Climate and hydrology

The Sava River Basin is characterized by the dominant moderate climate of the northern hemisphere, modified by the influence of the relief. Alpine climate prevails in the upper Sava Basin in Slovenia, moderate continental climate in the right tributaries’ catchment areas within Croatia, Bosnia and Herzegovina, Serbia and Montenegro, while moderate continental, mid-European climate primarily features in the left tributaries’ catchments in the Pannonian Plain. Cold and hot seasons are clearly defined. The winters can be severe with abundant snowfalls, while the summers are hot and long.
The precipitation amount and its annual distribution is fairly variable. The largest precipitations take place in far western catchments (Sava Dolinka and Sava Bohinjka Rivers) and at upper parts of catchments of the Kupa, Piva, Tara, Una, Vrbas, Drina and Lim Rivers. Areas with smallest precipitation are found in Slavonia, Srem, Semberija and the Kolubara River catchment.

Basic characteristics

Average annual air temperature approx. 9.5°C
Mean monthly water temperature lowest January: -1.5°C
highest July: 20°C
Average annual rainfall approx. 1,100 mm
Long-term average annual precipitation 600 mm up to 2,300 mm
Average evapotranspiration approx. 530 mm/year
Spatial distribution of runoff 150 mm/year (under 5 l/s/km2) up to 1,200 mm/year (almost 40 l/s/km2)
Average discharge of the Sava River
at the confluence
approx. 1,700 m3/s

In general, runoff largely follows pattern of spatial distribution of precipitation.
The right tributaries of the Sava River are characterized by much higher water yield:
  • Una River     - 23 l/s/km2
  • Vrbas River  - 19 l/s/km2
  • Bosna River - 19 l/s/km2
  • Ukrina River - 12 l/s/km2 
  • Tinja Rivers  - 12 l/s/km2
  • Drina River   - 40 and 50 l/s/km2as the largest tributary of the Sava River, the Drina River has a remarkably high water yield due to high precipitation, the long-term annual average is over 2,000 mm
The left tributaries (Krapina, Lonja and Orljava and Bosut Rivers) gets annually 700 – 1,000 mm of rain but relatively big evapotranspiration reduces unit-area runoff to just a few l/s/km2, which at the hilly regions can rise to 12 l/s/ km2.

A long-term average unit-area-runoff for the complete catchment area is of about 18 l/s/km2.