A hydrological view to the Ounianga lake region (NE Chad) evolution during the recent Holocene period is proposed. It relies upon multidisciplinary inputs combining hydrological characterization and modeling, results from climate modeling and from the recent sedimentary core analysis by Kroepelin et al. 2008 from lake Yoa. The latter shows that 4 major phases in drying have been observed in this Saharan region over the Holocene, including, (1) disappearance of tropical and altitude pollen species by 4300 cal yr BP, (2) evolution of the lake from a freshwater habitat to the present day hyper-saline oasis (around 3900 cal yr BP), (3) establishment of today’s terrestrial desert ecosystem as the result of continuous vegetation succession (5600 to 2700 cal yr BP), (4) evolution in regional wind regime with onsets of major dust mobilization at roughly 4300 cal yr BP and establishment of modern, near continuous northeasterly winds by 2700 cal yr BP.

The rate at which land surface soils dry following rain events is an important feature of terrestrial models. It determines, for example, the water availability for vegetation, the occurrences of droughts, and the surface heat exchanges. As such, surface soil moisture (SSM) “drydowns,” i.e., the SSM temporal dynamics following a significant rainfall event, are of particular interest when evaluating and calibrating land surface models (LSMs). By investigating drydowns, characterized by an exponential decay time scale τ, we aim to improve the representation of SSM in the ORCHIDEE global LSM. We consider τ calculated over 18 International Soil Moisture Network sites found within the footprint of FLUXNET towers, covering different vegetation types and climates. Using the ORCHIDEE LSM, we compare τ from the modeled SSM time series to values computed from in situ SSM measurements. We then assess the potential of using τ observations to constrain some water, carbon, and energy parameters of ORCHIDEE, selected using a sensitivity analysis, through a standard Bayesian optimization procedure. The impact of the SSM optimization is evaluated using FLUXNET evapotranspiration and gross primary production (GPP) data. We find that the relative drydowns of SSM can be well calibrated using observation-based τ estimates, when there is no need to match the absolute observed and modeled SSM values. When evaluated using independent data, τ-calibration parameters were able to improve drydowns for 73% of the sites. Furthermore, the fit of the model to independent fluxes was only minutely changed. We conclude by considering the potential of global satellite products to scale up the experiment to a global-scale optimization.

Over the last decade, many researchers around the world have realized the importance for society of the many services provided by soils, and a significant body of research has been devoted to estimating them using various types of proxies or relying on models. However, the field has suffered so far from a complete lack of actual measurements with which to evaluate available estimation methods. In this context, the key objective of the present research was to obtain, for the first time, direct measurements of several services provided by soils. The experimental site at which the research was carried out is an agricultural field located southwest of Paris (France), which presents the unique advantage that is it artificially drained, and therefore allows accurate mass balances to be computed over time for rain water, nutrients, and herbicides applied to the soil. A detailed methodology is presented to extract, from these data, quantitative measurements of 3 provisioning services (supply of water to nearby stream, provision of food by supplying, respectively, water and nitrogen to wheat crop), and 3 regulating services (flood mitigation, and filtration of, respectively, nutrients and herbicides). The results obtained with this methodology, and the relative significance of the different services are discussed. In particular, it is shown that, given the industrial-type of agriculture practiced at the site and its heavy reliance on fertilizers, the service related to the supply of nitrogen to the crops by the soil is not marginal, accounting for 22% of the total nitrogen consumption by the wheat. The availability of quantitative measures of selected soil services now paves the way for a detailed assessment of the various estimation methods currently in use, and the development of improved approaches.

