Evaporation and infiltration

Evaporation and infiltration are the two biggest losses between rain falling and water arriving at a reservoir or stream. Evaporation turns water into vapour from open water and soil; transpiration does the same through plants; together they form evapotranspiration. In hot, dry parts of India, reservoirs can lose a large depth of water each year to evaporation, a major item in reservoir planning.

Evaporation depends on temperature, humidity, wind, solar radiation and water quality. It is measured with pans, such as the Class A pan, and the pan value is multiplied by a pan coefficient, typically around 0.7, to estimate lake evaporation. It can also be calculated from energy balance or mass transfer methods, or combined methods like Penman. Evaporation from reservoirs can be reduced by monomolecular films, wind breaks, reducing exposed area or floating covers on small storages.

Infiltration is water entering the soil surface. Its rate starts high and decreases towards a steady value as the soil wets up. Horton's equation describes this decay; the Green-Ampt and Philip models are more physically based. Infiltration depends on soil texture, structure, moisture, vegetation, compaction and surface sealing. It is measured with double-ring infiltrometers in the field. In flood estimation, infiltration is often lumped into a φ-index. For groundwater recharge, rainwater harvesting and soak pits, high infiltration is what we want.