Penman Evaporation Research Articles

On the pressure dependence of the wind function in Dalton's and Penman's evaporation equations

A systematic analysis is presented on the influences of the pressure dependence of the psychrometric “constant” and the inherent pressure dependence of the wind function in expressions commonly used for calculations of evaporation from open water and crops. It is shown that the pressure dependence of the wind function has to be incorporated in the Penman equations in exceptional cases only. However, the results show that the pressure dependences of the wind function in Dalton-like evaporation expressions have to be different from those in use.

On Penman's equation for estimating regional evaporation

AbstractThe form of Penman's evaporation equation most extensively applied in hydrometeorology is discussed. It is shown that this equation underemphasizes the importance of ventilation relative to radiation in maintaining regional evaporation. A generalized ventilation term is proposed and a modified equation for evaporation derived. This new equation is calibrated to give the same annual total for short vegetation as Penman's original version. It predicts both an enhancement of winter evaporation at the expense of summer evaporation and a significant increase in annual total evaporation with vegetation size, especially in regions of frequent substantial rainfall.

An appraisal of advectional contributions to observed evaporation in Australia using an empirical approximation of Penman's potential evaporation

From data obtained in field studies of growth and water-use characteristics of cotton at Kimberley Research Station, Kununurra, Western Australia, a method is developed for obtaining an approximation of Penman's potential evaporation. The predictor used for this approximation is an adjusted version of a weighted vapour pressure deficit based on mean maximum screen temperature and found previously to provide an efficient basis for estimating evaporation from the standard (sunken) Australian evaporation tank. Using daily data, a distinctive linear relationship between the weighted vapour pressure deficit and Penman evaporation was identified for periods with low advection. An adjustment factor, based upon either the wet bulb depression or relative humidity, was then derived empirically to make this relationship applicable over a range of dryness conditions. When applied with mean data over ten-day intervals, the method was found to give close agreement with Penman evaporation at four Australian stations having differing climatic environments. At all of these stations the approximated value is more highly correlated with the Penman evaporation than are the estimates of tank evaporation obtained from empirical relationships having either the unadjusted weighted vapour pressure deficit or the simple saturation deficit as the independent variable. At two extra-tropical stations, the correlation between Penman evaporation and the approximated value exceeds that between Penman and observed tank evaporation, but at tropical stations the latter correlation is slightly higher. A broad regional application of the method is carried out, using published mean monthly climatic data for a network of 143 Australian stations. An index of advective strength derived as the ratio of the estimated tank evaporation (from the unadjusted weighted vapour pressure deficit) to the approximated Penman evaporation is also calculated. These parameters are mapped and discussed for summer and winter situations.

Estimates of potential evaporation using alternative data in Penman's formula

In an environment with a wide seasonal range of radiation and vapour-pressure deficit, the energy and aerodynamic components of Penman's evaporation formula were calculated daily for a year, and the potential evaporation was determined from several sources of data and by alternate modes of computation. Reference values for the energy component were obtained from measured net radiation over a well-watered crop. Reference values for the aerodynamic component were calculated from wet- and dry-bulb temperatures and windspeed using a relationship defined by Penman. Total radiation and relative duration of sunshine were found to be effective alternative sources of data for the energy component. When calculating the effective terrestrial radiation, it was found that equally satisfactory results could be obtained from a relationship based on proposals by Swinbank as by Penman's method which includes a vapour pressure term. Daily Piche and tank evaporimeter data were found to be satisfactory alternatives to the aerodynamic component. The conclusions reached are: ( 1) The minimal instrumentation needed to determine potential evaporation is a net radiometer, a Piche evaporimeter, and maximum and minimum thermometers, provided a suitable relation between the Piche and aerodynamic component is available. If a solarimeter is used in place of a net radiometer or sunshine recorder, a reliable albedo coefficient is necessary. When a sunshine recorder is used, a satisfactory relation between n N and Q t Q e is also required. is also required. ( 2) The use of inappropriate constants in the Penman formula is probably a greater source of error when determining potential evaporation than the deficiencies inherent in a particular type of instrument, e.g., a sunshine recorder. The constants appropriate to the particular climatic environment must be used.

Calculated and observed evaporation in a dry monsoonal environment

The annual regimes of calculated Penman evaporation, observed evaporation from the standard Australian tank and the Piche evaporimeters and several empirical evaporation predictors from a dry monsoonal environment are described and compared. The relationship between the evaporation predictors and the Penman and tank evaporation is examined. This analysis was carried out with specific reference to selected wet and dry periods of the year, each period being of 100 days' duration. It was found that evaporimeters and Dalton type relations show a slight lag behind Penman evaporation and that the Thornthwaite and Blaney and Criddle estimates are inadequate to describe either the Penman or tank evaporation regime throughout the year in this climatic environment. Marked differences in the efficiency of the various estimates in predicting Penman and tank evaporation over five-day intervals were found. No single relationship could be identified that would satisfy both wet and dry periods of the year. It is concluded that the use of a single generalized relationship in this type of environment for predicting either Penman or tank evaporation from any of the predictors used can only be justified where a high degree of approximation is acceptable.

The use of the Piche evaporimeter in the calculation of evaporation

AbstractWater loss measured by the Piche evaporimeter, E, was found to be highly correlated with the calculated aerodynamic term, Ea, in Penman's evaporation formula. The error introduced by using the more convenient Piche evaporimeter measurements was less than that caused by the use of a single daily observation of vapour pressure in the calculation of the aerodynamic term.No significant difference was found between the relationship in the Judean hills and the arid Negev region of Israel which wasA test of a simplified form of Penman's formula, based on local Piche evaporimeter readings for the aerodynamic term and a regional value for the energy term, gave close agreement with the evaporation values calculated using the full formula.