Abstract: In this paper a comparison between two agro-hydrological models, used to schedule irrigation of typical Mediterranean tree crops, is assessed. In particular the comparison between the model proposed by FAO, using a black box processes schematization, and the SWAP model based on the numerical analysis of Richards’ equation is initially presented for two irrigation seasons, 2005 and 2006, and two Mediterranean tree crops, i.e. grapevine (Vitis vinifera, L.) and olive oil (Olea europea, L.). The comparison mainly focuses on hydrological balance components and on soil water contents. After investigating and setting the scheduling parameters ordinarily practiced by the framers in the area (i.e ordinary management), the performance of the two models aimed to evaluate seasonal water requirement and irrigation time, is assessed. In the validation phase both the models satisfactorily simulated the soil water content, allowing to obtain quite comparable values of evapotraspiration fluxes. On the other hand, when the models are used for scheduling irrigation, the FAO 56 model usually overestimates the irrigation amount, as a result of an overestimation of the transpiration fluxes. On the contrary the SWAP model simulates values of crops water requirements and a number of irrigation corresponding to those evaluated in the ordinary scheduling at the investigated area. Finally, in order to improve the FAO 56 model performance, a modification of the stress function is presented and discussed. Keywords: Agrohydrological models, FAO 56, SWAP, Irrigation Scheduling,Vineyard and Olive grove Riassunto: Nel presente lavoro viene presentato un confronto tra due modelli di simulazioni agro-idrologica per la ge- stione dell’irrigazione in colture arboree mediterranee. Vengono in particolare confrontati il modello proposto dalla FAO che utilizza uno schema a serbatoio ed uno più complesso SWAP basato sulla soluzione dell’equazione di Ri- chards. Il confronto ha riguardato i valori delle componenti del bilancio idrologico ed i contenuti idrici del suolo rela- tivamente alle due stagioni irrigue 2005 e 2006 su colture di Vite ed Olivo. È stata inoltre valutata la performance dei due modelli sulla programmazione dell’irrigazione impostando i parametri di scheduling ordinari della zona. Sebbene nella fase di validazione entrambi i modelli hanno simulato in modo soddisfacente l’andamento temporale del conte- nuto idrico medio del suolo e stimato valori dei flussi evapotraspirativi del sistema suolo-pianta-bassa atmosfera del tutto confrontabili, nella fase di programmazione dell’irrigazione, il modello semplificato ha sovrastimato i volumi ir- rigui in conseguenza della sovrastima dei flussi traspirativi. Il modello SWAP ha invece simulato valori del consumo idrico delle colture e del numero di adacquamenti analoghi a quelli ordinari della zona oggetto d’indagine. Ai fini della programmazione dell’irrigazione viene quindi proposta una modifica della funzione di stress utilizzata dal modello FAO, in modo da migliorarne la performance. Parole chiave: Modelli Agroidrologici, FAO 56, SWAP, Programmazione dell’Irrigazione, Vigneto e Oliveto INTRODUCTION The quantification of crop water requirements of irrigated land has a fundamental importance, in particular in the Mediterranean regions characterized by semi-arid conditions. The knowledge of the evapotranspiration fluxes allows to correctly estimate crop water requirements and to dispose of irrigation management strategies able to increase the irrigation systems efficiency. For this purpose, to set out new procedures permitting the correct estimation 11 Italian Journal of Agrometeorology - 1/2010 Rivista Italiana di Agrometeorologia - 1/2010 Agro-Hydrological models to schedule irrigation of Mediterranean tree crops Giovanni Rallo*, Carmelo Agnese 1 , Federico Blanda 1 , Mario Minacapilli 1 , Giuseppe Provenzano 1 * Corresponding Author: e-mail: rallo.giovanni@gmail.com. Di- partimento di Ingegneria e Tecnologia Agro-Forestali (ITAF), Università degli Studi di Palermo. 1 Dipartimento di Ingegneria e Tecnologia Agro-Forestali (ITAF), Università degli Studi di Palermo. of crop evapotranspiration on large area, has addressed part of the scientific research towards the development of mathematical models able to provide a detailed description of the processes related to the exchange of mass and energy in the Soil-Plant- Atmosphere system, SPA (Feddes et al., 1978; Bastiaanssen et al., 2007). The advance of the knowledge related to the energy and mass exchange processes taking part in the SPA system, has carried to the characterization of a unique and dynamic system (continuum) in which the various variables involved in the processes must be considered as mutually dependent. The acquired information will allow to validate the available models aimed to supply a detailed description of the processes