Nature park Persina: Abiotic factors
According to the classification of climatic regions of Bulgaria the Svishtov-Belene lowlands lies in the temperate climatic zone. It represents a continuation of the Middle European temperate climatic zone. In that region the climate is formed by the impact of the wet oceanic aerial masses, transformed by the local relief. During the cold period the climate is strongly affected by the continental air, penetrating from the nonheast. The influence both of cold arctic and warm tropical climatic zones is negligible.
The reference climatic zone is characterised by strong frosts during the winter and dry hot periods during the summer. Mean monthly temperature in January is -1,8° and during the honest period – in July – it is 23,7°C. The mean annual temperature is 11,8°C. In that pan of the Danube plain the mean annual temperature amplitude is among the most highest ones for Bulgaria -25,5°C. Both the maximal and minimal extremes affect negatively the vegetation period of plant species. The high temperatures during the summer season cause increasing the transpiration and exhausting of the soil water, and the low temperatures during the winter season cause inversion phenomena.
The wind expression in the Danube plain and the predominant wind direction are determined not only by the situation and the migration of anticyclones, but also by the specific relief influence. The western and nonhwestem winds, related to the wet oceanic air transport predominate in the Svishtov – Belene lowlands. They are especially strong in the back part of the passing cyclones when ripening the clouds and decreasing of rainfalls is observed.
During the warm season these winds cause rapid soil drying. The influence of winds with eastern component is considerably lower. They are result of the anticyclonal formations beyond the Eastern Europe and by the barric depression in the aquatory of the Adriatic sea. Sometimes these winds meet cyclones moving to the east and usually such climatic situations in the early spring and in the autumn precede the abundant rainfalls. The mean number of the fog days is 43,2.
Different soil type could be found in the region of the park. At the upper parts around Svishtov and Nikopol and the terraces above the riverine forests, the carbonate Chemozems are predominant. The Humofluvisols and the Gleysols occupy the Danube islands and the riverine terraces, and the Fluvisols occur sporadically on the islands.
The Chemozems are characterised by a deep A horizon (50-60 cm), and the total depth ofA-B horizons could reach 100-150 cm. The mechanical composition of these soils is medium – to heavy silt-clay. The humus content in the wild Chemozems is 4-6%, and in the agricultural ones – 34%.
The Fluvisols are relatively young soil types and they are still at their initial stage of soil formation. The periodic washing-out by running water or floods and also new accumulations do not allow stable vegetation occupation and development of soil formation process. No genetic horizons are formed, but different layers only and primary humus (A) horizon is weakly formed. The soil characteristics depend completely on the composition of the layers accumulated. Typical vegetation formations on these soils are the meadows and some marshy communities.
The Humofluvisols are formed mainly on Fluvisol deposits. They have well-differentiated humus-accumulation horizon, with continuous transition into C-horizon, and deeply (below 100 cm), a process of sieving could be observed – profile A-N-G. The difference from the previous soil types consist mainly in the fact, that they are more distant from the river bed, the flooding period is shorter, and the underground waters are at depth about 1.5 m. The typical vegetation here is represented by hygrophytic meadow communities.
The Gleysols are formed mainly under the influence of specific hydrological factors. The underground waters are high and less mobile and this results in lack of oxygen and low fluctuation of their level. Also the salination processes are typical for these soil types, because of the underground waters salination. From the evolutionary point of view they represent the next stage after the Humofluvisoils. The soil profile is usually A-Go-Gr. Two gleyic horizons could be observed and the second one is wet and over-supplied by water.
The Histosols are organogenic soils, characterised by deeper than 30 cm peat horizon, containing more than 30 % organic matter and the profile structure is T-G. The main factor of the soil formation is the presence of shallow water layer on their surface in some abasements. Also very important condition is the water-impenetrable soil materials (mainly glay), or the presence of a solid rock beneath. In such a situation mainly marsh vegetation does occur -Phragmites. Scirpus, Typha, mosses etc. The process of soil formation undergoes two stages -peat accumulation and gleyine.
The territory of interest is limited between two riverbeds – the lower course of the Ossam River (the eight largest Bulgarian river) and the small river called Barata. The close disposition 2
of the Danube river, water penetrability of the soils and the plainy relief form one of the lowest water network densities for Bulgaria – below 0.2 km/km2 These factors also affect the low flow module – below 1 litre/sec/km2.
The mean annual precipitation for the territory of the Svishtov – Belene lowlands is 540 mm, which is close to the mean value for Bulgaria – 600 mm.
The precipitations are characterized by summer maximum 181 mm (month June – 85 mm and winter minimum of 101 mm (month February – 26 mm. Solid rains fall during the period December – March. The middle number of days with snow cover is 47,6.
Considerable changes in the natural hydrological regime of the territory are due to the dike construction works of the Svishtov – Belene lowlands near the Danube river, that resulted in 41 km coastal embankment as late as 1947. The direct link of the bogs with the Danube River is interrupted, which cause decreasing of their water level and drying.
The embankments also stopped the regular flooding of the lowlands during the high spring waters of Danube. These destructive consequences were forced also by the construction of Belene irrigation and drainage system. This system caused additional drying of 150 000 dka of the former flooded lands and marshes. The system is divided into two zones, containing 2 drainage, 3 irrigation and 7 zonal pump stations and a network of drainage and irrigation channels.