| dc.description.abstract | Evaporation of water from a pan (Ep) is an energy
driven process which integrates the effects of weather
factors, notably solar radiation, temperature , wind and
humidity. In their utilization of water (Evapotranspiration
or ET), plants are responding to the same stimuli,
but the actual rates of ET differ from those of Ep due to
the physiology of the plant. Additionally, ET rates vary
throughout the growing season with the extent of leaf
cover, for it is here that the atmospheric energy is
intercepted.
Nevertheless, at each growing stage, for a given crop
planted at 8 given density, a nearly constant ratio of
ET/Ep exists. When these ratios have been determined for
the entire season, they may be used to estimate maximum
evapotranspiration rates (,Et-m-); wherever the crop is to be
grown, simply by relating to Ep ,records from the growing site .
However, evaporation pans used in practice are
generally not well standardized. Thus, in order to use
pan evaporation rates as predictors of ET rates, it is
necessary to determine lipan factors" for the environments
in which pans exist (i.e. green grass, dry grass and bare
soil), and for the effects of screening and painting the
inside black. Therefore, an experiment was established at
Katumani Station (latitude 1° 35'S, longitude 370 141E,
and altitude 1575m) in Machakos District to investigate
the influence of these factors. The experimental field
was divided into three areas, one in which grass was
kept green by· irrigation, one in which grass was allowed
to dry when rains subsided; and one of bare soil.
A total of five class A pans were installed for the
study. One screened pan painted black inside was installed
in each of the three areas with the pan in green grass as
the standard. Two more pans were installed in the green
grass area of which one was unscreened but still painted
black inside and the other one was screened and painted
aluminium inside.
Daily evaporation readings were taken from these pans
for seven months (0pril 1st, 1981 to October 31st, 1981).
During the experiment the screens on the pan in the bare
soil area and the aluminium painted pan in the green grass
area were removed from 11th J:me to the End of June. The
screen was again removed from the aluminium painted pan for
the whole month of September.
It sometimes happens that pans ere not located where
ET rates need to be predicted. So an alternative is the
use of evaporation rates computed from meteorological
parameters. Hence, meteorological parameters to enable
computation of Penman Eo were recorded daily in an adjacent
agrometeorological station.
Average daily evaporation rates from each pan were
calculated for each month during the experimental period,
as was Penman Eo. These values were then expressed as
fractions of corresponding evaporation from the standard
pan.
In the course of the study especial 5-day period
was selected during which reading;s were taken from the
standard pan every three hours to enable determination
of the diurnal pattern of the rates of evaporation.
Hourly rates of evaporation were interpolated from the
readings and plotted versus time. For comparison
purposes, similar observations were made at Muguga
(latitude 10 13'5, longitude 360 38lE and altitude 2070m)
for two days.
Results show fairly consistent ratios of the pans
throughout the entire period of the experiment. Taking
the standard pan as 1.0, the unscreened pan in green
I
grass lost water 12 per cent faster, while the screened
aluminium painted pan (in green grass) lost 6 per cent
less. The screened pan in dry grass lost 8 per cent faster
while the screened pan in dry soil evaporated water 9 per
cent faster.
The Penman estimate ratios are also fairly consistent
with the monthly ratios being ± 2 per cent from the
average, and the rate on average being 4 per Cent faster
than the standard pan.
The diurnal pattern of the rates of evaporation
at Katumani shows that about 60 per cent of the daily
evaporation occurs during daytime with the maximum rate
at about 1700 hours.
The results from the study indicated no seasonal
variations in the pan factors. | en |
