Hydrology and climate

    Hydrology

The Czech Republic's only source of water is from atmospheric rain and snowfall. A11 the rivers, which have their source in Czech Republic territory, drain into neighbouring countries.

Three European watersheds cross the Czech Republic:

-  The Labe (Elbe) which drains into the North Sea (51,399 sq. Km).

-  The Odra (Oder) which drains into the Baltic Sea (4,721 sq. Km).

-  The Dunaj (Danube) which drains into the Black Sea (22,744 sq. Km).

North of the Moravskoslezské Beskydy is a ridge (peak: Petřkovická hůrka, 608m O.D.) separating the watersheds of the Baltic and Black Seas. To the north of this ridge the River 1. Odra flows to the Baltic Sea over a distance of 135 km on Czech Republic territory. To the south of this rim is the River Morava which flows into the Danube to the NW of Bratislava and on to the Black Sea. The Morava is one of the longest rivers in the country (350km) and one of its largest affluents is the 120km long River Bečva that has its source in Vsetín. The main river flowing through Vsetín is the Bečva, and along with the Senice contain more than 100 tributaries . Figure  lists a time series of mean annual total run offs (1888-1995) but does not imply any long-term trend since total run offs vary considerably from year to year.

    Climate 

The Czech Republic lies in the temperate climate zone of Europe. Four seasons regularly reoccur and are characterised by warm showery summers and cold, snowy winters. Planetary circulation causes westerly winds to prevail.The central geographic position of the Czech Republic in Europe results in mixed influences of continental and oceanic climates. The oceanic influence mainly affects the western part of the Czech Republic with the mountain ranges there catching the clouds generated by the Atlantic Ocean. Precipitation and cloudiness are therefore greater here than in the east. Farther to the east of the Czech Republic the continental character of the climate intensifies. Winters are colder, summers warmer (see figure) showing the extremes of winter and summer in Valašské Meziříčí), the range of day and night temperatures greater.

The mean annual temperature in the warmest lowlands of southern Moravia doesn't exceed 10°C (Hodonín, 180m O.D. = 9.5°C). The lowest temperatures are recorded at the top of mountains (highest mountain Sněžka, 1602m O.D.) but the mean annual temperature still does not drop below freezing point in these areas (Praděd, 1491m O.D. =0.9°C) (see Figure 4).

The mean annual precipitation for the entire Czech Republic is 693mm (see Figure 3) .

Precipitation is distributed unevenly because of the strong influence exerted by the country's relatively complex orography (see figure). The mountainous border zones receive a significantly greater amount of precipitation (approaching 1500mm) while the driest areas receive just over 400mm annually (see figure).  

Figures 3 and 4 show time series of mean annual precipitation and temperature. They exhibit a slight decrease in precipitation totals since the 1920s and moderately rising mean annual temperatures from the beginning of the 20th century. It is difficult to distinguish between anthropogenic and natural influences in these two trends.

The country's complex orography is also the cause of the considerable variability in local and regional climates. During winter a typical occurrence is that of temperature inversions with average elevation approximately 300-500m above ground level. These inversions mainly occur in NW and central Bohemia and Ostrava region (north Moravia). Weather in the Czech Republic is very unstable. This is mainly caused by very variable air pressures resulting from the dependence of main air streams over Europe on changes in the three major pressure systems:

-  Pressure depression over Iceland

-  Pressure head over the Azores

-  Pressure conditions over Asia