No abstract

[1] Plants in Mediterranean ecosystems have developed different strategies to cope with transient soil-moisture dynamics induced by the markedly out of phase seasonal behavior of rainfall and temperature. Deep-rooted plants use the soil moisture stored in the wet winter (extensive users), while shallower rooted plants exploit both the wet season storage and the more sporadic growing season rainfall (intensive users). Using stochastic models of soil-moisture dynamics, we present an analytical and numerical description of the probabilistic structure of the soil-moisture storage at the beginning of the growing season in relation to the dynamics of the wet season and then study its evolution during the subsequent growing season. Special attention is devoted to plant water stress as a function of the rooting depth and the soil-moisture storage at the beginning of the growing season. The existence of an optimal rooting depth for Mediterranean climates and its dependence on future hydroclimatic scenarios are discussed with reference to a test case in Sicily, Italy. Our analyses suggest that the forecasted decrease in rainfall for the Mediterranean regions might lead to a considerable increase in plant water stress and favor vegetation with shallower root systems.

This paper first presents a duration-dependent hypothesis regarding the parameters of the two component extreme value (TCEV) distribution and proposes a simple model to obtain the rainfall depthduration relationship both at ungauged sites and at short record gauged sites in the Sicilian region. Then, by using the annual maximum peak flood and the annual maximum mean daily discharge recorded in Sicily, a TCEV hierarchical regional procedure is developed. An empirical estimation criterion which links the mean annual flood with the watershed area, and a modified rational formula in which a mean runoff coefficient is introduced, are proposed. Finally, two relationships are established for estimating the mean runoff coefficient, making use of the permeable watershed area and the wooded area. Analyse régionales des pluies et des crues en Sicile grâce à la loi des valeurs extrêmes à deux composantes Résumé Ayant admis l'hypothèse d'une dépendance des paramètres de la loi des valeurs extrêmes à deux composantes à la durée considérée, les auteurs proposent un modèle simple permettant d'établir une relation hauteur-durée aussi bien dans les zones dépourvues de stations pluviométriques que dans celles qui en sont pourvues mais qui ne disposent que de brèves séries d'observations. Ensuite, utilisant les chroniques des maximums des débits de pointe annuels et des maximums des débits moyens journaliers, ils appliquent la loi des valeurs extrêmes à deux composantes à l'échelle régionale. Un critère empirique d'estimation reliant la crue moyenne annuelle et l'aire du bassin est alors proposé, ainsi qu'une formule rationnelle, modifiée par l'introduction d'un coefficient de débit moyen. Les auteurs présentent enfin deux relations prenant en compte la surface perméable et de la couverture forestière du bassin afin d'estimer le coefficient moyen de débit.

Extreme rainfall events have large impacts on society and are likely to increase in intensity under climate change. For design and management decisions, particularly regarding hydraulic works, accurate estimates of precipitation magnitudes are needed at different durations. In this article, an objective approach of the regional frequency analysis (RFA) has been applied to precipitation data for the island of Sicily, Italy. Annual maximum series for rainfall with durations of 1, 3, 6, 12, and 24 h from about 130 rain gauges were used. The RFA has been implemented using principal component analysis (PCA) followed by a clustering analysis, through the k‐means algorithm, to identify statistically homogeneous groups of stations for the derivation of regional growth curves. Three regional probability distributions were identified as appropriate from an initial wider selection of distributions and were compared – the three‐parameter log‐normal distribution (LN3), the generalized extreme value (GEV) distribution, and the two component extreme value (TCEV) distribution. The regional parameters of these distributions were estimated using L‐moments and considering a hierarchical approach. Finally, assessment of the accuracy of the growth curves was achieved by means of the relative bias and relative root‐mean‐square error (RMSE) using a simulation analysis of regional L‐moments. Results highlight that for the lower return periods, all distributions showed the same accuracy while for higher return periods the LN3 distribution provided the best result. The study provides an updated resource for the estimation of extreme precipitation quantiles for Sicily through the derivation of growth curves needed to obtain depth–duration–frequency (DDF) curves.

The "thermal inertia" method to retrieve surface soil water content maps on bare or sparsely-vegetated soils is analysed. The study area is a small experimental watershed, where optical and thermal images (in day and night time) and in situ data were simultaneously acquired. The sensitivity of thermal inertia to the phase difference between incoming radiation and soil temperature is demonstrated. Thus, to obtain an accurate value of the phase difference, the temporal distance between thermographs using a three-temperature approach is evaluated. We highlight when a cosine correction of the temperature needs to be applied, depending on whether the thermal inertia formulation includes two generic acquisition times, or not. Finally, the deviation in soil water content retrieval is quantifies for given values of each parameter by performing a sensitivity analysis on the basic parameters of the thermal inertia method that are usually affected by calibration errors.Key words thermal inertia; surface soil water content; remote sensing Analyse critique des approches fondées sur l'inertie thermique pour la cartographie de la teneur en eau de la surface du sol Résumé La méthode de « l'inertie thermique » pour la cartographie de la teneur en eau de la surface des sols nus ou à végétation clairsemée est analysée. La zone d'étude est un petit bassin versant expérimental, où des images optiques et thermiques (de jour et de nuit) et des données in situ ont été acquises simultanément. On a mis en évi-dence la sensibilité de l'inertie thermique à la différence de phase entre le rayonnement incident et la température du sol. Ainsi, pour obtenir une valeur précise de la différence de phase, nous avons évalué la distance temporelle entre thermographes en utilisant une approche à trois températures. Cet article montre la nécessité éventuelle d'une correction cosinusoïdale de la température, selon que la formulation de l'inertie thermique comprend ou non deux temps d'acquisition génériques. Enfin, l'article quantifie la variabilité de la teneur en eau du sol pour des valeurs données de chaque paramètre en effectuant une analyse de sensibilité sur les paramètres de base de la méthode de l'inertie thermique qui sont généralement affectés par des erreurs d'étalonnage.