Soil moisture plays a key role in water, carbon and energy exchanges between the land surface and the atmosphere. Therefore, a better representation of this variable in the Land-Surface Models (LSMs) used in climate modelling could significantly reduce the uncertainties associated with future climate predictions. In this study, the ESA-CCI soil moisture (SM) combined product (v4.2) has been confronted to the simulated top-first layers/cms of the ORCHIDEE LSM (the continental part of the IPSL Earth System Model) for the years 2008-2016, to evaluate its potential to improve the model using data assimilation techniques. The ESA-CCI data are first rescaled to match the climatology of the model and the signal representative depth is selected. Results are found to be relatively consistent over the first 20 cm of the model. Strong correlations found between the model and the ESA-CCI product show that ORCHIDEE can adequately reproduce the observed SM dynamics. As well as considering two different atmospheric forcings to drive the model, we consider two different model parameterizations related to the soil resistance to evaporation. The correlation metric is shown to be more sensitive to the choice of meteorological forcing than to the choice of model parameterization. Therefore, the metric is not optimal in highlighting structural deficiencies in the model. In contrast, the temporal autocorrelation metric is shown to be more sensitive to this model parameterization, making the metric a potential candidate for future data assimilation experiments.

Facing environmental issues, such as excess of nitrogen in the biosphere, water eutrophication, carbon emission in the atmosphere or hydrological impacts, peri-urban areas are not receiving as much attention as urban areas. In this research, we focus on a French peri-urban area in the Parisian region, the Saclay plateau. Following the territorial ecology approach, we consider this territory as a system organizing agricultural, food and waste flows, and we have analyzed the metabolism of this territory through its N, C, P and water flows. The results show first that the metabolism of the Saclay plateau is largely open, i.e. the main flows are input and output flows, and internal flows are small or nonexistent This is particularly the case for nitrogen and phosphorus flows. The same findings hold for carbon flows although primary production and respiration play the major role in the carbon metabolism. Besides the uncertainty of the available data, water flows also reveal a clear uncoupling between compartments of the system. The analysis of these material flow budgets reveals the high potential for reconnections of the different flows, mainly due to the presence of large areas of agricultural land and a large population. Implementation of biowaste recovery, recovery of human excreta at source, as well as rain recovery systems, could help to realize this potential provided that environmental and sanitary risks can be effectively managed. Developing a foresight approach based on scenarios of territorial metabolism can assist in the building process of a territorial project in periurban areas.

The SECHIBA module of the ORCHIDEE land surface model describes the exchanges of water and energy between the surface and the atmosphere. In the present paper, the adjoint semi-generator software called YAO was used as a framework to implement a 4D-VAR assimilation scheme of observations in SECHIBA. The objective was to deliver the adjoint model of SECHIBA (SECHIBA-YAO) obtained with YAO to provide an opportunity for scientists and end users to perform their own assimilation. SECHIBA-YAO allows the control of the 11 most influential internal parameters of the soil water content, by observing the land surface temperature or remote sensing data such as the brightness temperature. The paper presents the fundamental principles of the 4D-VAR assimilation, the semi-generator software YAO and a large number of experiments showing the accuracy of the adjoint code in different conditions (sites, PFTs, seasons). In addition, a distributed version is available in the case for which only the land surface temperature is observed.

Cette communication presente le processus de construction de deux guides, sur la caracterisation des echanges entre les cours d’eau et les nappes en relation, s’appuyant sur des travaux scientifiques menes avec des commanditaires gestionnaires de l’eau : 1) la Zone Atelier Bassin du Rhone (ZABR) et l’Agence de l’eau Rhone-Mediterranee-Corse (AERMC), 2) l’Office de l’Eau et des Milieux Aquatiques (ONEMA). Ces travaux visaient a mettre au point et a perfectionner une methodologie interdisciplinaire de caracterisation des echanges eaux souterraines/cours d’eau. Les methodes mises en œuvre relevent de l’hydrologie, de la geomatique, de la modelisation, de l’ecologie et de la geochimie. Ces travaux vont s’achever avec la publication de deux guides co-construits avec l’aide d’un panel de futurs utilisateurs : 1) dans un premier temps un guide methodologique faisant le point sur les connaissances et exposant de maniere detaillee la demarche en milieu alluvionnaire, 2) dans un second temps un guide technique, plus concis mais couvrant plus de situations.

