9 The Land Resources of North East Nigeria A/blume1 The ...

WIND
Strong winds often associated with squalls and storms do occur (Dorrell, 1947) but destructive
winds are not common. Direction and persistence of the wind (Text Maps 7a and 7b) are
however of paramount importance. It can be seen that, with the southerly movement of the
ITCZ from October to April the wind blows consistently from the north or, more often, the
northeast. During this period the area is exposed to very dry winds blowing from the
Sahara, the harmattan, often carrying a thick haze of wind-borne, conspicuously diamataceous,
dust (Bigelstone, 1958; McKeown. 1958). Prom May, throughout the summer rains until
September, the direction is reversed and the wind blows mainly across the area from the
southwest. Again the Benue Valley is somewhat exceptional. The dry season wind tends to
blow more directly from the north while the wet season wind is more westerly and even northwesterly
orientated and commences earlier in March and finishes later in October, than the
main part of the area.
Once this wind pattern is appreciated, the rationale of the rainfall/temperature pattern
falls into place. Local anticyclonic effects modify the generalised southwest direction of
wet season wind . in West Africa particularly in the south of the project area. The generalised
wind movements explain the near-diagonal oscillation of rain and temperature in the northern
sector. In the southern sector, the more westerly/northwesterly wind creates a pattern in
the wet season of up-wind exposure and down-wind rain shadow effects, fluctuating around the
critical 900-1 000 mm (35 - 40 in) isohyets. The high temperatures in the dry season in the
Benue Valley are probably due to low altitude and the effects of the land to the north
blocking the northerly orientated wind.
RADIATION
Available data of relevance to the project area in the form of 'hours of bright sunlight'
observations are restricted to Maiduguri, Bauchi, Ibi, Yola and Potiskum. Cochemé and
Franquin (1967) also give useful data for the surrounding territories.
Figure 3 shows the monthly variation in incoming radiation. The general picture is of radiation
rising to a maximum in November - February and falling to a minimum in August. An
interesting feature is the marked fall and recovery of the graph in the period February -
April. In the far north this occurs in March and is probably associated with overcast conditions
due to the meeting of the 'dust front' with the ITCZ (Hamilton and Archbold, 1945).
In the southern stations it is a month later and probably associated with increased rain
cloud and storm conditions as the ITCZ retreats southward. It should be noted that
Maiduguri, which occupies an intermediate position, merely shows a flattening of the graph
during this period.
Included in Figure 3 are graphs for mean cloud cover which show a clear inverse relationship
with the incoming radiation.
HUMIDITY
Humidity follows a simple relationship with the change of the seasons. Figure 4 shows the
monthly change in saturation deficit reaching a peak in the pre-rains hot period in March -
April and falling to a minimum in August in the mid-rainy season. It also follows a simple
inverse daily relationship with temperature reaching minimum values at night and maximum ones
at the height of the day.
WATER BUDGET
Unlike areas in the south of the country, radiation is not likely to be a limiting factor in
plant growth and it is the availability of water that is of prime agricultural importance
throughout the project area. Two approaches to the problem are possible; to consider the
balance between precipitation gain and various forms of water loss largely calculated from
climatic parameters, or to consider a level of precipitation based on past experience after
which the 'rainy season' can be said to have 'safely' started or to have finished.
14