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Various models have been proposed to predict changes in scalar concentrations and surface fluxes following an abrupt change in surface conditions. In most cases the boundary conditions used to describe the surface represent limiting cases such as a step change in either concentration, flux or surface resistance. Three main conclusions emerge from a review of experimental results published thus far. Firstly, a dry-to-wet transition can be treated as a step change in concentration, after which the difference between the downwind and upwind surface fluxes show a power-law dependence on distance from the leading edge with an exponent of the order of − 1 6 . Secondly, wet-to-dry transitions should be treated as resulting in a step change in flux. This has not always been appreciated. Thirdly, a dry-to-moist (e.g. irrigated) transition is shown to resemble more closely a step change in flux than a step change in surface resistance. This can only be explained if stomatal resistance varies in conjunction with downwind changes in saturation deficit. We propose a simple analytical model to describe the streamwise change in surface resistance in the limit of a step change in flux. A scarcity of published data suitable for validation of this model has led us to undertake a new experimental investigation. This experiment is the subject of a companion paper.

It has been suggested in Part I of this paper that, after a dryland-to-irrigated transition, the surface sensible and latent heat fluxes may exhibit little change with downstream distance from the leading edge. It was argued that such step changes in surface fluxes could be caused by a feedback mechanism between surface resistance and changing saturation deficit. A simple model was derived for describing the streamwise change in surface resistance leading to such a step change in flux. The aim of the present paper is to test these assumptions against experimental data acquired specifically for this purpose. Measurements were performed over a transition between dryland and irrigated barley. Eddy covariance determinations of sensible heat flux at a height of 1 m showed little variation downwind of the transition and, once corrected from the footprint effect, exhibited patterns roughly compatible with a step change in surface flux. Transects of radiative temperature, and air temperature and humidity above the crop, also showed downwind changes in good agreement with the expected variation. The most convincing results were obtained from measurements of stomatal and canopy resistance, whose downwind variation was close to that required by a step change in evapotranspiration.

Un système destiné à étudier les «effets photiques» du rayonnement en conditions artificielles sur la persistance des micro-organismes a été mis au point. Ce système est composé de plusieurs enceintes équipées de sources radiatives (lampes à halogénures métalliques). Des filtres optiques permettent de travailler dans des gammes de longueur d’onde déterminées. Le dispositif réalisé permet de maintenir constante la température de l’échantillon durant l’irradiation quel que soit le niveau d’éclairement reçu grâce à une plaque thermorégulable. Le système est équipé d’un ensemble de mesures des facteurs climatiques : température du support d’inoculum, humidité relative de l’air, rayonnement total et rayonnement ultraviolet. Les performances du banc d’irradiation ont été étudiées. Une cartographie des éclairements énergétiques en UVA et en UVB a été établie, ainsi qu’une cartographie thermique du plan d’irradiation. Le gradient des niveaux d’éclairement en UV sur le plan d’exposition des échantillons (3 600 cm2) dépend de la distance à la source radiative. Il se situe dans une gamme de dispersion de l’ordre de 10% à 1,50 m et de 20% à 0,50 m. Le gradient thermique n’excède pas 5% dans l’ensemble des situations éprouvées. Par ailleurs, la température du support des échantillons ne s’écarte pas de la consigne de plus de 1 °C. L’humidité relative, malgré l’absence de régulation, reste stable pendant la durée d’une irradiation. Le dispositif réalisé a été adapté pour des irradiations en conditions semi-contrôlées (support d’inoculum à température constante, rayonnement solaire)., A system was devised for separate analysis of the photic effect and the thermal effect of solar radiation on the persistence of pathogenic microorganisms under both artificial and natural light. The objective of the authors was also to investigate these effects in varying waveband and irradiance. The irradiation chambers consisted of controlled-environment incubators with 2 high-pressure metallic halogen lamps (400 W) irradiating a large temperature-controlled platform (3 600 cm2). The light quality was modified by using 2 long pass glass filters. In using a 295 nm cutting filter, the UVB irradiance ranged from 1.6 to 0.4 W.m-2 according to the distance from the source to the target plane. At 1.50 m distance the gradient of distribution of UVB irradiance on the target surface was less than 12%. The thermal gradient did not exceed 5%. The difference between the target temperature and the measured temperature at the sample surface support was less than 1°C. The relative humidity remained stable around 60% during the inoculum exposure. The device has been adapted for studies in ’semi-field’ conditions (constant temperature for the support of inoculum and solar radiation).

