Evaporation Process and Factors

Evaporation

• Evaporation is the process of a liquid changing to a gaseous state at a free surface, below boiling point. Heat energy is transferred during this process.
• Evaporation is a cooling process; the latent heat of vaporization (approx. 585 calories/gram of evaporated water) is removed from the water body.
• Factors affecting evaporation rate:
  • Vapor pressures at the water surface and the surrounding air
  • Air and water temperatures
  • Wind speed
  • Atmospheric pressure
  • Water quality
  • Size of the water body
• Sublimation is the transformation from solid to vapor without passing through a liquid state.
• Evaporation in the atmosphere occurs from free water surfaces (seas, lakes, rivers), moisture in solid surfaces (land, soil, vegetation, snowfields, glaciers), and falling rain drops.

Process of Evaporation

• Water molecules are constantly in motion, with velocity dependent on temperature.
• Intermolecular attraction causes surface molecules to escape into the atmosphere.
• Molecules with higher kinetic energy overcome attraction forces and evaporate (a process known as evaporation).
• Evaporation is a cooling process because escaping molecules carry heat energy.
• Molecules continuously move between the water surface and the atmosphere.
• Net movement of molecules from water to air is evaporation.
• When condensation (molecules returning to the water) exceeds evaporation, it leads to condensation.
• Evaporation is measured in terms of depth of water per unit area per unit time, e.g., mm/m²/h.

Necessity of Estimating Evaporation

• Large amounts are spent in constructing dams and reservoirs to store water and canals to carry water.
• Significant water is lost due to evaporation from reservoirs and canals (approximately 30%).
• Estimating evaporation loss is crucial to assess the available water for use at different times of the year.

Dalton's Law of Evaporation

• A thin layer of air saturated with vapor is immediately adjacent to the water surface.
• Vapor pressure is proportional to the difference between the saturation vapor pressure at the water temperature (e_w) and the actual vapor pressure in the air (e_a).
• The rate of evaporation is proportional to (e_w - e_a).
• Evaporation continues until e_w = e_a, i.e., saturation vapor pressure equals the actual vapor pressure, leading to a cessation of evaporation.
• The formula for Dalton's Law is commonly expressed as E_L = C (e_w-e_a) where C is a constant.

Factors Affecting Evaporation

• Temperature: Evaporation increases with increasing water and air temperatures. Relationship is complex as it depends on the difference in vapor pressure between the water and air.
• Solar Radiation: Evaporation relies on solar energy to heat the water. The rate changes throughout the day, night, and seasons depending on sunlight.
• Wind: Wind removes water vapor from the air, increasing evaporation. The impact is greater with higher wind speeds, but only up to a certain limit.
• Barometric Pressure and Altitude: Lower barometric pressure and higher altitudes increase evaporation due to reduced atmospheric pressure, although temperature changes can partially counteract this.
• Quality of Water: Dissolved solids in water reduce evaporation because they lower vapor pressure. Seawater evaporates more slowly than freshwater (2-3% decrease).
• Depth of Water: Deeper water absorbs more heat, potentially decreasing evaporation in warmer months and increasing it in colder months.
• Other Factors: Surface shape, surface area, color, flow velocity, and wave presence also influence evaporation, but their precise effects are not yet fully understood.

Evaporimeters

• Evaporimeters are water-containing pans exposed to the atmosphere.
• Evaporation loss in the pans is measured at intervals.
• Meteorological data (humidity, wind movement, air and water temperatures, precipitation) is also recorded.
• Common types of evaporimeters include Class A, ISI Standard, Colorado Sunken, and US Geological Survey Floating Pans.

Empirical Evaporation Equations

• Many empirical equations estimate lake evaporation using readily available meteorological data.
• Most are based on the Dalton type equation
• Common formulas include Meyer's Formula and Rohwer's Formula.
• These formulas use temperature, wind speed, and vapor pressure differences.

Transpiration

• Transpiration is the process where water from a plant leaves as vapor, exiting through the leaves.
• Factors affecting transpiration: vapor pressure, temperature, wind, light intensity, and plant characteristics (root and leaf systems).
• Transpiration is largely confined to daylight hours and is different from evaporation (which occurs night and day).
• Measurements of evapotranspiration can occur using lysimeters (special watertight tanks containing soil) or field plots.

Control of Evaporation from a Reservoir

• Evaporation from reservoirs can be reduced by up to 30% using various methods.
• Surface area reduction methods include minimizing reservoir surface area.
• Mechanical covers (floating materials above the surface) can also help reduce evaporation.

Explore the fascinating process of evaporation, where liquid transforms to gas. This quiz covers the key factors affecting evaporation rates, including temperature, wind speed, and atmospheric pressure. Understand how evaporation plays a vital role in nature and climate.