6 Evaporation and its measurement
Evaporation
Evaporation is the process by which water transitions from a liquid to a gaseous state (vapor) at its free surface, occurring below the boiling point.
Process Mechanics:
- Heat-Driven Transition: Solar heat causes water molecules to gain energy, break free from the liquid surface, and enter the atmosphere as vapor.
- Vapor Pressure: The motion of these vapor molecules exerts pressure on the water surface, known as vapor pressure.
- Saturation Vapor Pressure: As more molecules accumulate, the air above the surface can become saturated. The partial pressure exerted by the water vapor at this point is called saturation vapor pressure. Saturation pressure increases with temperature.
- Evaporation Dynamics: Evaporation continues as long as the vapor pressure in the air is lower than the saturation vapor pressure. Once they equalize, evaporation ceases, and condensation may occur if the air's vapor pressure exceeds the saturation pressure.
Dalton's Law of Evaporation:
This law states that the rate of evaporation is proportional to the difference between the saturation vapor pressure (es) and the actual vapor pressure (ea) in the air above the water surface:
- E = C (es - ea)
- Where:
- E = evaporation loss (mm/day)
- es = saturation vapor pressure
- ea = vapor pressure in the air (approximately 2m above the surface)
- C = a coefficient influenced by factors like barometric pressure and wind velocity.
Conditions for Evaporation:
- Continuous water supply.
- Constant heat supply.
- Vapor deficit (es > ea).
Components of Total Evaporation:
Total evaporation (E) encompasses various sources:
- Surface evaporation.
- Water surface evaporation (oceans, rivers, ponds, lakes).
- Atmospheric evaporation.
- Transpiration. Transpiration is the process of water being lost from the leaves of the plants from their pores.
Factors Affecting Evaporation:
- Temperature of Air: Higher temperatures increase evaporation rates.
- Wind Velocity: Increased wind speed enhances evaporation by removing saturated air.
- Atmospheric Pressure: Lower pressure promotes higher evaporation.
- Nature of Evaporating Surface: Different surfaces (soil, vegetation, water bodies) have varying evaporation potentials.
- Area of Water Surface: Larger surface areas result in greater evaporation.
- Depth of Water Body: Deeper bodies evaporate slower than shallow ones.
- Humidity: Higher humidity reduces evaporation.
- Impurities in Water: Dissolved salts decrease evaporation.
Evaporation losses can significantly impact water availability, especially in arid and semi-arid regions.
1. Reducing Exposed Water Surface:
- Optimizing Reservoir/Pond Shape:
- Designing water storage facilities to minimize surface area-to-volume ratio. Deeper, narrower bodies of water lose less water to evaporation than shallow, wide ones.
- Dividing large water bodies into smaller, deeper sections.
- Underground Storage:
- Storing water underground in aquifers or constructed underground tanks eliminates direct exposure to the atmosphere, drastically reducing evaporation.
2. Physical Barriers:
- Floating Covers:
- Using materials like plastic sheets, polystyrene boards, or specialized floating modules to cover the water surface.
These create a physical barrier, preventing water vapor from escaping.
- Using materials like plastic sheets, polystyrene boards, or specialized floating modules to cover the water surface.
- Shade Structures:
- Constructing shade structures using shade cloth, solar panels, or other materials to reduce solar radiation reaching the water surface.
This lowers water temperature and evaporation rates.
- Constructing shade structures using shade cloth, solar panels, or other materials to reduce solar radiation reaching the water surface.
- Windbreaks:
- Planting trees, shrubs, or erecting wind fences around water bodies to reduce wind velocity, which significantly contributes to evaporation.
- Planting trees, shrubs, or erecting wind fences around water bodies to reduce wind velocity, which significantly contributes to evaporation.
3. Chemical Methods:
- Monolayer Films:
- Applying thin films of chemicals, such as fatty alcohols, to the water surface.
These films create a barrier that reduces water molecule escape. However, environmental impacts must be carefully considered.
- Applying thin films of chemicals, such as fatty alcohols, to the water surface.
4. Water Management Practices:
- Efficient Irrigation:
- Using irrigation techniques that minimize water waste, such as drip irrigation or sprinkler systems with precise control.
- Using irrigation techniques that minimize water waste, such as drip irrigation or sprinkler systems with precise control.
- Leak Detection and Repair:
- Regularly inspecting and repairing leaks in water distribution systems to prevent water loss.
- Water Conservation:
- Promoting water conservation practices in households, industries, and agriculture to reduce the demand for stored water.
Key Considerations:
- The effectiveness of each method varies depending on factors like climate, water body size, and cost.
- Environmental impacts should be carefully evaluated before implementing any evaporation reduction method, especially chemical treatments.
- A combination of methods often provides the most effective solution.
By implementing these strategies, it's possible to significantly reduce evaporation losses and conserve valuable water resources.
Measurement of Evaporation:
- Pan Measurement Method:
- Uses evaporation pans (e.g., U.S. Class A, Colorado Sunken, I.S. Standard) to measure water loss.
- Requires a pan coefficient to adjust pan evaporation to actual lake evaporation.
- Uses evaporation pans (e.g., U.S. Class A, Colorado Sunken, I.S. Standard) to measure water loss.
- Empirical Formulae:
- Utilizes equations (e.g., Mayer, Rohwer, Lake Hefner) based on meteorological data.
- These equations are based on Dalton's law.
- Water Budget Method:
- Balances water inflows, outflows, and storage changes to estimate evaporation.
- Energy Budget Method:
- Analyzes incoming and outgoing energy to determine the energy used for evaporation.
- Uses Bowen's ratio to calculate sensible heat loss.
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