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Bulgarian Journal of Soil Science, Agrochemistry and Ecology
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Climatic characteristics of optimum Agro-climatic regions for maize irrigation in Bulgaria
Zornitsa Popova, Maria Ivanova, Emil Dimitrov, Ivan Varlev
Abstract: Precipitation and reference evapotranspiration calculated by the Penman-Monteith equation PM -ET0 are indispensable in crop irrigation requirements and irrigation scheduling studies. In our former research optimum Agro-Climatic (AC) regions have been identified by unification of the traditional 97 irrigation regions (Zahariev et al., 1986) on the basis of average reference evapotranspiration for July and August PM – ET0July-Aug relative to the period 1951-2004. The aim of present study is to provide climatic characteristics of these unified AC regions in the form of: a) maps with average precipitation and average reference evapotranspiration ET0 for the territory of the country and b) some quantitative analyzes of inter-seasonal and inter-regional variability of precipitation and ET0.
It has been found that ET0July-Aug totals in a normal year (having 50% probability of exceedance) range from 270 to 340 mm over Bulgarian plains (fig. 4b) while precipitation normality relative to the same months varies from 60 to 110 mm (fig. 3b). Moreover, precipitation totals for July and August in the dry years (having probability of exceedance P > 90%, same fig. 3b) are less than 10-20 mm.
The study also shows that monthly precipitation has a large “inter-annual” and also “inter-seasonal” variability with smaller precipitation in July, August and September when “flowering” and “yield formation” maize development stages occur (figs. 2a, 2c, 2e and 2g). Precipitation relative to “May-September” and peak demand “July-August” period are the lowest in AC region V (Sandanski, figs. 3a and 3b), followed by AC region IV (Plovdiv) and AC region III (Varna and Lom) and AC regions I (Sofia) and IV (Pleven) where seasonal precipitation are the highest ranging from 120 to 550-700 mm (figs. 3a and 3b).
Compared with precipitation, the variability of PM–ET0May-Sept relative to “May-September” is significantly lower. It is the highest in the extremely dry AC region V (Sandanski), varying from 740 to 840 mm over the studied 54-year period (fig. 4a). PM–ET0May-Sept is about 30-110 mm less in AC region IV (Stara Zagora, Plovdiv and Pleven), decreases by 70-90 mm more in the AC regions III and II (Lom, Varna and Silistra) and is the lowest in AC region I (Sofia). During the months of peak irrigation demands July and August the difference between locations in terms of PM–ET0July-Aug is about half than that relative to PM–ET0May-Sept (fig. 4b).
Spatial distribution of seasonal rainfall “April-September” is characterized by considerable non uniformity (fig. 5a). Most of Eastern regions (Silistra and Dobrich, AC region II) and those from the Black Sea coastal zone (Varna and Burgas, AC regions III and IV) are highly vulnerable to drought (Alexandrov (Ed.), 2011). In summer the extreme South-West part of the country (Sandanski and Petrich), as well as Haskovo and Svilengrad (AC region V), Plovdiv, and Burgas (AC region IV) and Sofia (AC region I), South Bulgaria, and a part of North Bulgaria (Silistra, Kavarna and Shabla – AC regions II and I) are the most vulnerable to drought due to the scarce precipitation (fig. 5b) combined with a high evapotranspiration demand ET0July-Aug (figs. 6b, 6c).
Statistical and graphical analyzes of ET0 in the unified AC regions show that the highest maximum ET0July-Aug values and the lowest summer precipitation totals are inherent to the Transition Mediterranean climate of AC region V (Sandanski, figs. 2i 2j, 3b and 4b). In summer the lowest minimum, maximum and average values of ET0 are inherent to the conditions of the Moderate continental/Black sea climate in the I (Sofia, fig. 2b), II (Silistra, fig. 2d,) and III (Varna and Lom, fig. 2f) AC regions. The highest ET0 values are found for the Transitional continental (Pleven and Plovdiv, AC region IV, fig. 2h) and Mediterranean climates (Sandanski, AC region V, fig. 2j).
Reliable 1:1 relationships (R2 ≥0.80, RMSE≤ 5.8 mm/month and a slope coefficient b ≥0.90, fig. 7), derived from monthly ET0 normalities relative to two periods (1951-2004 and 1981-2010), show that both data series could provide trusty characteristics of present climate conditions in June, July and August.
Keywords: agro-climatic regions; climatic maps; net irridiation; net requirements; precipitation; probability analyzes; reference evapotranspiration PMET0
Date published: 2018-10-03
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