John laing, mass media and misisi areas

3.1Drainage

Drainage of the area reveals an essentially radial pattern (Fig. 9). This pattern appears consistent with the domical-type relief, which conforms to the basin and swell structural concept applied by HOLMES (1965) to explain the relief of Africa, with the Lusaka plateau forming a minor swell.

One of the most conspicuous features of this plateau is the scarcity and/or complete lack of surface drainage particularly in its central part. Thus, rainwater drains into fissures and/or infiltrates through the overburden to join the underground water. Only surface water in excess of the infiltration capacity is drained into minor seasonal streams.

Fig. 9: Map showing the city boundary, topography and drainage of the Lusaka plateau.

3.2Potential Evapotranspiration, ET_pot

ET_pot = 16  (mm d^-1)

Where: t ≡ average monthly temperature (^oC); J ≡ Heat index, the sum of 12 months’ values of thus, J ≡ ;

a = or a = 6.75  10^-7  J³ – 7.71  10^-5  J² + 1.792  10^-2  J + 0.49239

For the period under review, values obtained from the Thorntwaite Formula and corrected by a factor that varies with the number of days in a month and geographic location, are given in Table 2.

Table 2: Potential evapotranspiration values calculated by the Thorntwaite Formula.

1. 
   Less than potential evapotranspiration, in which case, there is a resultant water deficit. During times of such water deficiencies, plants on the Lusaka plateau shed off their leaves as a measure to reduce evapotranspiration.
   
2. 
   Equal to potential evapotranspiration, giving no resultant change in the soil moisture content, and
   
3. 
   Greater than potential evapotranspiration, resulting in a net excess in the soil water content, in which case, that part of the rainfall, which is not evaporated, enriches the moisture content of the soil until the field capacity is attained. Excess water, after fulfilment of the field capacity, trickles down to the groundwater store.