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Design of a geographic information supported database for the management of pressurised irrigation systems at the plantation du Haut Penja, Cameroon

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par Chick Herman AZAH
University of Dschang - Agric engineer 2009
  

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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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