Isabelle Braud

HYMEX , a 10-year Multidisciplinary Program on the mediterranean water cycle.

The Mediterranean countries are experiencing important challenges related to the water cycle, including water shortages and floods, extreme winds, and ice/snow storms, that impact critically the socioeconomic vitality in the area (causing damage to property, threatening lives, affecting the energy and transportation sectors, etc.). There are gaps in our understanding of the Mediterranean water cycle and its dynamics that include the variability of the Mediterranean Sea water budget and its feedback on the variability of the continental precipitation through air–sea interactions, the impact of precipitation variability on aquifer recharge, river discharge, and soil water content and vegetation characteristics specific to the Mediterranean basin and the mechanisms that control the location and intensity of heavy precipitating systems that often produce floods. The Hydrological Cycle in Mediterranean Experiment (HyMeX) program is a 10-yr concerted experimental effort at the international level that aims to advance the scientific knowledge of the water cycle variability in all compartments (land, sea, and atmosphere) and at various time and spatial scales. It also aims to improve the processes-based models needed for forecasting hydrometeorological extremes and the models of the regional climate system for predicting regional climate variability and evolution. Finally, it aims to assess the social and economic vulnerability to hydrometeorological natural hazards in the Mediterranean and the adaptation capacity of the territories and populations therein to provide support to policy makers to cope with water-related problems under the influence of climate change, by linking scientific outcomes with related policy requirements.

A simple soil-plant-atmosphere transfer model (SiSPAT) development and field verification

Abstract When examining the various soil-plant-atmosphere models proposed in the literature, it becomes obvious that, according to the speciality of their authors, one or several compartments of the model are generally very detailed, whereas the other compartments remain crude. The aim of this work was first, to build a model, including the main physical processes, but with equivalent degrees of simplification for all the compartments and, second, to provide a validation as complete as possible for the various compartments. The resulting model, driven by meteorological forcing at a reference level (incoming solar and long-wave radiation, air temperature, humidity and wind speed, and rainfall), can be divided into four main compartments. In the soil, coupled heat and mass transfer equations, including liquid and vapour phase transfer, are solved. In the atmosphere, stability is taken into account in the calculation of the aerodynamic resistances. At the soil-plant-atmosphere interface, one vegetation layer is considered, with two energy budgets: one for the bare soil fraction of the plot and one for the vegetated fraction. In the soil, root uptake is modelled using an electrical analogue scheme with various resistances (soil, root, xylem). Finally, in the case of rainfall (or irrigation), interception, infiltration and runoff is calculated. The model is first described and then compared with field data collected on a soybean plot of 0.72 ha. The soil is composed of three horizons, the hydraulic and thermal properties of which were determined experimentally. The atmospheric forcing and the net radiation were measured. The sensible heat flux above the canopy was deduced from wind speed and temperature profiles. In the soil, water pressure, water content and temperature were measured at several depths. Temperature profiles also allowed for the derivation of the soil heat flux at the ground surface and the latent heat flux was obtained from the energy budget. Plant height, leaf area index and leaf water potential were also recorded on several days. Seven days of complete measurements were available: 2 days were under dry conditions (19–20 August 1991) and 5 days under wet conditions (24–28 August 1991) following a rainfall of 46 mm on 22 August 1991. Missing parameters were calibrated using the first 3 days of the wet period (24–26 August 1991) and the model was validated on the remaining days. A fair agreement between the model and the data was observed for both atmospheric fluxes, for soil variables (water content and temperature) and for leaf water potential, provided only an accurate determination of the parameters was made.

HyMeX-SOP1: The Field Campaign Dedicated to Heavy Precipitation and Flash Flooding in the Northwestern Mediterranean

The Mediterranean region is frequently affected by heavy precipitation events associated with flash floods, landslides, and mudslides that cause hundreds of millions of euros in damages per year and often, casualties. A major field campaign was devoted to heavy precipitation and flash floods from 5 September to 6 November 2012 within the framework of the 10-year international HyMeX (Hydrological cycle in the Mediterranean Experiment) dedicated to the hydrological cycle and related high-impact events. The 2- month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy, and Spain. The observation strategy of the field experiment was devised to improve our knowledge on the following key components leading to heavy precipitation and flash flooding in the region: i) the marine atmospheric flows that transport moist and conditionally unstable air towards the coasts; ii) the Mediterranean Sea acting as a moisture and energy source; iii) the dynamics and microphysics of the convective systems producing heavy precipitation; iv) the hydrological processes during flash floods. This article provides the rationale for developing this first HyMeX field experiment and an overview of its design and execution. Highlights of some Intense Observation Periods illustrate the potential of the unique datasets collected for process understanding, model improvement and data assimilation.

