Evaporation from natural nonsturated surfaces
Granger, Raoul Joseph
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Evaporation from nonsaturated surfaces is investigated. The concept of potential evaporation is first examined; a series of definitions are developed and classified, and it is shown that the relationship between potential and actual evaporation rates depends on the controlling variables in the chosen definition for potential evaporation. Extending the work of Penman (1948) to the unsaturated case, a general equation is derived to describe evaporation from nonsaturated surfaces. Applying Bouchet's (1963) hypothesis with a consistent set of definitions also leads to the same general equation. To account for departures from the saturated condition, the equation makes use of the concept of relative evaporation, the ratio of the actual evaporation rate to the rate which would occur under the prevailing atmospheric conditions if the surface was saturated at the actual surface temperature. A relationship is established between the relative evaporation, G, and a dimensionless parameter called the relative drying power, D, the ratio of the drying power (the evaporation rate which would occur if the surface was saturated at the actual air temperature) to the sum of the drying power and the net available energy. The relationship is non-dimensional and appears to be single-valued. An experimental investigation of evaporation from bare soil and growing wheat is carried out; data from an energy balance installation show that the G-D estimates of evaporation are in close agreement with calculations obtained using the Bowen ratio approach. The data are also used to refine the relationship between relative evaporation and relative drying power. The G-D estimates of evaporation also agree closely with independent estimates obtained from the soil water balance at three sites during the growing season. The combination of this G-D relationship with the derived general evaporation equation constitutes a simple model for obtaining estimates of evaporation from nonsaturated surfaces; no prior estimate of the potential evaporation is required, and the surface conditions of temperature and humidity need not be known. A preliminary relationship is found for the vapour transfer coefficient (used in a Dalton type transfer equation) for daily time periods. Algorithms are presented for the estimation of daily net radiation and for soil heat flux. A new approach is proposed for the application of remotely-sensed surface temperature data to the estimate of regional evaporation. A relationship is derived between the surface temperature and the vapour pressure deficit in the air. The relationship allows for the use of remotely-sensed data in evaporation models such as that presented above. The method is shown to provide superior results to the simplified energy balance approach currently being applied.