The method of indirect evapotranspiration measurements was based upon aerodynamic (profile) approach with use of the Automated System for Data Sampling and Assimilation (ASDSA) operated at Valday, Russia. Sensors of the system were installed on stake masts above a meadow grass surface. Measurements and data processing were carried out with a computer control. Solar radiation and long-wave radiation, air temperature and air humidity, wind speed and precipitation rates were determined half-hourly by using the ASDSA sensors.

The actual evapotranspiration rates were computed by using a simple turbulent transfer scheme. The potential evaporation was defined by Penman's approach with using of the radiation flux data. Received are the data of daily courses of evaporation and main factors conditioned it during a summer season (Figure 1).

The climatic indices of wetness are then evaluated, such as well-known Budyko's index (ratio of the net radiation income and latent heat flux R/LE), the Bowen ratio, the ratio of actual and potential evaporation (E/E0). The latter is suggested as the most suitable to operational estimating the climatic wetness indices. Daily evaporation ranges from 0,9 to 5 mm, and potential evaporation from 2,5 to 12 mm. The ratio of E/E0 oscillates from 0,25 to 0,85 around its average value equal to 0,5. Correlation (as well as discordance) between diverse climatic indices (Figure 2) are of particular interest.

The values received may be expanding to a large scale, based upon suitable spatial averaging the common meteorological data and on remotely sensed radiation flux and so-called vegetation index. Actual problems however remain when going on evaporation measurements.

• Choice between two versions: either detail profile measurements, or using a bulk-scheme, which assimilates meteorological data only at a single height and on surface.
• Experimental estimating the boundary conditions (surface temperature and air humidity). It is very important to obtain the moisture of the uppermost soil layer to assign the near-surface humidity (Monsi et al., 1990). Perhaps only the remote sensing (Kalma and Jupp, 1990) would be preferable.
• Research in transpiration activity of the plant cover. An alternative appears to be again either to study it in all details, or only to evaluate remotely the vegetation index (VI) as corresponding to biophysical parameter of plant communities.
• More precise investigation of the relationship between evaporation and soil water content since it affects the spatial variability in evaporation (Shutov, 1998).

References

• Kalma, J., Jupp, D. (1990) Estimating evapotranspiration from pasture using infrared thermometry. - Agr. and Forest Meteorol., vol. 51, No 3-4, 223-246
• Monsi, N., Hamotani, K. and Omoto, Y. (1990) Dynamic bechavior of the moisture near the soil-atmosphere boundary.- Bull. of Univ. Osaka, vol.. 42, pp. 4-69
• Shutov, V.A. (1998) Experience and problems of experimental studies of evaporation from land surface. - Meteorol. and Hydrol., No 1, 82-93 (in Russian)
• Shutov, V.A. (2000) Direct and indirect evapotranspiration measurements and data processing. - In: Catchment hydrological and biochemical processes in the changing environment. Proc. of the Liblice Conference (22-24 Sept., 1998). IHP-V Technical Documents in Hydrology, No. 37. UNESCO Press, Paris, 237-249

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