2.3.3 Meteorological factors determining
evapotranspiration
The meteorological factors determining ET are weather
parameters which provide energy for vaporization and remove water vapor from
the evaporating surface (Allen et al., 1998). The principal weather
parameters to consider are:
· Solar radiation
The ET process is determined by the amount of energy
available to vaporize water. Solar radiation is the largest energy source and
is able to change large quantities of liquid water into water vapor. The
potential amount of radiation that can reach the evaporating surface is
determined by its location and time of the year. Due to differences in the
position of the sun, the potential radiation differs at various latitudes and
in different seasons. The actual solar radiation reaching the evaporating
surface depends on the turbidity of the atmosphere and the presence of clouds,
which reflect and absorb major parts of the radiation. Not all-available energy
is used to vaporize water part is used to heat up the atmosphere and soil
profile.
· Air temperature
The solar radiation absorbed by the atmosphere and heat
emitted by the earth increase the air temperature. The sensible heat of the
surrounding air transfers energy to the crop and exerts as such a controlling
influence on the rate of evaporation. In sunny warm weather, the loss of water
by ET is greater than cloudy and cool weather.
· Air humidity
While the energy supply from the sun and surrounding air is
the main driving force for the vaporization of water, the difference between
the water vapor pressure at the evapotranspiring surface and the surrounding
air is the determining factor for the vapor removal. Well-watered fields in hot
dry arid regions consume large amount of water due to the abundance of energy
and the desiccating power of the atmosphere. In humid tropical regions,
notwithstanding the high-energy input, the high humidity of the air will reduce
the ET demand. In such an environment the air is already close to saturation,
so that less additional water can be stored and hence the ET rate is lower than
in arid regions.
· Wind speed
The process of vapor removal depends largely on wind and air
turbulence, which transfer large quantities of air over the evaporating
surface. When vaporizing water, the air above the evaporating surface gradually
becomes saturated with water vapor. If this air which is the driving force for
water vapor removal is not continuously replaced with drier air ET rate
decreases.
2.4 Maximum Production
Banana cultivation is mainly geared towards the production of
fruits, for the production of beer, animal feeds and not the least the
production of fibers for textile industry (Marty, 1983). In this study, the
objective of banana cultivation is for the production of fruits. Many models to
estimate the potential production of crops have been developed. Doorenbos
et al., (1986) presented the Dewit model in 1965 as a model of
estimation of the potential production of crops. This model unfortunately has
not yet been adopted for the banana crop surely due to the complexity of this
perennial herbaceous plant. Smith et al., (1996) concluded that
agricultural production is affected by the level of water stress that will be
experienced by the crops during development. This model goes from
E
linear regression, to bring in the rate of production reduction
to arrive at a relation that
? 1 ?
? E x
Smith et al (1996) called Crop Water Yield Function?
(CWYF). The summarized equation is as follows:
(2.9)
In this equation Ya is the actual production in
tons/hectare (t/ha); Ymax is the maximum production in t/ha;
ETa is the actual evapotranspiration in mm/season; ETmax
is the maximum evapotranspiration in mm/season and Ky is the yield
response factor and has no unit. Ky describes the reduction in
relative yield due to water stress. Doorenbos et al., (1986) suggested
that for banana, Ky will have a value between 1.2 and 1.35,
represents the rate of evapotranspiration reduction, and
represents the rate of production reduction due to water
stress
The problem with this model is the determination of the
maximum production of a banana plantation with respect to only the response to
water, indifferently from other factors of production such as soil factors,
crop factors, climate and topography.
Beernaert and Bitondo (1993) in their effort to consider many
factors for the estimation of the potential production of crops presented a
model called Modèle d`évaluation des terres?. This model
considers factors such as winds, fungus or nematodes diseases, the crop species
cultivated and others, and is more reliable than other models.
They have used this model on many crops including banana. They
estimate that in ideal conditions (without limitations) the potential
production of a banana plantation ranges between 40 and 60 tons/ha.
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