Jun 7, 2017-Around the second week of June, if conditions are normal over the Asiatic mainland and surrounding water bodies, the South Asian Monsoon (SAM) enters Nepal and covers the entire country over a period of about two weeks. It starts to weaken by the third week of September, then departs, returning the following June. Westerly disturbances active from December to February bring some rains to Nepal. Rains from these atmospheric systems sustain the regional hydrology that supports the lives and economy of about 650 million inhabitants who live in the Ganga River Basin (GRB), which includes all of Nepal and parts of China, India and Bangladesh.
Not all parts of the GRB receive the same amount of rainfall, however. Wide spatial variations that exhibit intra- and inter-seasonal and inter-annual variability exist. The monsoon keeps the regional hydrological cycle going, which begins with the interception of precipitation by vegetation, storage in soil, infiltration to groundwater aquifers and surface runoff to streams. The network of streams and rivers convey water to the ocean and seas. Within this cycle, dry spells, high flows and sedimentation are regular features that produce droughts and flood disasters. Uneven social structures, differential local economies and poor governance in the countries within the GRB worsen the impact of these disasters.