Yoshida - 1981 - Fundamentals of Rice Crop Science

104 FUNDAMENTALS OF RICE CROP SCIENCE
for many regions. It has also been used to estimate the potential evapotranspiration
and water balance of rice crops in monsoon Asia (Maruyama 1975).
A fundamental defect of Thornthwaite’s method is the degree of correlations
between temperature and the energy available for evaporation, depending on
weather conditions. Thornthwaite’s method fits areas where the climates are
similar to those in central and eastern US.
2.5.11. The van Bavel method
Van Bavel (l966) modified the Penman equation in such a way that it contains no
empirical constants or functions, and is not restricted to grass or to any other
specified set of surface conditions other than unrestricted water supply.
For practical purposes, the van Bavel equation takes the following form:
(2.32)
The van Bavel method of estimating PE is perhaps the most comprehensive
approach that has ever been attempted. It is applicable to a wide range of
environments where advection is present. The ratio of measured vs computed
evapotranspiration varies from 0.87 to 1.05 with a weighted mean of 0.96. This
disagreement, however, appears to be within instrumental and measurement errors
over a range in daily evaporation from 3 to 12 mm.
2.5.12. Water balance
Knowing water gains and losses, we can calculate the balance between the two.
Water balance can be examined at the root zone for a given field, or it can be
viewed on a much larger scale, such as a hydrologic cycle, as illustrated in Figure
2.20.
In its simplest form the water balance states that, in a given volume of soil, the
difference between income ( W in ) and outgo ( W out ) during a certain period is equal
to the changes in storage ( D W ) of the soil during the same period:
or
Income – oputgo = D storage
W in – W out = D W
(2.33)
where D W can be changes in soil moisture in upland fields or changes in water
depth in irrigated lowland fields.