Évapotranspiration | EPFL Graph Search

vignette|Représentation schématique du bilan évapotranspiration/alimentation de la nappe/ruissellement. À l'échelle mondiale, l'évapotranspiration potentielle ETp provient de différents flux d'eau retournant vers l'atmosphère sous forme de vapeur : transpiration (64 % de l'ETp), interception évaporée (27 %), évaporation des sols (6 %) et des étendues d'eau à l'intérieur des terres (3 %). vignette|Pluviométrie mondiale mensuelle. Dans les forêts tropicales humides, 90 % de la vapeur d'eau atmosphérique est générée de l'évapotranspiration, cette dernière contribuant de manière importante aux précipitations forestières (d'où le concept de bassin de précipitations, équivalent de bassin versant aérien), et à la redistribution de flux de vapeur souvent comparés à une rivière volante (rivière aérienne exportant ces flux au-delà du bassin). L'évapotranspiration (abréviation scientifique ET) est le processus biophysique de transfert d'une quantité d'eau vers l'atmosphère, par l'évaporation au n

Développement de méthodes de modélisation des flux turbulents dans un environnement alpin en utilisant des réseaux de stations micrométéorologiques

Romain Mage

Les caractéristiques spatiales et temporelles générales associées aux circulations atmosphériques induites par des effets thermiques locaux des terrains montagneux sont bien étudiées et le comportement général des systèmes de vents diurnes de montagne (vents catabatiques et anabatiques de vallée et de pente) est globalement bien compris. Toutefois, les détails liés à l'impact de l'hétérogénéité spatiale ne sont pas aussi bien compris et étudiés, en particulier l’effet de l’hétérogénéité spatiale et temporelle des circulations associé à un relief complexe sur les flux de surface à une échelle local. Dans ce travail sont présentés des résultats issus d’une campagne hydrologique menée durant l’été 2009 dans le Val Ferret, près du col du Grand Saint-Bernard dans les Alpes suisses sont présentés. Les données issues d’un réseau de 12 stations micrométéorologiques sans fil (stations Sensorscope) ainsi que d’une station de mesure de corrélation turbulente sont présentées, incluant des mesures des flux turbulents de chaleur sensible et de chaleur latente. Sur la base des résultats, deux modèles de flux turbulents à la surface ont été développés puis appliqués aux stations Sensorscope. L’objectif est de pouvoir fournir de bonnes estimations des flux turbulents de chaleur sensible et latente ainsi que de l’évapotranspiration dans un environnement alpin en utilisant uniquement des mesures simples issues de réseaux de stations micrométéorologiques sans fil, faciles à déployer dans des terrains montagneux et permettant un suivi des mesures. Cela a été fait à travers une extension du modèle de Penman-Monteith. La comparaison avec les mesures de corrélation turbulente a indiqué un haut niveau de corrélation avec le nouveau modèle (0.97 pour la chaleur latente et 0.95 pour la chaleur sensible).

2010

BIOPHYSICAL CONTROLS ON EVAPOTRANSPIRATION: NUMERICAL MODELS AND EXPERIMENTAL OBSERVATIONS

Federico Croci

Water and food are basics of human life, unfortunately climate changes and water pollution have reduced the amount of water available for human uses. In this context the study of the hydrologic cycle is becoming of growing importance in order to optimize the anthropogenic exploitation of water resource. Evapotranspiration is the term used to describe the part of the water cycle which removes liquid water from an area with vegetation into the atmosphere by the processes of both transpiration and evaporation. Evapotranspiration is an important part of the hydrologic cycle because it represents a considerable percentage of water losses in watersheds. This process is also one of the main consumers of solar energy at the Earth's surface. The estimation of evapotranspiration it is of great importance for agriculture and water resources management, for example, by knowing the rate of water loss from a region, farmers will be better placed to efficiently manage the irrigation strategies. The rate of evapotranspiration at any location on the Earth's surface is controlled by several factors such as energy availability, humidity gradient, wind speed immediately above the surface, water availability, type of vegetation, stomatal resistance and soil characteristics. The problem of estimating evapotranspiration has been widely studied. During the past several decades, different approaches have been proposed and many formulas are available in the literature to fulfill this task. The complexity of the phenomena and its great variability all over the world make difficult to find a universal and robust method for the evapotranspiration prediction and estimation. Evapotranspiration can be obtained by many estimation methods. Some of these methods need many weather parameters as inputs while others need fewer. Factors such as data availability, the intended use, and the time scale required by the problem must be considered when choosing the evapotranspiration calculation technique. The Penman-Monteith equation was derived from the classic Penman equation adding to the terms already considered in the Penman formulation (aerodynamic resistance and energy balance) the canopy resistance. This approach requires many meteorological data, such as maximum and minimum air temperatures, relative humidity, solar radiation, and wind speed. If some of these data are not available, alternative techniques have been developed to overcome this drawback. However, this estimations of meteorological data affect the quality of the result, alternatively, in such cases other simpler methods may be used. The Penman-Monteitu equation is assumed to be the most reliable because is based on physical principles and because it considers all the climatic factors, which affect evapotranspiration.

2013

Experiments in hydrologic solute transport

Pierre Claude Jean Queloz

This thesis deal with experimental, field and modeling studies on reactive solute transport within hydrologic control volumes. The substances used in open environments for plant protection and other purposes are likely to be flushed by rainfall and enter into the hydrologic system if they are not disposed of through a sewage network. This represents a significant hazard for receiving ecosystems. The concentrations observed in surface waters are often highly dynamic, because of the variability of the hydrologic drivers controlling solute transport. This makes surface water monitoring particulary laborious, thus calling for a better understanding, and description of hydrologic transport at relevant spatial and temporal scales. The features of hydrologic transport are investigated under controlled conditions in a large weighing lysimeter, where all input (rainfall) and output (evapotranspiration and bottom discharge) fluxes are closely monitored. Random rainfall and large evapotranspiration (ET) fluxes dictated by two willows induce highly variable hydrologic conditions in the system. Fluorobenzoic acids (FBAs) as well as isotopically labeled water were used to unequivocally mark selected rainfall events. Chemical analyses were performed on samples of discharge and soil water collected through sampling ports in order to estimate the breakthrough curves of each tracer. Besides revealing unreported reactive behavior of the FBAs, these results demonstrate non stationary tracer responses that not only result from the transient precipitation forcing, but also from the variability in the ET-induced water deficits and how the output fluxes sample the water and solutes in storage under the prevalent moisture conditions. The experimental data were further explored using a model based on travel time distributions (TTDs). Describing hydrologic transport using TTDs is becoming increasingly common in catchment hydrology, because it allows a collective measurement of many processes (climate forcing, internal mixing, flow pathways, etc.) into a single stochastic descriptor, therefore removing the need to estimate numerous physical parameters and circumventing the limited applicability of point-scale laws for structurally complex and heterogenous environments. This parsimonious model is able to reproduce the different tracer responses by keeping track of the age composition of the stored water and assessing the ways ET and discharge sample the available water ages in time. The results emphasize the effects of ET on solute transport, because it samples water of different ages (i.e. residence times) in the transport volume in regards to discharge and thereby strongly modifies the resulting TTDs. In this way it is possible to compare the respective roles of the hydrologic variability and reactivity attributes (namely plant uptake and microbial degradation) which affect the tracer responses and prevent a direct interpretation of the experimental breakthrough curves as TTDs. \ These experimental and modeled results highlight the importance of non stationary transport, often overlooked to interpret tracer data in catchment hydrology. This thesis demonstrates the benefits of tracer experiments towards the understanding of hydrologic systems undergoing reactive transport.

EPFL2015