PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

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of 65-70% (Ward, 1975; cited by Schulze, 1995). With regards particularly to
the A-pan as a reference crop evaporation, Schulze (1995) emphasised the fact
that:
• the Penman and Penman-Monteith equations are de facto accepted
standard methods for Er;
• that crop water use is related more closely to solar radiation than to A-pan
evaporation (Turner and Wiersma, 1964), and;
• that the A-pan has been shown to overestimate crop reference evaporation
by up to 30% (van Zyl et al., 1989).
However, its incorporation in the ACRU model according to the developers
(Schulze, 1995) is because of the following reasons:
1. The Class Apan is universally the most common evaporation pan in usage,
having been adopted as the standard evaporation pan since the
International Geophysical year of 1957/8.
2. When operated properly it is accepted as a reasonably reliable, simple,
inexpensive integrator of atmospheric evaporative demand over a time
period and used commonly as a reference for potential evaporation (Green,
1998).
3. In Southern Africa, where the ACRU model has been used most frequently,
crop coefficients relating consumptive water use of the plant/soil continuum
under different growth stages to a reference evaporation, have been tried
and tested most widely against the Apan.
4. Mathematically elegant physically based methods of estimating Er, such as
the universally accepted Penman based methods (e.g. Penman 1948) make
high demands on input data, including net radiation, wind and vapour
pressure deficits. These data are not yet readily available in many stations in
Southern Africa. On the other hand, there is often at least a rudimentary
network of Apans in Southern Africa (e.g. over 750 Apans exist in Southern
Africa) (Schulze and Maharaj, 1991).
Emmanuel Obeng Bekoe Phd Thesis Chapter 3 Catchment-Hydrol,& Model Selection