By: Tayyaba Yousaf
Natural Evaporation is a pprocess by which water changes from a liquid to a gas or vapor. Evaporation is the primary pathway that water moves from the liquid state back into the water cycle as atmospheric water vapor. It is called naturaevaporation.
Harold Penman published his transformational paper in 1948, virtually no attempt had been made in applying mechanistic expressions to the challenge of predicting water loss via evaporation from natural surfaces. Therefore, there was little guidance in approaching the problem. He suggested that resolving evaporation expressions for open water surfaces was the primary step in developing expressions for bare soil and grass. His stated aim was “to approach the problem of the dependence of evaporation from bare and cropped soil on weather conditions through a study of evaporation from open water, seeking an absolute relation between weather elements and open water evaporation, and comparative relations between losses from the soil and losses from open water exposed to the same weather” Penman realized that there were two considerations in attempting to describe open water surface evaporation:
“Two requirements must be met to permit continued evaporation. There must be a supply of energy to provide the latent heat of vaporization, and there must be some mechanism for removing the vapor, i.e., there must be a sink for vapor”. The idea of estimating evaporation using an energy balance approach was originally presented by Cummings and Richardson (1927). Penman fleshed out the components of the energy balance to solve for water surface evaporation. He combined the basic energy balance equation with the sink strength equation to derive what he called the “combination” equation. The combination equation is the familiar Penman energy?balance equation widely used in defining a potential, or reference, evaporation. This equation includes additive terms dependent on net radiation and on vapor pressure deficit.
Attempts by Penman to relate the evaporation predictions to natural surfaces showed considerable variability. For conditions of “freshly wetted bare soil” (Penman, 1948, p. 137), the evaporation rate was 90% of that of an open water surface. However, for other conditions, the bare soil evaporation was overpredicted by the open water surface model, with the base soil evaporation decreasing to 54% of the open water surface. Penman did not offer a solution in accounting for soil drying in estimating bare soil evaporation. Evaporation rates from wet bare soil and from turf with an adequate supply of water are obtained as fractions of that from open water, the fraction for turf showing a seasonal change attributed to the annual cycle of length of daylight.
The ratio of evaporation from a grass surface to open water was even more variable than that observed for the ratio of bare soil to water surface evaporation. In a 2-yr study, Penman divided the season into several periods of roughly 2 to 4 week and calculated the ratio of turf evaporation to open water evaporation. The ratio turned out to be quite variable, with the ratio ranging from 0.31 to 0.98. When he estimated the ratio over periods of several months, the mean of the ratio became more stable, ranging from 0.6 in midwinter to 0.8 in midsummer. Penman suggested that the annual value for the ratio was 0.75. However, he offered no solution to overcome the variability when making evaporation estimates for short time periods relevant to agronomic practices for most crops.
He finally concluded for natural surfaces that “there must inevitably be something of the time and place at which the experimental work was done included in the equations”
In an attempt to account specifically for transpiration from leaf canopies, John Monteith (1964) modified Penman’s energy balance equation by including a canopy conductance term. This modification was based on the implicit assumption that all, or nearly all, crop water loss was due to transpiration from the plant canopy, and that the conductance term reflected leaf regulation of transpiration by stomata. Hence, the Penman–Monteith equation was actually derived as a description of canopy transpiration rate. Crop canopy conductance, however, remains a major unknown, since its valuation is essentially unresolved in representing canopy water loss and no straightforward approach has been developed to obtain in situ estimates of canopy conductance.