According to the global water balance (figure below), South Africa is situated in a negative runoff zone (latitude 22°-35°, Southern Hemisphere), which means that annual evaporation always exceeds rainfall.

Global water balance (Miller, 1993)

According to the Strakhov model (1967), South Africa is further situated in a semidesert / desert latitude zone, which has almost no weathering and soil formation, due to the drop in precipitation and vegetation cover, and the higher temperatures and evaporation rate  - below.

The Strakhov model (Selby, 1985)

WATER IN GREATER JOHANNESBURG

Johannesburg straddles the main watershed divide for the subcontinent and is one of the few cities of its size and kind in the world, which is not situated on a major watercourse (DWAF, 1999). The catchment areas within the metropolitan area consists of the Jukskei Catchment in the north and the Upper Klip River catchment in the South.

REGIONAL CLIMATE

Average annual rainfall in the Klip River catchment ranges from about 650mm near the Vaal Barrage to 750mm on the Witwatersrand ridge in the north. The average total monthly rainfall recorded at Johannesburg International Airport is shown in the table below.
Annual Average Rainfall, Evaporation and Temperature for Johannesburg Airport (1961-1990) (Weather Bureau, 1991).

JUKSKEI CATCHMENT

The Jukskei catchment is situated within the Pretoria-Witwatersrand area. A range of users from recreational activities to industrial users are using the Jukskei River, with different water standards catering for each category. The catchment is divided into eight sub catchments to ensure that no user or activity was overlooked (Jukskei Catchment, 1996).  The Jukskei River Catchment in the north is one of the main tributaries of the Crocodile River, upstream of the Hartbeespoort Dam. The catchment area drains a large part of the Witwatersrand. The source of the Jukskei River is located within the Eastern Metropolitan Local Council (EMLC), with a population of 790 000 around this catchment in 1991.

UPPER KLIP RIVER CATCHMENT

Water in the southern parts of Greater Johannesburg drains to the Vaal Barrage and the Atlantic Ocean via the Klip River  (DWAF, 1999). The Klip River catchment is one of the most heavily impacted river systems in South Africa and is subjected to almost every type of pollution. It furthermore serves all five recognised user groups identified by DWAF (domestic, agricultural, recreation, industrial and the natural environment).

The natural run-off of the Klip River catchment is estimated to be 111 x 10⁶m³/annum (Stewart Scott et al, 1996).

CRITICAL WATER MANAGEMENT PROBLEMS FACING THE JUKSKEI AND UPPER KLIP RIVER CATCHMENT AREAS

Water is a scarce and critical resource for the whole Greater Johannesburg. However, potable or drinking water is of a high standard throughout the metropolitan area. Johannesburg is dependent on transfers of water from the Vaal River System for its water needs. This means that the city has to play its part in conserving water and using it efficiently, as well as ensuring that the Vaal system is kept free from pollution.

The Upper Klip River in the south is located in an area of urban development and past mining activity, and is subject to intense pressure from human activities. In addition to water scarcity, a large percentage of drinking water is lost due to degradation of the water supply infrastructure, water wastage and leakage.

However the main concerns from an environmental perspective are the impacts of the increasing demands on water resources, and the impact of pollution on downstream impoundment and on users of this water source.