This paper compares the relative advantages of improvements in practices and changes in cropping system patterns in order to manage water resources at catchment basin level and thus preserve the cropping systems over the long term. When water for irrigation comes from large aquifers belonging to the catchment basin itself, the cropping systems pattern is the crucial factor because it determines the annual supply of water to the aquifers in order to match the total water demand. When water for irrigation is surface water coming from rivers or other reservoirs, both practices and cropping systems play an active role in fitting water demand to water supply. Whatever the origin of irrigation water, a greater diversity of cropping systems will be an advantage when there is a shortage of water.

Une etude experimentale de la gelivite des bourgeons de vigne in situ a ete entreprise sur le vignoble champenois (Vitis vinifera L Chardonnay) de 1987 a 1989 en utilisant l'analyse thermique differentielle dans des chambres de refroidissement. L'analyse de la gelivite a consiste a etudier l'influence sur la temperature de rupture de surfusion de facteurs tels que le stade phenologique, le mouillage des organes vegetaux et la teneur en bacteries dites glacogenes. Au vu des resultats il apparait que stade phenologique et mouillage sont des variables determinantes, tandis que l'action des bacteries est beaucoup plus tenue et difficile a interpreter (...)

International audience; The Penman-Monteith equation (PM) provides a direct and logical route to explaining rates of crop water consumption without the need of resorting to the artificiality of a reference crop and ill-defined crop factors. However as an operational irrigation tool, the PM is often impractical because of uncertainties about stomatal behaviour and turbulent transport. While recent progress is providing better descriptions of these processes, it is questionable whether such detail is always warrented for practical irrigation. Another option for evaporation prediction is the Priestley and Taylor equation (PT). This approach enjoys reasonable empirical support in humid regions and our efforts here are devoted to better defining its limits of applicability. We begin by presenting previously published and new evidence in support of Priestley and Taylors' value of 1.26 for the coefficient α indicating proportionality between evaporation rate and available energy. While α may depart from this value depending on windspeed and saturation deficit, it is shown to be fairly insensitive to both over a reasonable range of conditions. A daytime mean saturation deficit of 10 g m–3 is suggested as a likely upper limit for the range of applicability of the PT; below this limit, the extra data inputs required for the PM will not be rewarded with a better estimate of evaporation. In arid regions and under stable conditions, fluxes well beyond the PT estimate have been measured and no simple prediction alternative to the PM exists.

Evapotranspiration is a major term of the soil water balance. Knowing the amount of water directly evaporated from the soil or through transpiration of plants is a point of interest not only for agronomists but for meteorologists, hydrologists and irrigation project managers. A large number of papers have been published in the last thirty years concerning both mechanisms, measurements and estimation techniques (for review see e.g. Brutsaert 1982, Seguin et al. 1982, Hatfield and Fuchs 1990, Jensen et al. 1990, Allen 1994). The aim of this paper is not only to review what is available today, for different time and space scales but also to see how it could be used by farmers or more generally by project managers. Recommendations are made to improve both knowledge and transferability to users.

International audience; Previous measurements of water loss from small-dish evaporimeters mounted at the height of irrigated crops grown under conditions of extreme local advection in the Sudan are reexamined. From these evaporimeter measurements, it is possible to calculate fractional changes in the saturation deficit. Relationships between canopy conductance and saturation deficit are briefly reviewed and introduced into the Penman-Monteith equation to calculate transpiration rates as a function of distance downwind of the boundary between the upwind desert and the irrigated crop. In contradiction to most theoretical predictions, these new calculations show rates of transpiration to undergo only modest changes with increasing fetch. This occurs because of the feedback interaction between saturation deficit and stomatal conductance. This result is in good accord with a recent study suggesting that a dry-moist boundary transition may be best modelled as a simple step change in surface fluxes and further that the advective enhancement of evaporation may have been overestimated by many advection models. Larger effects are expected on dry matter yields because of the direct influence of saturation deficit on the yield-transpiration ratio.

Les auteurs décrivent 2 chambres de refroidissement utilisées pour les études de gélivité des bourgeons in situ. La première est une chambre de refroidissement convectif élaborée à partir d’éléments de congélateurs. La deuxième cherche à reproduire le phénomène de refroidissement radiatif qui a lieu au cours des nuits de gelées de printemps afin de permettre les dépôts de rosée et de givre qui se produisent dans les conditions naturelles., The authors describe 2 cooling chambers used to study frost sensitivity of vine buds. The first is a classical convective cooling system which can be run in the field (fig 1). The second is designed to provide buds with the radiative cooling which occurs during spring frosts. This chamber reproduces part of the system designed by Went (1976), Gill and Waister (1976) and Marcellos (1981). Figure 3 reproduces the schema of the chamber. Radiative cooling is provided by means of evaporating nitrogen at roof level in the upper part (3). Warm air is circulated in the middle part in order to maintain the polyethylene walls (4) free of water and transparent to thermal infrared radiation. This radiative cooling system makes dew or rime deposition possible, as in natural conditions. This is illustrated in figure 4 where bud temperature (----- and ---) is kept lower than both air temperature (-) and dew point (-·-) during cooling.

