Validation

The results presented so far offer a qualitative description of the moisture state of the surface. In order to be able to assess the validity of the results quantitatively, physical parameters like, for example, the rates of evapotranspiration given in figure 4, have to be derived. These values can be compared to on-site measurements or other independent data. However, independent measurements of such physical parameters are rare in Sicily, which makes validation problematic. Therefore, three different approaches have been selected in order to attempt an evaluation of the EVA model results.

Firstly, daily rates of reference evaporation according to Priestley-Taylor {ETpt) have been calculated from the same meteorological data as applied in the EVA model, but without using remote sensing data. This comparison is expected to reveal the differences in the parameterisation of the turbulent fluxes, since the available energy term is identical to a large extent. Figure 5 shows time series of ETa and ETpt for the class 'non-irrigated arable land' in the four years from 1989 to 1992. In spring and autumn, when evapotranspiration is mainly limited by the available energy, both methods agree well with corresponding results. In summer the reference evaporation (ETPr) shows its maximum in accordance with the peak in solar radiation and thus in available energy. The modelled actual evapotranspiration however, is significantly lower than the reference evaporation due to the restricted moisture supply in these months. Apparently, ETa accounts more realistically for the actual influence of the surface, that is, the well-known limited water availability in the summer months. Here, the evaporation according to Priestley-Taylor reveals its 'potential' character by overestimating daily rates, because it does not account for a restriction in surface moisture supply. Hence, the comparison with this method shows the advantage of the EVA results, but cannot give a quantitative assessment of their quality.

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Figure 5. Comparison of time series from 1989 to 1992 of daily rates of évapotranspiration as estimated by the EVA model (ETa) and by the reference method of Priestley-Taylor (ETpt) for the CORINE land cover class 211 (non-irrigated arable land)

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Figure 5. Comparison of time series from 1989 to 1992 of daily rates of évapotranspiration as estimated by the EVA model (ETa) and by the reference method of Priestley-Taylor (ETpt) for the CORINE land cover class 211 (non-irrigated arable land)

Secondly, point measurements of relevant parameters can serve for validation purposes to a certain extent, even if the difficult comparison of point values versus gridded model results is inherent to this approach. Unfortunately, only three stations were able to provide Class-A-Pan evaporation (Epan) measurements, and clearly pan evaporation is not a very suitable parameter to validate estimates of actual évapotranspiration; Epcm determines at best a potential rate of evaporation. The land cover type 'complex cultivation pattern' (CORINE land cover class 242) that is dominant within a 5 km by 5 km area surrounding each of the three stations, has been used for the model calculations. Epa„ measurements have been taken as representative for these areas. The comparison of Epan time series with ETa and ETpt for the station ofNoto (figure 6) confirms the quasi-potential evaporation rates computed with the Priestley-Taylor approach; Epa„ and ETPr are in good agreement. For the actual évapotranspiration, however, no quantitative assessment of the accuracy could be derived, since the measurements clearly did not experience any restriction in water supply in summer, and hence measure a distinctly different quantity than the one produced by the EVA model.

Comparison o(Pan Evaporation lo Model Results for Sicily 19S0 - 1992 (CORINE class 242)

Comparison o(Pan Evaporation lo Model Results for Sicily 19S0 - 1992 (CORINE class 242)

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Figure 6. Comparison of daily pan evaporation measured at the station Noto (Epan), south-east Sicily, with Priestley-Taylor evaporation (ETpt), and with daily actual évapotranspiration from the EVA model (ETa). For all ET calculations the CORINE class 242 (complex cultivation pattern) has been assumed as the underlying land cover type

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Figure 6. Comparison of daily pan evaporation measured at the station Noto (Epan), south-east Sicily, with Priestley-Taylor evaporation (ETpt), and with daily actual évapotranspiration from the EVA model (ETa). For all ET calculations the CORINE class 242 (complex cultivation pattern) has been assumed as the underlying land cover type

Thirdly, gridded model output of an independent Global Circulation Model (GCM) has been considered as a potentially suitable tool for validation. For this purpose, parameter fields of the daily energy fluxes at the surface level have been obtained from the TOGA Extension Data Set of the ECMWF in Reading, U.K. Five parameters, latent and sensible heat flux, solar and thermal radiation as well as evaporation, were available with spatial resolutions of 1.125° and 0.5° for the period from 1991 to 1992. For Sicily this resolution corresponds to a grid cell size of 50 km at best, which is rather coarse as compared to the EVA model resolution of a few kilometres (see figure 7). The comparison of the ECMWF model output with the EVA model results for 1991 is presented in figure 8. Here, average values for the land cover class 'non-irrigated arable land' are plotted for both models.

The evaporation estimates differ considerably in that the ECMWF output shows decisively lower values than the modelled The daily evaporation estimates are in agreement only in spring and for a few days during summer. However, it remains difficult to assess which model is right and which is wrong. Considering the coarse resolution of the ECMWF grid, it must be questioned whether surface conditions can be correctly represented for a re-

gion like Sicily, which is especially true for the 1.125° resolution in 1991. Soil moisture availability, vegetation types, or landuse are highly variable in space, and hence produce a highly variable pattern of resulting turbulent fluxes. Such patterns can certainly not be represented by the spatial resolution of the available ECMWF data.

Figure 7. Example of the ECMWF output resolutions (background grids) and comparison with the EVA model resolution (centre image) for Sicily in 1991/92 Until 16.09.91 the spatial resolution ofthe ECMWF data is 1.125°, from 17.09.91 on they are available at 0.5° resolution

Figure 8. Time series of daily evaporation from ECMWF, Priestley-Taylor (EPT) and EVA (ETa) models, and comparison of short-wave (SW) and long-wave (LW)

radiation from the ECMWF and EVA models for Sicily in 1991. Values shown for the EVA model correspond to average values of the parameters for the CORINE land cover class 211 (non-irrigated arable land)

Figure 8. Time series of daily evaporation from ECMWF, Priestley-Taylor (EPT) and EVA (ETa) models, and comparison of short-wave (SW) and long-wave (LW)

radiation from the ECMWF and EVA models for Sicily in 1991. Values shown for the EVA model correspond to average values of the parameters for the CORINE land cover class 211 (non-irrigated arable land)

The daily radiative fluxes on the right hand side of figure 8 are in better agreement than the evaporation. Both EVA results and ECMWF output show the same magnitudes and a corresponding course over the year. Within the ECMWF data a stronger scattering is visible; short-wave radiation is somewhat lower than the results of the EVA model, but the long-wave component has a similar average behaviour. Hence, for the incoming radiative fluxes that do not depend on the surface parameterisation, ECMWF data can be regarded as a suitable tool for validation. For evapotranspiration, however, the model output of a Global Circulation Model does not seem to be an appropriate means of validation, because the high spatial variability of the surface energy partitioning process is not sufficiently accounted for.

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