Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 99
water transpired in a unit of time by a short green crop of uniform height, which
completely shades the ground and is never short of water. Penman preferred to use
the term potential transpiration for PE. The Penman concept suggests that PE
represents the maximum possible evaporative loss from a vegetation-covered
surface. The actual evapotranspiration, however, may exceed PE when there is a
supply of advected energy (Chang 1968, Ward 1975). Advection is known as the
oasis effect and may occur where a moist area is surrounded by or adjacent to hot,
dry land. Under such conditions, sensible heat will be transferred to the moist area
and its evapotranspiration rate will be increased.
In a humid climate, such as during the rainy season in monsoon Asia, advected
energy is probably negligible and, hence, the PE defined by Penman may apply to
actual evapotranspiration. In an arid or semiarid climate, the existence of large
amounts of advected energy renders the concept of PE inexact and unrealistic.
Pruitt (1960) developed the notion of potential maximum evapotranspiration to
allow for the situation in which advected energy is an important factor in determining
evapotranspiration rates.
More recently, van Bavel (1966) developed an equation relating PE to net
radiation, ambient air properties, and surface roughness. As an improvement over
the Penman equation, the proposed model contains no empirical constants or
functions. The van Bavel equation is applicable to a wide range of weather
conditions, under which advection of sensible heat to the evaporating surface
occurs. Thus, the refinement of both theoretical and experimental treatments of PE
has caused the meaning of PE to change, and caution should be taken when using
the term.
2.5.7. Pan evaporation
Up to the present, different kinds of pans — size, design, and installation — have
been used to measure the evaporation rate from a free water surface (Chang 1967,
Ward 1975). The data obtained from using these pans are quite different. For
instance, a small-sized pan gives a higher evaporation rate than a large-sized pan
under the same weather conditions. Hence, extreme care must be exercised when
interpreting pan evaporation data collected from different sources.
To eliminate the effects of pan design and installation on evaporation rates, the
World Meteorological Organization (WMO) adopted the U.S. Weather Bureau
Class A pan as the interim standard for the International Geophysical Year. Since
that time, the class A pan has been widely accepted in many countries.
The U.S. Weather Bureau Class A pan is a cylinder 120.7 cm (47.5 inches) in
inside diameter and 25.4 cm (10 inches) deep. It is constructed of galvanized iron
or monel metal, preferably the latter, in areas where the water contains large
amounts of corrosive substances. The evaporation pan is placed on a wooden
platform with the bottom approximately 10 cm above the ground so that air may
circulate beneath the pan. The site should be fairly level, soded, and free of
obstructions; the grass should be frequently watered. The pan should be filled to a
level 5 cm below the rim, and refilled when the water has evaporated 2.5 cm.