Sommaire A partir d'un mod&le analytique de diffusion, nous calculons l'evolution horizontale et verticale des concentrations et des flux d'aerosols emis par une sur-face ou un couvert vegetal. Ce modele s'applique i deux cas particuliers: - soit concentration constante au-dessus de la zone d'emission, - soit flux d'hission constant sur toute la zone. Les flux de concentration en aerosols sont calcules tout d'abord sur la zone d'kmission et i l'inttrieur de sa propre couche limite de diffusion. Dans un deuxikme temps, on s'interesse i ces mCmes quantites au-deli de la zone d'tmis- sion de maniere i calculer les retombees. Pour une mCme concentration sur la zone d'emission et une mCme rugositt de surface, ces retombees dependent de quatre paramPtres: la vitesse du vent, la vitesse de sedimentation des aerosols, la dimension de la zone d'emission et la distance i cette zone. Reference est faite i un exemple d'application concernant quelques maladies i dissemination anemophile telles que l'oldium de l'orge et la rouille du bli et nous discutons ducomportement des particules en fonction de valeurs de la vitesse du vent.

Using a simplified formula previously derived by B. Itier and Ch. Riou for turbulent fluxes under unstable conditions, the authors give a modified version of Dalton's formula for evaporation or sensible heat flux. In the classical version of Dalton's formula the “bulk transfer coefficient” is written as A + BU where A and B are regression coefficients. The authors show that A is a function of the ratio of the difference in temperature between the air and the surface to the wind velocity. Values of A found in the literature are discussed. The increase of its value when adjusting a Dalton formula to over barren soil instead of over water is presented here in terms of instability effects rather than roughness effects. The results obtained by means of the new modified Dalton's formula are compared to independently measured values of sensible heat flux deduced from the energy balance, using a net radiometer (Rn) and soil temperature profiles (G) on a dry sand in southern Tunisia.

The influence of pure water artificial fog on air and bud temperatures during night cooling under a clear sky is reported. Fog generation transforms the temperature profile near the ground into one which is nearly isothermal with height, compared to the normal inverted night temperature profile. The overall temperature reached is relatively constant and close to the wet bulb temperature of the air. Within the fog, the difference between air and bud temperature tends to 0.1–0.2°C, rather than to 0.5–2°C as outside the fog, depending on net radiation. The mechanisms whereby energy is transferred to buds to reduce air-bud temperature difference are discussed. Increases in incoming radiation and dew or frost deposits seem not to be high enough to reduce it in such a way. In addition, the dependence of fog stability on wind conditions is analysed.

International audience

The practical experience obtained by our participation in the European project TELLUS (a part of the HCMM programme) led us to state that the thermal inertia concept and sophisticated models are useful for understanding basic processes and for performing informative simulations, but cannot be used for a real estimation of evaporation On the other hand, a simplified procedure like that proposed by Jackson et al. (1977) appears feasible, but its theoretical basis and its field of application are subject to discussion. Two main questions arise, namely, the conditions underlying the use of one instantaneous measurement to estimate daily integrated values, and the significance of the adjustment parameter B in the simplified relationship ETd = Rnd − B(Ts − Ta)i which relates the daily evaporation to daily net radiation by means of one measurement of the surface and air temperature, at a given time of day These questions are discussed with reference to results of recent experiments. A theoretical analys...

Une experience realisee a Coruche (Portugal) sur une culture de tomate a permis de tester plusieurs criteres de contrainte hydrique relatifs a la plante et a son microclimat. Le potentiel hydrique de base et la conductance stomatique sont des criteres tres sensibles a un arret de l'irrigation. Les criteres microclimatiques (temperature et flux de surface) peuvent etre interessants, si certaines precautions relatives a la dimension de la parcelle etudiee sont prises [...]

