Conclusion And Recommendations

In this study, the surface energy partitioning has been modelled by a rather simple one-layer resistance approach for the region of Sicily and for the period 1989-1992. The aim was to characterise the moisture availability for the whole island with a spatial resolution of about one kilometre and a time-step of a few days. The results provide a general description of spatial and temporal behaviour of the energy balance components, that correspond to the pattern to be expected for a Mediterranean region. It could be shown that resulting physical parameters like the evaporative fraction can be used for environmental applications such as monitoring the moisture state of the surface cover. The chosen approach, the data used and the spatial and temporal resolution of the model can be considered as being adequate for regional applications.

The derivation of absolute quantities of physical parameters from the model like evapotranspiration, however, is limited by two principal drawbacks. Firstly, empirical relationships have had to be introduced into the model due to a lack in physical input parameters at the spatial resolution of the model. Secondly, the validation of the model results proved to be very difficult, since suitable reference data have not been available. The latter problem is certainly the more serious one, since 'ground truth' of spatially resolved physical parameters will - realistically - never be available, other than in regions studied in detail for research purposes. Therefore, different as well as new approaches of validation will have to be considered. One possible way for validating model results in the future is the comparison with Regional Circulation Models (RCM). Goyette et al. (this volume) show that nowadays a spatial resolution of 1 km can be achieved with such RCMs. Improvements are, however, still necessary in deriving suitable high-resolu tion surface information and in demonstrating the validity of the representation of atmospheric processes at these small scales.

With respect to the lack of input parameters for the model, remotet sensing might provide a solution in the future, or at least steps ahead towards this objective. If quantitative, physical parameters can be measured directly and with a known accuracy from space-based sensors, many approximations in simple models, such as EVA, can be avoided. Additionally, more sophisticated models like the ones proposed by Kustas and Norman (1997) or Anderson et al. (1997) could then be applied operationally. These models describe the energy exchange of the surface by two- or more layer resistance schemes that require spatially and temporally resolved information on the structure of the surface cover as well as on the boundary layer development. Until today, the application of such models is restricted to studies where detailed knowledge of atmospheric and surface properties is available from intensive field campaigns. Certainly, the spatial and temporal resolution of remote sensing data will not increase without limits in the future. However, with a spatial resolution of 1 km as implemented in this study the surface energy balance of a region can be represented in an appropriate way. Such spatial resolution, together with a temporal resolution of several images during daytime, will considerably increase the model performance, since then the temporal evolution of the boundary layer can be taken into account. The availability of data from the second generation of Meteosat satellites (MSG) in the near future will be a major step in this direction (e.g. EUMETSAT 1999). Much more important, however, is the derivation of absolute quantities of relevant parameters like fractional vegetation cover, LAI, or surface temperature; especially the latter parameter cannot be derived reliably from the current Meteosat data. In the near future, the quantitative datasets derived from e.g. MSG will not only increase the model accuracy, but also reduce the necessity of extensive validation in new study regions.

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