The annual rainfall in Karnataka varies roughly from 50 to 350 cm. In the districts of Bijapur, Raichur, Bellary and southern half of Gulbarga, the rainfall is lowest varying from 50 to 60 cm. The rainfall increases significantly in the western part of the state and reaches its maximum over the coastal belt. The south-west monsoon is the principal rainy season during which Karnataka receives 80% of its rainfall. Rainfall in the winter season (January to February) is less than one per cent of the annual total, in the hot weather season (March to May) about 7% and in the post-monsoon season about 12%.
South-West monsoon normally sets in over the extreme southern parts of Karnataka by about 1st of June and covers the entire state by about 10th of June. The rainy months July and August account individually to about 30% and 18% of annual rainfall. There are about 26 rainy days (with daily rainfall of atleast 2.5 mm) in the south-west monsoon begins from the northern parts of Karnataka around 2nd week of October and by the 15th October monsoon withdraws from the entire state.
The retreating monsoon current i.e. the north-east monsoon (October to December) effects the eastern parts of South Interior Karnataka and accounts for about 30% of rainfall in this region. Out of the 14 heavy rainfall stations in India, with annual rainfall of more than 500 cm., four stations are situated in Karnataka. They are Agumbe in Tirthahalli taluk of Shimoga district (annual rainfall-828 cm) and Bhagamandala (603 cm), Pullingoth (594 cm) and Makut (505 cm) in Kodagu district.