Evaporation is a fundamental process in the hydrological cycle, playing a critical role in the redistribution of water on the Earth's surface. In hydrology and water resources engineering, understanding evaporation is essential for estimating water losses from reservoirs, lakes, irrigation systems, and other open water bodies. Evaporation also impacts agricultural planning, dam design, and catchment area management. This chapter deals with the principles, factors affecting, measurement, and estimation methods of evaporation in detail.

Evaporation is the process by which water changes from liquid to vapor phase due to the absorption of energy (usually from solar radiation). It primarily occurs at the surface of water bodies and moist soils. The rate of evaporation depends on climatic conditions and surface characteristics.

Several factors influence the rate of evaporation:
1. Solar Radiation: Primary energy source for evaporation. Directly proportional to evaporation rate. Greater solar radiation increases the energy available for converting water to vapor.
2. Temperature: Higher air and water temperatures accelerate evaporation. Warm air holds more moisture, enhancing the gradient for vapor movement.
3. Humidity: Inversely related to evaporation. High relative humidity reduces the vapor pressure gradient between water surface and air.
4. Wind Speed: Wind removes the saturated air layer near the water surface, maintaining a high vapor pressure gradient. Increases evaporation rate significantly in open water bodies.
5. Atmospheric Pressure: Lower atmospheric pressure reduces the boiling point of water, thereby increasing evaporation.
6. Water Quality: Impurities and dissolved salts can alter evaporation rates. Saline water evaporates slower due to reduced vapor pressure.
7. Surface Area: A larger exposed surface area leads to higher total evaporation.

Evaporation can be measured using direct or indirect methods. One common method is the Class A Evaporation Pan: a standard instrument made of galvanized iron. This pan is installed in an open area and the daily evaporation is measured by changes in water level. However, evaporation from a pan is generally more than from a natural water body due to greater exposure and heating. Other methods include:
1. ISI Standard Pan - Modified for Indian conditions.
2. Floating Pan - Provides a more accurate representation of real conditions as it's placed on the water's surface.
3. Lysimeter - Measures evaporation from soil and plants (evapotranspiration).
4. Atmometer - Also known as an evaporimeter, measures evaporation from a wet porous surface.

When direct measurement is not feasible, evaporation is estimated using empirical or analytical formulas. Key methods include:
1. Water Budget Method: Uses the continuity equation: E = I + P – O – ΔS (where E is evaporation) to assess water losses in larger bodies of water.
2. Energy Budget Method: Calculates evaporation based on energy conservation principles using net radiation and the energy consumed in different processes.
3. Penman Equation: Combines energy and aerodynamic factors into a formula that accounts for various atmospheric conditions.
4. Empirical Formulas: Such as Mayer’s Formula and Rohwer’s Equation, which depend on local measurements.

In regions with water scarcity, it is crucial to reduce evaporation losses, especially from reservoirs and canals. Techniques include:
1. Physical Methods: Such as using floating covers or shading (like trees).
2. Chemical Methods: Using monomolecular films to suppress evaporation by covering the water surface.
3. Structural Measures: Enhancing reservoir design by deepening, lining, or adding windbreaks.