Après avoir passé en revue les différentes méthodes de mesure de l’évapotranspiration réelle, qu’il s’agisse des méthodes directes telles que la pesée ou la mesure du stock en eau du sol ou des méthodes indirectes telles que la méthode du rapport de Bowen et la méthode aérodynamique combinée, on développe une méthode de mesure simple cherchant à allier les avantages de la méthode du rapport de Bowen (deux niveaux de mesure dans l’air seulement) et ceux de la méthode aérodynamique combinée (pas de mesure d’humidité de l’air). On montre (cf. fig. 1) comment l’ETR peut être calculée aisément à partir de la méthode aérodynamique combinée dans laquelle le flux de chaleur sensible, quel que soit l’état thermodynamique des couches de surface, peut toujours être obtenu directement à partir de ΔU et ΔT, les écarts de vitesse du vent et de température entre deux niveaux de mesure dans l’air. On compare la méthode proposée avec la méthode aérodynamique classique (fig. 2) et avec la méthode du rapport de Bowen utilisée par le système BEARN (fig. 3 et 4). Le bon accord obtenu permet de préconiser l’utilisation de la méthode proposée pour sa facilité d’emploi et sa fiabilité dans le temps, une fois précisées les conditions d’emploi et les limitations., This method tries to add the advantages of the classical combined method (no measurement of air humidity) to those of the Bowen ratio method (only two levels of measurements in the air). The sensible heat flux H is obtained by means of measurements of wind velocity and of the difference in air temperature at two levels only. Whatever the stability conditions are, the flux H can be expressed as a function of Ho = K ΔU ΔT, the Thornthwaite-Holzman relationship :- under stable conditions, H is progressively reduced by H = Ho [1 - ( Ri/Ric)]2 to H = Ho/10, arbitrary fixed value (fig. 1) - under unstable conditions, for small values of Ri (- Ri < 1), three formulas are proposed, adding buoyancy effects to forced convection : either H = Ho + K’ ΔU α|T ΔT|β(ITIER 1980-1982) or H = Ho(1 + K"Ri)y (Riou, 1982) for higher values of Ri (- Ri > 1), we retain the Priestley formulation using DYER (1965) and CRAWFORD (1965) values for h* : H = A|ΔT|3/2 Fig. 2 shows values of H obtained by the simplified method under unstable conditions versus those given by the classical selfconsistent technique using mast measurements at six levels. Fig. 3 and 4 show values of AET obtained by the simplified combined method as compared to values given by the Bowen ratio method. The good agreement added to the fact that this technique can be considered inexpensive, easy to use and able to stand the test of time, allows us to suggest its use for people working in evapotranspiration measurements.

The evapotranspiration of a wheat crop was studied over two consecutive years and determined by an automatic system of energy balance (B.E.A.R.N.). Results were analysed to examine the role played by meteorological factors (excluding indirect biological effects), and separately by physiological factors, on the variations in evapotranspiration. Even with the large difference in rainfall, and consequently the water balance, in the two years 1975 and 1976, it clearly appears that the critical evapotranspiration (P′Rn/(P′ + γ) plays dominant role together with the regular physiological development related to the ageing of the crop, as indicated by the mean crop resistance. The good relationship between this mean crop resistance and the critical resistance defined by meteorological factors alone, may be justified by the classical response of stomatal aperture to light and atmospheric humidity. Nevertheless, this relationship possibly suggests the definition of a specific ratio between actual evapotranspiration and the critical value of evapotranspiration, a result which can be very useful in practice.

This letter follows up the theoretical analysis by Seguin and Itier (1983) of a simplified linear relationship between daily evaporation and midday surface temperature. The use of a basic set of equations describing surface fluxesallows us to obtain sets of combined (ETd, Ts—Ta) values for various surface roughness (z0) values ETd. being the daily evaporation and (Ts—Ta)the difference between surface and air temperature. The procedure, applied to two published experimental studies, allows us to refine the empirically obtained relationship. It is thereafter used to simulate the effect of surface roughness upon the coefficients of the simplified relationships: the origin a is found to be insensitive to z0 (with a constant value of 1–27mm/d) and the slope b may be computed as a simple function of z0, thus allowing us to propose a formulation applying to a large range of surface roughness (from 1mm to 10cm).

The objectives of this paper are to describe the learning of the use of the Laser Doppler Velocimeter (LDV) as well as the Particle Image Velocimeter (PIV), including the fine points in their usage for instructional purposes. The application is to measure the velocity distribution across a flow of water bounded by a layer of oil using lasers. The characteristics at the oil-water interface are very interesting. It would be significant to measure the velocity distributions around this region. Such a scenario occurs during oil spills and spills of oily chemical pollutants in the sea or open ocean. The LDV is a well established method for measuring both laminar and turbulent flows. In this method, tracer particles are used to assist in measuring velocity profiles. This method was pushed to the limit by measuring the velocity boundary layer in the open channel flow. The average free-stream velocity is measured by other conventional means as a check on the LDV measurements. The PIV method is an optical method used to obtain instantaneous velocity measurements and related properties in fluids. The fluid is seeded with tracer particles and it is the motion of these seeded particles that is used to calculate the velocity information of the flow being studied. The PIV produces two dimensional vector fields. The simple PIV system was pushed to the limit by using it to measure the velocities in the oil-water interface of an open channel flow bounded by oil on the surface. The major difference between the LDV and the PIV is that the LDV measurements are done at a point, whereas the PIV measures the velocity of a region. Furthermore, PIV produces two dimensional vector fields while the LDV produces only a velocity measurement.Copyright © 2009 by ASME