--- title: "Importance of hydrology in Nepal, global water balance" subject: "Engineering Hydrology" chapter: 1 key: "semester_5/engineering_hydrology:hydrology-importance-global-balance" source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/hydrology-importance-global-balance site: BCT Engineering --- # Importance of hydrology in Nepal, global water balance Engineering Hydrology · Chapter 1: Introduction - Asked in: 2070 Asar, 2+2 marks - Video: [Introduction to Engineering Hydrology and its Applications [Year - 3]](https://www.youtube.com/watch?v=Sds3dB-hA8E) — Mtutor & Mschool - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/hydrology-importance-global-balance - Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-1 ## Question Why is the study of hydrology important for engineers for planning and designing of water resources projects in Nepal? Explain the significant features of global water balance with necessary equation. ## Short answer In Nepal, hydrology is essential for designing hydropower, irrigation and flood control because 80% of rainfall occurs in monsoon. It estimates design floods, dry-season flows and GLOF risks. The global water balance features that ocean evaporation exceeds precipitation, land precipitation exceeds evaporation, and the surplus returns as runoff (P_l – E_l = R), while globally total evaporation equals precipitation (P_global = E_global). ## Answer #### 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%. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is hydrology important for Nepal's water projects? A. Rainfall is uniform all year B. Nepal has no rivers C. Rainfall is highly seasonal, about 80% in June to September D. Nepal has no snow Answer: C. Seasonal flow makes estimates of flow and floods essential. (Key point: Seasonality) 2. What fraction of earth's water is fresh? A. About 50% B. About 97.5% C. About 25% D. About 2.5% Answer: D. 97.5% is saline. (Key point: Fresh water) 3. For oceans, which holds? A. Evaporation exceeds precipitation B. Precipitation exceeds evaporation C. They are equal always D. No evaporation occurs Answer: A. Ocean E > P and land P > E. (Key point: Ocean balance) 4. Over the whole earth: A. Evaporation exceeds precipitation B. Total evaporation equals total precipitation C. Precipitation exceeds evaporation by runoff D. There is no balance Answer: B. P_global = E_global; land surplus returns as runoff. 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