The prime sources of rain in Nepal is the monsoons blowing from the Bay of Bengal. These monsoons blowing from south-east direction into Nepal are forced to rise up along the slopes of mountains and hills. These contribute major proportion (60% to 90%) of the total annual rainfall received by various geographic regions of the country.
As regards its regional distribution, the terai or madhesh region receives moderate rain (100 cm. to 170 cm.). But all terai or madhesh sections do not get the same amount of rain. It generally declines from east towards west. The eastern section gets on an average 170 cm., the central section 150 cm. and the western section less than 150 cm. The reason for this variation is that the moisture bearing monsoons blow from south-east direction. As it proceeds more and more towards west, it contains less and less amount of moisture, because much of its water vapour is spent in eastern parts of the country.
In its distribution from south towards north, it increases and decreases irregularly, depending on relief features. The windward slopes of the Mahabharat and Churia Ranges receive higher rainfall, ranging from 200 cm. to 250 cm. The east-west river basis such as those of Sunkoshi, Gandaki and Karnali lying on the lee side of the ranges get slightly lesser amount. The midland region too, leaving a few exceptional cases, receives moderately high amount of rain, generally about 150 cm. From there northward to the Himalayan region, rain increases in general and is true for central and eastern sections. Some of the well-exposed slopes of the Himalayan Ranges receive as high as 200 cm. Likewise, certain transverse river basins such as those of upper Arun and upper Sunkosi in the eastern sector permit the entry of moisture laden monsoons and get copious amount of rain (over 300 cm.). But the parts higher than 5,000 m. receive less than 100 cm. and that too is mostly in the form of snow. The reason for this low amount of precipitation is that those lofty parts lie above the level of monsoons. This then shows that the distribution of rain along north-south direction is very uneven.
Agriculture of Nepal depends on the monsoon as irrigation is done by the monsoon rain. Monsoon rain wets ground and helps to maintain the level of underground water. Rainfall also helps to produce hydroelectricity in our country.
Sometimes excessive rainfall causes flood, landslide and soil erosion in Nepal. For that disasters, people need to get ready for different environmental conservation methods like afforestation.