Evaporation from natural nonsturated surfaces
Abstract
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.