Adaptation to an uncertain climate change: cost benefit analysis and robust decision making for dam dimensioning

Climate models project large changes in rainfall, but disagree on their magnitude and sign. The consequences of this uncertainty on optimal dam dimensioning is assessed for a small mountainous catchment in Greece. Optimal dam design is estimated using a Cost-Benefit Analysis (CBA) based on trends in seasonal temperature and precipitations from 19 IPCC-AR4 climate models driven by the the SRES A2 emission scenario. Optimal reservoir volumes are modified by climate change, leading to up to 34% differences between optimal volumes. Contrary to widely-used target-based approaches, the CBA suggests that reduced rainfall should lead to smaller water reservoirs. The resulting change in the Net Present Value (NPV) of water supply is also substantial, ranging from no change to a large 25% loss, depending on the climate model, even assuming optimal adaptation and perfect foresight. In addition, climate change uncertainty can lead to design errors, with a cost ranging from 0.3 to 2.8% of the NPV, depending on site characteristics. This paper proposes to complement the CBA with a robust decision-making approach that focuses on reducing design-error costs. It also suggests that climate change impacts in the water sector may reveal large, that water reservoirs do not always provide a cost-efficient adaptation strategy, and that alternative adaptation strategies based on water conservation and non-conventional water production need to be considered.     

Propagation transhorizon au-dessus de la Méditerrannee Sud-Orientale (Mer de Libye)

Sommaire Une étude radiométéorologique en mer close, a été initiée par le Groupe Radiométéorologique de l'OTAN. Pour completer les resultats des mesures au dessus du Canal d'Otranto (Martina Franca-Corfou) publiées en collaboration avec le Groupe Italien du Service de l'Aeronautique Militaire, des mesures au dessus de la mer de Libye sur la partie SE de la Mediterranée, d'une liaison transhorizon de 725 km entre le Caire et la ville d'Ierapetra, ont été enterprises.Pour expliquer le mecanisme de cette propagation transhorizon, on a appliqué une méthode d'analyse, basée sur les courbes de distribution du champs reçue en fonction du pourcentage du temps, indiquée par l'un des auteurs (Anastassiadis). Les courbes de distribution Rayleigh ou SuperRayleigh, obtenues pendant la période chaude, indiquent un mécanisme de reflection diffusée, tandis que les courbes avec des pentes superieures d'une Rayleigh pure, pendant la periode froide indiquent un mécanisme de guidage. L'hypothèse ci-dessus est supportée par le fait que les variations en altitudes des surfaces isobares suivant les periodes chaudes ou froides, presentent un fort coefficient de correlation avec les intensités du champ reçu.

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Summary A radiometeorological study of VHF propagation along transhorizon paths over several parts of the Mediterranean sea, was initiated by the Radiometeorological Panel of NATO's Science Committee. After a study over the Otranto channel (Martina Franca-Corfu) performed in collaboration with the Italian group, in the present paper is investigated the 725 km, mainly over the Lybean sea transhorizon path Cairo City to lerapetra, located in the South cost of Creta island. Three years fieldstrengths measurements were analysed in Ierapetra center. Following the observed results, during the warm period of the year it is a good reception, while during the cold period the reception was very poor or inexistant.In order to explain the mechanism of this transhorizon propagation, a method of analysis based on the distribution curves, as indicated by one of the authors (Anastassiadis) was used. Rayleigh or super-Rayleigh distribution curves of the received signalstrength versus time percentage observed during the warm period indicate a diffused reflection mechanism while Sub-Rayleigh distribution observed during the cold period indicate a ducting mechanism. These results are supported by the close correlation observed between the variation in altitudes of isobaric surfaces and the location of volume following the warm or cold period of the year in which the diffused reflection mechanism is produced.