Importance of hydrology in Nepal, global water balance

Importance of hydrology in Nepal

Nepal has more than 6,000 rivers and a large hydropower potential, but rainfall is highly seasonal (about 80% in June to September). Hydrology is therefore important to:

• Hydropower: estimate mean flow, flow duration and design flood for projects (Arun, Kali Gandaki, Upper Tamakoshi).
• Irrigation: find dry-season dependable flows for canals such as the Mahakali and Sunsari-Morang schemes.
• Water supply: assess the yield of springs and rivers for towns (Melamchi).
• Flood and landslide control: design embankments, bridges and culverts; handle monsoon floods and GLOF risk.
• Planning and climate change: assess snowmelt, glacier retreat and drought for sustainable use.

Global water balance

About 97.5% of the earth's water is saline (oceans) and only 2.5% is fresh, most of it locked in ice and groundwater. Features of the global balance:

• Evaporation from the ocean is greater than precipitation on it, and the reverse holds over land.
• The surplus vapour carried to land (about 10% of ocean evaporation) returns to the sea as river runoff.
• Oceans: $E_o > P_o$; land: $P_l > E_l$; and $P_l - E_l = R$.
• Over the earth as a whole, total evaporation equals total precipitation.

$$P_{global} = E_{global}, \qquad P_l - E_l = R_l \ (\text{runoff and groundwater to oceans})$$

Approximate values (per year): ocean evaporation about 85% and ocean precipitation 77% of the global cycle; land precipitation about 23% and land evaporation about 15%.