A simple water and energy balance model designed for regionalization and remote sensing data utilization

A simple soil‐vegetation‐atmosphere transfer (SVAT) model designed for scaling applications and remote sensing utilization will be presented. The study is part of the Semi-Arid Land Surface Atmosphere (SALSA) program. The model is built with a single-bucket and single-source representation with a bulk surface of mixed vegetation and soil cover and a single soil reservoir. Classical atmospheric forcing is imposed at a reference level. It uses the concept of infiltration and evaporation capacities to describe water infiltration or exfiltration from a bucket of depth dr corresponding to the average infiltration and evaporation depth. The atmospheric forcing is divided into storm and interstorm periods, and both evaporation and infiltration phenomena are described with the well-known three stages representation: one at potential (energy- or rainfall-limited) rate, one at a rate set by the soil water content and one at a zero rate if the water content reaches one of its range limits, namely saturation or residual values. The analytical simplicity of the model is suitable for the investigation of the spatial variability of the mass and energy water balance, and its one-layer representation allows for the direct use of remote sensing data. The model is satisfactorily evaluated using data acquired in the framework of SALSA and a mechanistic complex SVAT model, Simple Soil-Plant-Atmosphere Transfer (SiSPAT) model.

Unidimensional modelling of a fallow savannah during the HAPEX-Sahel experiment using the SiSPAT model

Abstract In the framework of the HAPEX-Sahel experiment, a data set was gathered on a fallow savannah site of the Central East Supersite. This includes 54 days of atmospheric forcing (air temperature and humidity, wind speed, solar and long-wave radiation and rainfall), net radiation, sensible, latent and soil heat fluxes and soil temperature series at a time step of 20 min. Furthermore, 17 soil moisture profiles, the evolution of the leaf area indices and some soil characteristics were available. The data set was used, at the field scale, to calibrate and validate the SiSPAT (simple soil plant atmosphere transfer) model, a 1D model of coupled heat and mass transfer in the soil-plant-atmosphere continuum. The objectives of the study were (i) to assess the performances of the model in the prediction of the diurnal cycle of net radiation, turbulent fluxes, soil temperatures and the evolution of soil water content over a period of 54 days (day of the year 239–292, 1992), characterized by early stage intense rainfall events and fast drying afterwards, (ii) to analyse the influence of soil surface crust on the water balance and (iii) to identify the 1D modelling limits when the surface area consists of two strates: a ground sparse herb layer, characterized by a large spatial variability of surface properties and water content with scattered bushes. The model was calibrated over a 2-week period and then run over the whole 54-day period. We were able to reproduce the main characteristics of the observed net radiation, turbulent fluxes, soil temperature and soil moisture for the intense rainfall events and for an elongated dry period. Nevertheless, when the crust was not taken into account, the rainfall-runoff-infiltration process and the evapotranspiration after rain were poorly predicted (overestimation of evapotranspiration of infiltration). When a crust was considered to model the water balance at the field scale, its influence was found to be substantial on the runoff generation and the infiltration, and consequently on the bare soil evaporation. However, runoff predictions were much larger than the observations. Indeed, at the field scale, no runoff was generally observed. Lateral redistribution of water between crusted and noncrusted zones was observed in the plot. However, this cannot be taken into account with the presented 1D deterministic modelling. Hence further model development is needed to yield a better representation of soil water fluxes at the field scale.

A stochastic approach to studying the influence of the spatial variability of soil hydraulic properties on surface fluxes, temperature and humidity

The assessment of mass and energy fluxes at the soil-biosphere-atmosphere interface is a key point for the improvement and reliability of climate model predictions. At the local scale, numerous models have been developed to predict surface fluxes. These models are generally deterministic and the extension of their results to larger scales, such as the catchment scale or the scale of an atmospheric or climatic model mesh, is greatly complicated by the large variability of surface properties. To address this issue, stochastic approaches have been proposed to enable prediction of fluxes in terms of probability density functions. This paper is concerned with the influence of the spatial variability of soil hydraulic properties. This variability is expressed through one parameter, the scale factor for which a log-normal distribution is assumed. Values of the scale factor, drawn from this distribution, are introduced into a unidimensional model, called SiSPAT, which describes the soil-plant-atmosphere coupled heat and water transfers. Two land uses are considered: a bare soil and a rather densely vegetated one for which the same forcing is applied over 7 days. Comparison of the two cases shows that the vegetation tends to smooth the influence of the spatial variability of soil properties limiting the number of observations required to estimate a spatial mean with a prescribed degree of accuracy. Comparison of the deterministic and stochastic solutions also shows that the former approach leads to a bias for the bare soil case, the differences between the two being almost negligible in the presence of vegetation.

