John laing, mass media and misisi areas

3.3Actual evapotranspiration, AET

One of the methods used to determine AET is that of TURC (In: MATTHESS & UBELL, 1983). It takes into account annual precipitation and the average annual temperature, as follows:

Wherein:

R = annual rainfall [mm]; J = 300 + 25  t + 0.05  t³ = 820; t = mean annual temperature [⁰C].

This gives an annual AET-value of 594.8 mm. This may also be regarded as representing the long-term average for the period of 31 years (1991/92 – 2000/01) under review.

3.4Groundwater Recharge, GW_R

In Lusaka, recharge is derived from rainfall, which falls directly on the plateau. When rain falls on the soil surface, both gravity and capillary potential tend to cause its downward movement by infiltration.

Although earlier studies have indicated the recharge area to lie south-east of the city, the presence of well-developed karstic features over the entire marble terrain would indicate that recharge occurs over the whole city area underlain by the marble. The conspicuous lack of permanent surface-drainage over the Lusaka plateau is evidence that the area imbibes most of the rain water that falls on it, which implies that the area is mainly drained underground except during periods of excessive rains, when rates of infiltration may be overwhelmed.

However, very little information is available about groundwater levels in the Lusaka aquifer because there are piezometers or wells from which these recordings would be obtainable, other that those from which groundwater is abstracted for supply to the city. In addition, measuring gauges that would allow determination of groundwater levels as influenced by precipitation are not available. Consequently, the estimation of groundwater recharge is only based upon general hydrological parameters, mainly precipitation, potential and actual evapotranspiration.

On the basis of the acquired and calculated hydrologic data, the components of total runoff (S_TRO), which is composed of groundwater recharge (GW_R) and surface runoff (S_RO), can be determined as follows:

GW_R + S_RO = R – AET (mm/a)

GW_R + S_RO = 819.7 – 594.8 (mm/a)

GW_R + S_RO = 224.9 (mm/a)

Because of problems pertaining to the partitioning of GW_R and S_RO, two optimistic assumptions have been made as regards the amount of groundwater recharge:

• 
  That the total runoff constitutes groundwater recharge. The ubiquitous lack of surface drainage on the Lusaka plateau may justify the assumption.
  
• 
  The calculated total runoff of 224.9 mm/a represents 27% of the annual rainfall. Coincidentally, von HOYER at al. (1978) and UBELL (1961) determined for the Lusaka and Hungarian karst areas, respectively, that groundwater recharge contributes 22% of the total annual rainfall. Implicitly, although the two areas lie in different geographic and climatic zones, similarities in quantities of groundwater recharge would probably be compensated by components of runoff and evapotranspiration. On the basis that 22% of the annual precipitation constitutes recharge, a resultant annual replenishment to the groundwater store of 180 mm is attainable.
  

The observation of water levels in some of the Lusaka Water and Sewerage Company (LWSC) boreholes for a period of seven years (1995-2001) indicates a general rise of groundwater levels between December and January (Figs. 11), which is also indicative of the time the field capacity in the soils is attained. This implies that rainfall events at the beginning of the rainy season contribute wholly towards evapotranspiration as well as replenishing soil moisture.

Fig. 11: Response of the groundwater table to rainfall in one of the Lusaka Water and Sewerage Company boreholes (1995 – 2001)

Thus, if the water table is close to the surface and sufficient water is supplied, the moisture may reach the water table and add to the groundwater.

hydrogeology

‘Concealed’ subsurface solution feature

Fig. 12: Dissolution features in the Lusaka marbles (A) concealed subsurface solution feature (B) Intersection of solution features at depths in boreholes.

Fractures-sizes in marbles appear to have subsequently and progressively increased in comparison with those of the rest of the fissures and other primary voids in the rock, where water circulation may not have been so intense. The presence of these features has transformed these rocks into a favourable and comparatively cheap source of water supply to the city and they appear to have exerted a lot of control on groundwater flow in the aquifer.

In addition, shearing along the marble-schist lithologic contact must have given rise to open conduits, which together with fracture zones, formed essential channel-ways for water into the rock mass. Even some of the Lusaka Water and Sewerage Company (LWSC) borehole-drilling records indicate an evident relationship between water strikes and large scale fracturing and/or faulting/shearing (Table 3).

In this regard, the Lusaka terrain forms a distinctive landform arising from a combination of relatively high rock solubility and well-developed permeability. Table 4 gives a summary of average hydrogeological properties of the aquifer in Lusaka.

Table 3: Influence of geologic features on borehole yield.

Thus, the hydrogeology of Lusaka indicates that:

• 
  The Lusaka aquifer has the best groundwater potential to support the City of Lusaka water supply for large-scale exploitation if well managed.
  
• 
  The flow in the aquifer is directed mainly towards the north-west, south-west and north-east Fig. 13. Generally, Misisi and John Lang settlements are located on a relatively flat area thus resulting in stagnant waters during rain season, which eventually infiltrate to the ground store. Natural discharges occur in certain areas along the schist-dolomite contact through small intermittent springs and seepages.
  

Fig. 13: Hydrogeology of the Lusaka area. Modified from Nyambe and Maseka, 2000