PhD Thesis Emmanuel Obeng Bekoe - Cranfield University

Emmanuel Obeng Bekoe Phd Thesis Chapter 2 Hydrol, Humid Tropics & Study Area
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Jackson (1978) noted that 10-15% of rain days account for about 50% of rainfall
in the tropics.
Hayward and Oguntoyinbo (1987) alluded to this variability in rainfall and
observed that much rainfall in West Africa is derived from storms or showers
lasting three to six hours, when possibly 100 drops in excess of 2 mm diameter
may fall per cm 2 , assuming the intensity of 25 mm hr -1 , which will produce runoff
and soil loss in open farmland. Similarly, Manley and Askew (1993) and
Masiyandima et al. (2003) corroborated the intense nature of the rainstorms,
with the former realizing that the rainfall intensities can be high in the tropics
inducing erosion and flooding whilst the latter reported that rainfall events can
last less than 30 minutes in the humid and dry zone of West Africa. This
variability in rainfall intensities according to Jackson (1978) can exist between
nearby locations and this he illustrated in a tropical catchment in Tanzania (East
Africa) where for time periods of a month or longer, rainfall totals differed by
between 16%-33% at nearby locations (from < 1 km–20 km apart), even when
the rainfall stations are in an area of uniform relief and their long term averages
are similar. Although spatial rainfall variability is universal and not confined to
tropical areas, it is less important in temperate regions where rain is often
general over wide areas (Jackson, 1978). Jackson (1978) concluded that this
degree of ‘localness’ of spatial rainfall variability patterns is important in the
tropics because of the small scale peasant rain-fed agriculture practised in such
regions, and has considerable implications for rainfall measurement and
analysis. McGregor and Nieuwolt (1998) also observed that other rainfall
characteristics, such as its seasonal and diurnal distribution, intensity, duration
and frequency of rain-days also demonstrate spatial and temporal variations,
where intensities can be high but with short durations.
2.3.2 Evaporation and Evapotranspiration
The combination of two separate processes whereby water is lost from the soil
surface by evaporation and from the crop by transpiration is referred to as
evapotranspiration (Allen et al., 1998). Balek (1983) noted that one of the main