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

2.3.1 Measurement of evapotranspiration

ET is not easy to measure. Specific devices and accurate measurements of various physical parameters or the soil water balance in Lysimeters are required to determine ET (Allen et al., 1998). The methods are often expensive, demanding in terms of accuracy of measurement and can only be fully exploited by well-trained research personnel. Although the methods are inappropriate for routine measurements, they remain important for the evaluation of ET estimates obtained by more indirect methods. The methods used in the measurement of ET are: Energy balance and Microclimatological methods, Soil water balance, Lysimeters, Meteorological data, Pan Evaporation. Owing to the difficulty of obtaining accurate field measurements, ET is commonly computed from weather data. A large number of empirical or semi-empirical equations have been developed for assessing crop or reference crop ET from meteorological data. Some of the methods are only valid under specific climatic and agronomic conditions and cannot be applied under conditions different from those under which they were originally developed.

Numerous researchers have analyzed the performance of the various calculation methods for different location. As a result of an Expert Consultation to compare several methods of calculation of ET_o in May 1990, the FAO Penman-Montheith method is now recommended as the standard method for the definition and computation of reference evapotranspiration, ET_o (Allen et al., 1998)

2.3.2 The Penman-Montheith equation

In 1948, Penman combined the energy balance with the mass transfer method and derived an equation to compute the evaporation from an open water surface from standard climatological records of sunshine, temperature, humidity and wind speed (Allen et al.,

1998). This so-called combination method was further developed by many researchers and extended to cropped surfaces by introducing resistance factors.

From the original Penman-Montheith equation and the equations of the aerodynamic and surface resistance, the FAO Penman-Montheith method to estimate ET_o is:

(2.8)

Where,

ET_o reference ET [mmday^-1],

R_n net radiation at the crop surface [MJm^-2day^-1],

G soil heat flux density [MJm^-2day ^-1],

T mean daily air temperature at 2 m height [^oC],

U2 wind speed at 2 m height [ms^-1],

e_s saturation vapor pressure [KPa],

e_a actual vapor pressure [KPa],

(e_s - e_a) represents the vapor pressure deficit of the air [KPa],

ñ_a is the mean air density at constant pressure,

cp is the specific heat of the air,

Ä represents the slope of the saturation vapor pressure temperature relationship, ã is the psychometric constant,

The equation uses standard climatological data of solar radiation (sunshine hours), air temperature, humidity and wind speed. To ensure the integrity of computations the weather data should be collected at 2 m above the extensive surface of green grass, shading the ground and not short of water.