--- title: "Hydrological losses statement, factors affecting evaporation" subject: "Engineering Hydrology" chapter: 3 key: "semester_5/engineering_hydrology:losses-not-actual-evap-factors" source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/losses-not-actual-evap-factors site: BCT Engineering --- # Hydrological losses statement, factors affecting evaporation Engineering Hydrology · Chapter 3: Hydrological Losses - Asked in: 2082 Bhadra, 2+3 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/losses-not-actual-evap-factors - Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-3 ## Question Justify the statement "Hydrological losses are not actual losses". Briefly describe factors influencing evaporation. ## Short answer Hydrological losses are not actual losses because water is only removed from direct runoff; infiltrated, intercepted and evaporated water remains in the hydrologic cycle. Evaporation is influenced by vapour pressure deficit, temperature, wind speed, solar radiation, atmospheric pressure, water quality, size/depth of water body and soil surface condition. ## Answer #### Hydrological losses are not actual losses Hydrological losses (interception, depression storage, evaporation, transpiration, infiltration) are "losses" only from the point of view of direct surface runoff. The water is not destroyed; it only moves to another part of the hydrologic cycle (conservation of mass). - Infiltrated water recharges soil moisture and groundwater and later appears again as baseflow in streams and springs. - Evaporated and transpired water goes to the atmosphere, forms clouds and returns as precipitation. - Interception and depression storage are eventually evaporated or infiltrated. - Water used by plants (transpiration) is useful for agriculture and forests. So these are only losses from the surface runoff volume; the water remains in the system and is available again. #### Factors influencing evaporation 1. **Vapour pressure deficit** $(e_s - e_a)$: evaporation follows Dalton's law, $E \propto (e_s - e_a)$. 2. **Temperature** of water and air: higher temperature raises $e_s$. 3. **Wind speed:** removes saturated air above the surface and increases turbulence. 4. **Solar radiation:** provides the latent heat (about 2.45 MJ/kg). 5. **Atmospheric pressure:** low pressure at high altitude increases evaporation. 6. **Quality of water:** dissolved salts reduce vapour pressure (sea water evaporates about 2 to 3 % less than fresh water). 7. **Size and depth of the water body**, and heat storage in deep lakes. 8. **Soil surface condition** (moisture, colour, cover) for evaporation from land. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why are hydrological losses called 'not actual losses'? A. The water is destroyed but quickly replaced by rain B. They are losses only with respect to direct surface runoff; the water stays in the hydrologic cycle C. They only occur in dry seasons D. They are measured wrongly in the field Answer: B. By conservation of mass the water is not destroyed; it moves to soil, groundwater or atmosphere and is available again. (Key point: Why losses are not real losses) 2. What happens to infiltrated water according to the answer? A. It recharges soil moisture and groundwater and later returns as baseflow B. It becomes surface runoff in the same storm C. It evaporates immediately D. It is permanently lost to the stream Answer: A. Infiltration feeds soil moisture and groundwater, which later appear as baseflow and springs. (Key point: Fate of infiltrated water) 3. According to Dalton's law, evaporation is proportional to which quantity? A. (e_s - e_a), the vapour pressure deficit B. Water depth C. Atmospheric pressure squared D. Wind speed only Answer: A. E is proportional to (e_s - e_a), the difference between saturation and actual vapour pressure. (Key point: Vapour pressure deficit) 4. How does dissolved salt affect evaporation? A. It doubles it B. It increases it by about 20 % C. It has no effect D. It reduces vapour pressure, so sea water evaporates about 2 to 3 % less than fresh water Answer: D. Dissolved salts lower the vapour pressure, so evaporation is slightly reduced. 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