (ΔP) between the sampled sub-periods, whose length was set to 8-year. The GDSST allows a larger number of discontinuous periods to be sampled, and is computationally more effective than the basic bootstrap to identify the most climatically contrasting conditions. In addition, the GDSST offers a larger continuum of climatic conditions and a better spread of validation periods than the benchmark techniques, which is essential to test the parameter transferability of hydrological models. As supplementary material, a package file containing MATLAB© scripts to run the three benchmark SSTs and the proposed GDSST, as well as an application example on the five catchments, can be freely downloaded.•An enhanced split-sample test based on an oriented bootstrap to assess transferability of hydrological models.•The proposed split-sample test is computationally more effective than the basic bootstrap to identify the most climatically contrasting conditions.•MATLAB© code of the proposed GDSST and four benchmark SST, with application example

A bootstrap-based differential split-sample test to assess the transferability of conceptual rainfall-runoff models under past and future climate variability

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Robustness of conceptual rainfall-runoff models to high-resolution climate projections in northern Tunisia

International audienceRobust hydrological models with temporal transposable parameters are needed to evaluate the impact of climate change on water resources at the catchment scale. This study thus aims to assess the transferability of three conceptual rainfall-runoff models (GR4j, HBV and IHACRES) under contrasted climate conditions in view of hydrological scenarios based on high-resolution climate projections in Northern Tunisia. For this purpose we developed an original General Split Simple Test (GSST) based on an oriented bootstrap allowing for more contrasted sub-periods to be sampled. When applied to five catchments in northern Tunisia, the enhanced GSST provided clearer limits of models' transferability under changing precipitation (P) and temperature (T) conditions in comparison to other techniques. The transferability limits call for selecting a past sub-period as close as possible to future climate to identify calibration parameters that can be used for hydrological projections. These limits of transferability were then confronted to the climate projections from nine high-resolution Eurocordex and Medcordex Regional Climate Models (RCMs) over the studied catchments. The RCMs precipitation and temperature simulations over the historical 1970‒2000 period were first assessed in order to select the more realistic ones for future projections. Given the insufficient realism of mean seasonal and annual precipitation from the RCMs, a delta-change monthly correction was used to perturb the observed climate series according to climate simulations under two Radiative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) for two horizons (medium 2040‒2070 and long-term 2070‒2100 horizons).The effects of the selected past calibration period on the hydrological projections were analysed. We found that models calibrated on the whole observed period lead to underestimate the climate change impacts on runoff by 5 to 20% in comparison to their calibration on sub-periods with mean annual P and T closer to future climate conditions

Evaluating the robustness of conceptual rainfall-runoff models under climate variability in northern Tunisia

International audienceClimate change is likely to have significant impacts on runoff in the Southern rim of the Mediterranean basin that already suffers from scarcity of water resources. To investigate how hydro-climatic conditions could evolve in this region, not only future projections of climate are necessary but also robust rainfall-runoff models that are able to be fairly reliable under changing climate conditions. This study thus aims at assessing the robustness of two conceptual rainfall-runoff models (GR4j and IHACRES) under long-term climate variability on eight basins covering the main hydrographic characteristics in Northern Tunisia (High Medjerda, Zouaraâ, Ichkeul and Cap bon). The catchment areas are between 81 km² and 418 km². The streamflow regime of the basins can be considered as natural since these basins are located upstream from storage-dams and withdrawals. A 30-year common period (1970-2000) was considered to capture a large spread of hydrological conditions. The models were calibrated and validated using various goodness-of-fit criterions: NSE, KGE and Bias. Their efficiency was evaluated according to a 10-day time step using three differential split sample tests that aimed at assessing the models under various precipitation and PE conditions. Results showed that the two models were able to correctly reproduce runoff from the basins with a good agreement of the hydrograph shapes, of the high and low flows as well as the water volumes. However, the models prove a limited transferability of their parameters under contrasted climate conditions in almost all basins. Consequently, many parameter sets will have to be considered to represent the hydrological uncertainties associated in the assessment of possible climate change impact on runoff in the basins

Sub-chapter 2.3.1. Hydrological impacts of climate change in North African countries

Introduction Countries in North Africa with a semi-arid Mediterranean climate are facing water scarcity and high inter-annual variability of their water resources. Many dams and reservoirs have been built in recent decades to collect surface water and improve the management of existing water resources (Figure 1). However climate projections for this region indicate a possible decrease in precipitation together with an increase in temperature that could result in increased evaporation (Schilli..