Following the annual course of the evaporative fraction, moisture conditions over the entire island of Sicily (approx. 25,000 km2) have been monitored with a time-step of a few days, depending on the availability of cloud-free images. Plots of the spatial distribution of this parameter are presented in figure 1 for selected days in 1991. Light grey colours indicate areas with ample water supply where evapotranspiration is limited only by the available energy. Dark grey and especially black colours indicate a lack of moisture at the surface cover, which seriously restricts evapotranspiration. Since clouds obviously limit the use of optical remote sensing data, the days presented were chosen according to minimum cloud cover.
A clear and realistic evolution of EF can be seen over the year. In March and April EF values are high, indicating that evapotranspiration occurs throughout Sicily in an almost unlimited way. However, from June onwards, an increasing area exhibits dark grey and black colours with EF values as low as 0.1. These values represent regions where evapotranspiration is strongly limited by the available water. Only the mountainous regions in the north and north-east of Sicily remain with a sufficient water supply to ensure a high level of evapotranspiration. It is not before October that the situation is relieved, and only in November sufficient moisture is available throughout Sicily.
Opposite to 1991, which represents an average year regarding precipitation amount and distribution in Sicily, the year 1989 can be considered a drought year (G. Rossi, Univ. Catania, pers. comm.). This event is clearly seen in figure 2. In contrast to the situation in 1991 (figure 1) evapotranspiration is restricted over large areas of Sicily already in early spring 1989. In Sicily, low values of the evaporative fraction are a normal situation during the summer months. The early decrease of EF in 1989, however, indicates a large moisture deficit with severe consequences for agriculture and natural vegetation. Water stress becomes less severe only in October 1989 as shown by EF values increasing to levels from 0.6 to 0.9.
Time series of the evaporative fraction and of the temperature difference between surface and air temperature (AT) have been plotted in figure 3 for the period of 1989 to 1992. The data shown refer to the CORINE land cover type 'non-irrigated arable land' (CORINE 1993), which is the most frequent land cover type in Sicily. In general, the annual course of the evaporative fraction is characterised by low values from 0.3 to 0.5 in summer and by high values of up to 0.9 in spring and autumn. During the drought in 1989, however, values are as low as 0.1, indicating extraordinarily dry conditions. As expected, the temperature difference shows the opposite annual cycle, AT governs the sensible heat flux and thus also the evaporative fraction; however, it can explain only part of the scatter in the annual evolution of the evaporative fraction.
Rates of daily evapotranspiration (ETa) for 1990 as estimated by the method described above are shown in figure 4. In early spring as well as in autumn the daily rate ofETa takes values of 0 rrnnd"1 to maximal 3 mmd"1, while in the summer months in the mountainous regions values as high as 8 can be reached. The regional and temporal pattern as well as the magnitude of the rate of evapotranspiration are presented realistically. Considering the topography and the spatial distribution of the input parameters, ETa values are expected to fall in the given range. In general, two types of minimum are encountered: one due to limited energy supply in early spring and autumn, the other due to a lack of moisture in summer. The latter only occurs in regions where water supply is severely restricted (western and central-southern Sicily), whereas the former is evenly distributed over Sicily, since it depends mainly on the uniformly incoming global radiation as the forcing function in the energy budget. By contrast, the mountainous region in the north-east (Nebrodi Mts.) and Mt. Etna experience their maximum in evapotranspiration during summer when energy supply peaks and water availability is not severely limited.
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