Low-flow simulation and forecasting remains a difficult issue for hydrological modellers, and intercomparisons can be extremely instructive for assessing existing low-flow prediction models and for developing more efficient operational tools. This research presents the results of a collaborative experiment conducted to compare low-flow simulation and forecasting models on 21 unregulated catchments in France. Five hydrological models (four lumped storage-type models - Gardenia, GR6J, Mordor and Presages - and one distributed physically oriented model - SIM) were applied within a common evaluation framework and assessed using a common set of criteria. Two simple benchmarks describing the average streamflow variability were used to set minimum levels of acceptability for model performance in simulation and forecasting modes. Results showed that, in simulation as well as in forecasting modes, all hydrological models performed almost systematically better than the benchmarks. Although no single model outperformed all the others for all catchments and criteria, a few models appeared to be more satisfactory than the others on average. In simulation mode, all attempts to relate model efficiency to catchment or streamflow characteristics remained inconclusive. In forecasting mode, we defined maximum useful forecasting lead times beyond which the model does not bring useful information compared to the benchmark. This maximum useful lead time logically varies between catchments, but also depends on the model used. Simple multi-model approaches that combine the outputs of the five hydrological models were tested to improve simulation and forecasting efficiency. We found that the multi-model approach was more robust and could provide better performance than individual models on average.

This study evaluates the impact of the spatial density of the flow gauging network on hydrological computations in an ungauged catchment context. The considered computations are simulation of daily streamflow, estimation of long-term average streamflow and extreme streamflows (10-yr and 100-yr floods and some characteristics on low flows). In this context, we used the lumped rainfall-runoff model GR4J. The regionalization applied here, is based on the spatial proximity of catchments. The aim of the study is to evaluate the impact of the distance at which the first donor stations are located, on the efficiency of the hydrological computations. This evaluation shows that for all the computations of streamflow considered here, the efficiency of the regionalization process decreases when the flow gauging network density is reduced (i.e. when we "remove" the closest catchments). Results also show that this drop in efficiency similarly impacts all the hydrological variables analyzed here.

Within the Explore 2070 project, an evaluation of the possible impacts of climate change on surface water between the 1961-1990 reference period and the 2046-2065 period was carried out in continental France and in overseas departments on the basis of the A1B greenhouse gas emission scenario, seven general circulation models and two hydrological models (Isba-Modcou and GR4J). In continental France, results indicate: (1) a possible increase in air temperature between +1.4 degrees C and +3 degrees C; (2) an uncertain evolution of precipitation, most models however agreeing on a decreasing trend in summer precipitation; (3) a significant decrease (10% to 40%) of mean annual flows at the country scale, especially pronounced in the Seine-Normandie and Adour-Garonne districts; (4) a strong decrease in summer low flows in most basins; (5) more heterogeneous and less significant evolutions for floods. A special care was given to the quantification of the uncertainties linked to these results. They provide an indication of the significance of projected changes. The evolutions calculated in the overseas zones can be considered non-significant given the level of uncertainty linked to the hydro-climatic modelling chain. These results urge to implement adaptation strategies based on a better management of water resources, among others.

Cet article etudie la variabilite au cours des 50 dernieres annees de la nappe des sables quaternaires (NSQ) dans la region des Niayes au Senegal, afin de deceler les liens possibles de cette variabilite avec le climat mais aussi les activites anthropiques. L'analyse interannuelle de la morphologie de la nappe a partir des cartes piezometriques multidates montre que les plus fortes baisses piezometriques ont ete enregistrees au cours de la secheresse des decennies 1970 et 1980, avec une diminution progressive moyenne de pres de 0,51 m tous les 10 ans. A l'echelle saisonniere, les variations piezometriques sont plus prononcees a la fin de la saison pluvieuse. L'analyse du bilan hydrologique moyen pour l'annee 1974 suggere que les causes naturelles constituent les principaux facteurs de destockage de la nappe avec plus de 96 % des sorties d'eau, dont 90 % par evapotranspiration. Les prelevements anthropiques ne depassent guere 4 % des sorties. Toutefois, l'evolution de la NSQ est aussi perturbee par les activites anthropiques, via notamment les amenagements hydrauliques (barrages), dans le secteur nord de la region des Niayes ou le niveau de la nappe n'a baisse que de 0,11 mm entre 1984 et 1994 en depit de la persistance des deficits pluviometriques des annees 1980 et 1990.