Ecohydrology in Mediterranean areas: a numerical model to describe growing seasons out of phase with precipitations

摘要

The probabilistic description of soil moisture dynamics is a relatively newtopic in hydrology. The most common ecohydrological models start from astochastic differential equation describing the soil water balance, wherethe unknown quantity, the soil moisture, depends both on spaces and time.Most of the solutions existing in literature are obtained in a probabilisticframework and under steady-state condition; even if this last conditionallows the analytical handling of the problem, it has considerablysimplified the same problem by subtracting generalities from it.The steady-state hypothesis, appears perfectly applicable in arid andsemiarid climatic areas like those of African's or middle American'ssavannas, but it seems to be no more valid in areas with Mediterraneanclimate, where, notoriously, the wet season foregoes the growing season,recharging water into the soil. This moisture stored at the beginning of thegrowing season (known as soil moisture initial condition) has a greatimportance, especially for deep-rooted vegetation, by enabling survival inabsence of rainfalls during the growing season and, however, keeping thewater stress low during the first period of the same season.The aim of this paper is to analyze the soil moisture dynamics using asimple non-steady numerical ecohydrological model. The numerical model hereproposed is able to reproduce soil moisture probability density function,obtained analytically in previous studies for different climates and soilsin steady-state conditions; consequently it can be used to compute both thesoil moisture time-profile and the vegetation static water stresstime-profile in non-steady conditions.Here the differences between the steady-analytical and the non-steadynumerical probability density functions are analyzed, showing how theproposed numerical model is able to capture the effects of winter rechargeon the soil moisture. The dynamic water stress is also numericallyevaluated, implicitly taking into account the soil moisture condition at thebeginning of the growing season. It is also shown the role of differentannual climatic parameterizations on the soil moisture probability densityfunction and on the vegetation water stress evaluation.The proposed model is applied to a case study characteristic ofMediterranean climate: the watershed of Eleuterio in Sicily (Italy).