Bernard Itier

Modelling the local climate in island environments: water balance applications

In small volcanic islands the local scale climate is influenced by the regional scale climate and by the orography and orientation of air masses movement over the islands. A model was developed in a GIS environment to generate local scale climate variables from those observed at the synoptic scale, from coastal weather stations. An advective submodel, based on the Foehn effect and assuming the conservation of mass and energy, computes local scale air temperature, relative humidity, clouds occurrence and precipitation. A radiative submodel, using information generated by the advective submodel, computes local scale global radiation. A rotational terrain model allows that computations be performed according to the direction of wind. Because the model works within a GIS, results concern the spatial distribution of all climatic variables on the island territory. Results of the validation of temperature, relative humidity, global radiation and rainfall are presented. For agro-meteorological purposes, an application of generated data to perform the sequential water balance is also analysed by comparing results from computations using simulated and observed data at a control weather station located at medium altitude. Results support assumptions utilised in the model and the further use of generated local climate fields for water management and environmental studies in small island environments.

Parameterisation of the Shuttleworth-Wallace model to estimate daily maximum transpiration for use in crop models

In crop models maximum transpiration is an important component of the computation of water stress factors. It depends on reference climatic variables and leaf area index, and also on soil evaporation which modifies the actual air properties around the plants. This last effect is not accounted for in classical approaches used in crop models. Yet Shuttleworth and Wallace theory offers a framework to simulate canopy and soil evaporation fluxes in a coupled way. In this paper an adaptation and a parameterisation of the basic equations from Shuttleworth and Wallace is proposed, allowing use of the model to calculate maximum transpiration by using daily variables. The adaptation concerns soil evaporation. A potential soil evaporation is calculated assuming that, when the soil surface is wet, total evaporative flux consumes the whole available energy. It is used as an input to a two-staged model to calculate actual soil evaporation. The parameterisation relies on two field experiments performed on well-irrigated soybean. Measurements of net radiation balance show that radiation extinction within the canopy is less than generally admitted. Simulations of daily soil evaporation exhibit the same dynamics as microlysimeter measurements, which can be high even when the crop is fully developed. Bulk canopy resistances derived from Bowen ratio measurements agree closely with values obtained from classical formulae using a mean stomatal resistance of 250 ms−1. The modified and properly parameterised model shows that the contribution of plants to total evapotranspiration is highly variable as a result of the interactions between direct soil evaporation and plant transpiration.

Effet du rayonnement solaire sur la persistance des conidiospores de L'hyphomycète entomopathogène,Nomuraea rileyi, à la surface d'un couvert végétal

RésuméLa persistance des conidiospores deNomuraea rileyi (F.) Samson à la surface du couvert végétal a été étudiéein situ en culture de féverole («Féverole 1 et 2») et de chou. Leffet du rayonnement solaire sur la survie du germe a été analysé, dune part, dans des microparcelles recouvertes décrans sélectifs arrêtant les radiations de longueurs donde inférieures à 320 nm (UV A+) ou à 400 nm (UV−) ou par un écran total vis-à-vis du rayonnement direct (RYT−) et, dautre part, dans une microparcelle découverte (RYT+). Les conditions microclimatiques ont été suivies pendant toute la durée des expérimentations et le contrôle de lactivité résiduelle du champignon a porté essentiellement sur le dénombrement des spores viables sur milieu nutritif après mise en suspension dun broyat des échantillons de feuilles prélevés sur le terrain. Dans le cas des variantes expérimentales (RYT+) et (RYT−) le potentiel infectieux a été éprouvé sur des larves deSpodoptera littoralis Boisd. (Lépidoptère Noctuidae) alimentées pendant 24 h avec des rondelles de feuilles recueillies dans le couvert.La durée de demi-vie de linoculum sporal est étroitement liée à lensoleillement; par temps clair elle peut atteindre un minimum de 3,6 h («Féverole 1»), alors que dans les microparcelles totalement protégées du rayonnement direct, elle est supérieure à 40 h. Les données dactivité pathogène confirment linactivation de linoculum soumis au rayonnement solaire.Parallèlement, le recours aux écrans transparents (UV A+) ou opaques (UV−) au proche ultra-violet (320–400 nm) a permis de démontrer clairement leffet léthal de lUV A naturel sur les conidiospores deN. rileyi, dont les demi-vies peuvent être réduites jusquà 4 fois («Féverole 1»: XUV A +=11,6 h au lieu de XUV−=48,2 h).A partir de ces résultats, on estime que par temps ensoleillé en été, le potentiel dinoculum deN. rileyi exposé au rayonnement solaire direct peut chuter de 104 fois en une semaine, alors que par temps couvert cette réduction naurait été que de 100 fois.SummaryField persistence of conidia ofNomuraea rileyi (F.) Samson was studied at the top of the vegetation in order to evaluate the influence of solar radiation. Field trials were conducted on pigeon bean (2 experiments) and on cabbage (1 experiment) in 4 m2 plots. Four treatments were tested; they consisted of (1) one plot covered with a screen cutting the direct sunlight (RYT−) (2) one plot covered with a glass screen transmitting solar radiation between 320 and 2 500 nm (UV A+), (3) one plot covered with a glass screen coated with a UV A and B blocking film cutting wavelengths above 400 nm (UV−), and (4) one uncovered plot exposed to the direct sunlight (290 to 2 500 nm) (RYT+), respectively. The estimate ofN. rileyi survival was based mainly on viable conidia counts and, in the cases of both (RYT+) and (RYT−) configurations, the pathogenic activity was assayed on larvae ofSpodoptera littoralis Boisd. Logarithmically transformed viable spores counts were analyzed using a linear model and results were expressed in terms of viable conidia half-life. Microenvironmental parameters monitored in the field included sunlight, hours of sunshine, air temperature, leaf surface temperature, relative humidity, leaf wetness duration and precipitation.The viable spore half-life appeared to be dependent on the sunlight intensity. Under sunny conditions the half-life decreased to 3,6 hr, on the other hand, when plots were covered with a screen blocking the direct sunlight (RYT−) it could be of 40 hr or more. The data of pathogenic activity (angular values) of the spores over time declined as did their viability.The use of selective screens transmitting the UVA radiations (UVA+) or blocking the wavelengths above 400 nm (UV−) demonstrated clearly the lethal effect of solar UV A radiations on spores deposited on leaves exposed to the direct sunlight in field. For example, the half-life of conidia was reduced 4 times when exposed to UV A.An equation was formulated which predicted that in very sunny conditions a high concentrated inoculum ofN. rileyi viable conidia (3×106 spores/cm−2) could be reduced 10 000 fold over 7 days, whereas viability decreased only 100 times during a cloudy period.

Effects of temperature and solar radiation interactions on the survival of quiescent conidia of the entomopathogenic hyphomycetePaecilomyces fumosoroseus (Wize) Brown and Smith.

The detrimental effect of solar radiation on the survival of conidia of the entomopathogenic fungusPaecilomyces fumoroseus was studied by monitoring germinability and ability to form colonies (CFU) of conidia irradiated at two temperatures, 25 and 35 °C, harmless to shaded conidia. There was no apparent effect when spores were exposed to a high level of artificial radiation (0.66 W m−2 UVB). However, at a lower level of irradiance (0.33 W m−2), effects of radiation occurred more quickly at 35 °C than at 25 °C. Under natural solar radiation, the rate of decrease in germinability or viability was doubled at 35 °C as compared to 25 °C, indicating an interaction between temperature and radiation effects under natural conditions. This interaction was not detected in indoor experiments, indicating that the spectral distribution of UV radiation has to be taken in account as well as its irradiance when studying its effects.