An assessment of effective land surface parameterisation in regional-scale water balance studies

Abstract Numerical experiments have been carried out with a soil–vegetation–atmosphere transfer (SVAT) model to study the impact of spatial variability in soil and land surface parameters on regional-scale water balance components. A statistical-dynamical approach has been used to account for the spatial variability of the selected parameters and to determine the seasonal evolution of the impact on the water budget. Point data are used to derive probability-density functions for the thickness of the permeable top soil layer, coefficients for the water retention and hydraulic conductivity curves, the leaf area index and the minimum stomatal resistance. These distributions are incorporated in the model’s mathematical framework in order to generate univariate distributions (sensitivity patterns) for evaporation, transpiration, total evaporation and runoff. The means of these univariate distributions of outputs yield catchment-scale averages. Next, the study obtains catchment-scale evaporation estimates by running simulations with aggregated parameters obtained as statistical descriptors of parameter distributions. The difference between the catchment-scale averages and values obtained with aggregated parameters describes the non-linear response of the model to spatial variability of the particular parameter. Finally, the study also investigates several effective parameters based on recently described hydrometeorological aggregation rules. Results show significant differences in sensitivity patterns between individual parameters and between seasons.

Towards multi-scale integrated hydrological models using the LIQUID ® framework. Overview of the concepts and first application examples

Distributed hydrological models are valuable tools that can be used to support water management in catchments. However, the complexity of management issues, the variety of modelling objectives, and the variable availability of data require a flexible way to customize models and adapt them to each individual problem. Environmental modelling frameworks offer such flexibility; they are designed to build and run integrated models on the basis of reusable and exchangeable components. This paper presents the LIQUID^(R) framework, developed by Hydrowide since 2005. The purpose of developing LIQUID^(R) was to provide both easier integration of hydrological processes and preservation of their characteristic temporal and spatial scales. It suits a wide range of applications, both in terms of spatial scales and of process conceptualisations. LIQUID^(R) is able to synchronize different time steps, to handle irregular geometries, and to simulate complex connections between components, in particular involving feedback. The paper presents the concepts of LIQUID^(R) and the technical choices made to meet the above requirements, with focuses on the simulation run system and on the spatial discretization of process components. The use of the framework is illustrated by five application cases associated with contrasted spatial and temporal scales.

Spatial variability of surface properties and estimation of surface fluxes of a savannah

Abstract During the HAPEX-Sahel experiment in 1992, a data set including atmospheric forcing, soil temperature at several depths, surface fluxes and surface soil moisture from 0 to 15 cm was collected on a degraded fallow savannah over a period of 18 days. The SiSPAT SVAT model was calibrated using this data set to provide a realistic reference set of parameters. The sensitivity of surface fluxes to the specification of surface properties based on this reference set of parameters was quantified focusing on runoff, evapotranspiration and soil moisture at the field scale. Runoff and latent heat flux, predominantly bare soil evaporation, were found to be the most sensitive processes in relation to soil parameters. For transpiration, even with such a sparse vegetation, leaf area index was the most sensitive factor. A stochastic approach was used to analyze the sensitivity of surface fluxes to the spatial variability of surface parameters. For a variation of ±50% of the parameters, no significant bias was obtained between the mean of the stochastic simulations and the 1-D simulation performed with the median values of the parameters. The analysis indicates that the variations of the components of the water budget are linearly related. For larger variations of the parameters, the bias is significant; therefore, simple aggregation rules fail to capture the nonlinearities induced by water transfer into the soil. When diurnal cycles are considered, the standard deviation of bare soil evaporation and surface soil moisture was found to be maximum for the intermediate wetting range. For this period, it would be valuable to parameterize the spatial variability of surface properties into larger scale models. Finally, the SiSPAT model, which solves equations derived from the Richards equation [Richards, L.A., 1931. Capillary conduction of liquids through porous mediums. J. Phys. 1, pp. 318–333.]. for soil water distribution is shown to be very sensitive to the specification of soil parameters. This result would hold for similar models, showing that the Richards equation should be used with caution within large-scale models if a robust estimation of the long- term water budget is to be obtained.

Heat and water exchanges of fallow land covered with a plant-residue mulch layer: a modelling study using the three year MUREX data set

Abstract The MUREX (Monitoring the Usable Soil Reservoir Experimentally) experiment was conducted on fallow land in the Southwest of France. A three year continuous data set, including climatic variables, energy fluxes, surface water content, soil moisture profiles, surface and soil temperature, and evolution of vegetation characteristics was collected. The field possessed a plant-residue mulch layer, formed naturally by the accumulation of decaying and dead biomass. The three-year data set was used to analyse and model the long-term water and heat exchanges of the field using the SiSPAT (Simple Soil Plant Atmosphere Transfer) model. The original version was modified to take into account heat and water transfer within the plant-residue mulch layer. The 1995 data set was used for calibration of unmeasured parameters. Years 1996 and 1997 were used for validation of the approach, using the same parameter set obtained in 1995. Model results and observations were in good agreement for the three years when the plant-residue (mulch) layer effect was considered. The model properly reproduced contrasting responses to different rainfall conditions. Model simulations were used to understand some physical processes modified by the mulch layer. A decrease of 5–10% of annual total evaporation was obtained, as compared to the residue free case, associated with a decrease in soil evaporation and increase of transpiration. The decrease in soil evaporation was responsible for higher surface soil moisture. Daily soil and air temperature profiles were shown to be considerably modified by the mulch layer, an inversion occurring within the mulch, leading to colder averages and a smaller amplitude for soil temperature.