Improving irrigation water use efficiency is crucial for sustainable greenhouse and nursery crops, as annual water requirements of these crops is huge ranging from 5,000 to 12,000 m3 ha-1. Most of these crops are conducted using micro-irrigation systems and therefore higher irrigation efficiency can be achieved only through improved irrigation scheduling and, at least in soilless culture, by recirculation of drainage water. Crop evapotranspiration (ET) is the primary process affecting crop water requirements, therefore the proper estimation of this parameter is essential for efficient irrigation management. One approach to the determination of crop ET is modelling. Herein, some models of ET of greenhouse and nursery crops are presented. The papers is mainly based on the research works performed by the Authors in the last years. In a series of experiments conducted in the area of Pistoia (Tuscany, Italy), which is the most important centre in Europe for the cultivation of landscaping ornamentals, ET was modelled in different woody ornamental species (Forsythia intermedia, Photinia x fraseri, Prunus laurocerasus and Viburnum tinus) grown outdoors in container filled with a peat-pumice mixture. The classical relationship between ET and potential evapotranspiration (ET0) was chosen, since it appeared more suitable for application in commercial nurseries. Four separate models were developed and used to schedule irrigation in an experiment conducted using a multiple-crop production scheme, that is by simulating the typical arrangement of commercial nurseries in Pistoia area, where different species are grown in the same irrigation sector. The scheduling method based on ET model was compared with the standard grower’s practice, which entails the use of simple timer adjusted during the growing season according to grower’s experience. Compared to the timer method, the application of ET model nearly halved the seasonal water use as a result of the lower irrigation frequency, with no significant effects on plant growth and market grade. Crop ET was also modeled in tomato and gerbera plants grown in rockwool slabs in the typical greenhouse conditions of Mediterranean regions in autumn and spring season. In gerbera, the Penman-Monteith equation and an empirical regression model (based on global radiation and vapour pressure deficit as independent variables) were used to predict diurnal ET, which was measured with a weighing gutter. A sub-model was implemented to estimate the evolution of leaf area as a function of thermal time (i.e. growing degree days). Both Penman-Monteith model and simplified equations allowed good estimation of ET and appeared suitable for automated irrigation control of gerbera soilless culture. In gerbera and tomato, a very simple model of ET as function of leaf area index and indoor solar radiation provided satisfactory estimate of crop water requirements.
Keywords: protected crops, irrigation, Penman- Monteith equation, soilless culture