There is an escalation in the frequency and severity of extreme events due to a number of environmental and/or anthropogenic factors. Droughts and exceptionally wet periods are regional phenomena, which are considered as major environmental extremes, especially in semiarid regions of the world, such as Greece. The development of severity-duration-frequency (SDF) relationships of droughts and wet periods over Greece is important in contemporary hydroclimatic and agroclimatic design and planning in the country. The Palmer Drought Severity Index (PDSI) is used for a quantitative description of droughts and wet periods. Statistical tests and visual inspection indicate that the EV1 (Gumbel) frequency distribution fits satisfactorily all the identified durations of droughts and wet periods, respectively. Moreover, the SDF curves show that decreasing frequencies (i.e. increasing recurrence intervals) correspond to increasing severities of droughts and wet periods, respectively. The developed SDF relationships are used to produce tables and isoseverity maps of Greece for each identified duration and all the selected return periods or frequencies, which constitute an essential aid for design purposes. The results of the study indicate that there is a decreasing pattern of the severities of droughts and wet periods from west to east and that, for similar durations and return periods, the wet spells are, in general, more extreme than droughts in Greece.Analyse sévérité-durée-fréquence des périodes sèches et humides en GrèceRésumé La fréquence et la sévérité des événements extrêmes dus à différents facteurs naturels ou/et anthropiques augmentent. Les sécheresses et les périodes exceptionnellement arrosées sont des phénomènes régionaux, considérés comme des phénomènes extrêmes de première importance, en particulier dans les régions semiarides comme la Grèce. L'établissement de relations sévérité-durée-fréquence (SDF) s'y révèle importante à l'heure actuelle pour la conception et la planification hydro et agroclimatique. L'indice de sévérité de sécheresse de Palmer (Palmer Drought Severity Index-PDSI) a été utilisé pour décrire quantitativement les périodes sèches et humides. Les tests statistiques et l'appréciation visuelle montrent que la distribution de Gumbel (Extreme Value 1-EV1) rend bien compte de l'ensemble des durées observées des périodes sèches et humides. Les courbes SDF montrent de plus que les événements les plus sévères sont ceux dont la fréquence est la plus faible (c'est à dire ceux dont la durée de retour est la plus longue). Les relations SDF établies ont été utilisées pour élaborer à l'échelle de la Grèce des tables et des cartes d'égale sévérité pour différentes durées et pour différentes durées de retour qui constituent un outil essentiel d'aide à la conception. Les résultats de cette étude indiquent qu'il existe, pour une même durée et une même durée de retour, une tendance à la décroissance de la sévérité d'ouest en est, et que les épisodes humides sont, en général, plus sévères que les épisod...
Abstract. The growing number and effectiveness of Earth observation satellite systems, along with the increasing reliability of remote sensing methodologies and techniques, present a wide range of new capabilities in monitoring and assessing droughts. A number of drought indices have been developed based on NOAA-AVHRR data exploiting the remote sensing potential at different temporal scales. In this paper, the remotely sensed Reconnaissance Drought Index (RDI) is employed for the quantification of drought. RDI enables the assessment of hydro-meteorological drought, since it uses hydrometeorological parameters, such as precipitation and potential evapotranspiration. The study area is Thessaly, central Greece, which is a drought-prone agricultural region characterized by vulnerable agriculture. Several drought features are analyzed and assessed by using monthly RDI images over the period 1981-2001: severity, areal extent, duration, periodicity, onset and end time. The results show an increase in the areal extent during each drought episode and that droughts are classified into two classes, namely small areal extent drought and large areal extent drought, respectively, lasting 12 or 13 months coinciding closely with the hydrological year. The onset of large droughts coincides with the beginning of the hydrological year, whereas the onset of small droughts is in spring. During each drought episode, the maximum occurs usually in the summer and they all last until the end of the hydrological year. This finding could justify an empirical prognostic potential of drought assessment.
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