Annual potential evapotranspiration
Potential evapotranspiration (ETo) includes both the potential evaporation from the soil surface and the transpiration by plants. The rate of ETo depends on the existing climatic conditions including radiation energy of the sun, wind speed, air temperature, vapor deficit of the air, and temperature. ETo values are obtained when the water available for this process is non-limiting. In case that soil water availability is limited then the actual evapotranspiration (ETa) occurs receiving values lower than ETo. The ratio of ETa/ETo can be used as an important indicator for assessing aridity of an area. Furthermore, another important climatic index of the average water available in the soil is the ratio between mean annual precipitation (P) and mean annual evapotranspiration (ETo). These are critical environmental factors affecting the evolution of natural vegetation. Rainfall and soil water availability in comparison with ETo values can be used as determinant factors for assessing desertification vulnerability of an area.
The potential evapotranspiration rate was mainly calculated for the various study sites from meteorological data received from the National Meteorological Services. The Penman-Monteith method as modified by Allen (1986) has been applied using the following meteorological data: air temperature, relative humidity, wind speed, and solar radiation. The following classes of potential evapotranspiration have been considered in this study: (a) ETo<500 mm, (b) ETo ranging from 500-800 mm, (c) ETo ranging from 800-1200 mm, (d) ETo ranging from 1200-1500 mm, and (e) ETo>1500 mm.
Data for this indicator were calculated for 1339 field sites, corresponding to 15 study sites (Table 1). Based on the obtained data (Fig. 6), the majority of the study field sites (45.2% of the study field sites have dry climatic conditions with annul potential evapotranspiration rates ranging between 1200-1500 mm per year, in the study sites areas of Novij Saratov-Russia, Djanybek-Russia, Secano Interior-Chile, Santiago Island-Cape Verde, Guadalentin Basin Murcia-Spain, Zeuss Koutine-Tunisia, and Crete-Greece. The next most important class of potential evapotranspiration rate (ETo 800-1200 mm/yr) was defined in 28.2% 0f the study field sites, corresponding to the study sites of Boteti Area-Botswana, Novij Saratov-Russia, Djanybek-Russia, Santiago Island-Cape Verde, Mação-Portugal, Cointzio catchment-Mexico, Gois-Portugal, and Crete-Greece. The wettest climatic conditions were found in 21.0% of the study field sites, corresponding to the study sites of Nestos Basin Maggana-Greece, Eskisehir-Turkey, Mamora Sehoul-Morocco, and Konya Karapinar plain-Turkey. Finally the driest climatic conditions with annual potential evapotranspiration rate greater than 1500 mm was found in 5.5% of the study field sites, corresponding to the study sites of Boteti Area-Botswana, and Zeuss Koutine-Tunisia.
Fig. 6. Distribution of potential evapotranspiration rate prevailing in the study field sites
