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Hydrology of the Limpopo River Basin: Water Balance

As stated in the Principles of Hydrology section, Water balance is the concept used to understand the availability and the overall 'state' of water resources in a hydrological system, considering all of the inflows and outflows into and from the basin.

The information below presents an initial water balance budget. However, a more complete and holistic water balance estimate is required if the objectives of integrated water resources management are to be met in the Limpopo River basin.

The LBPTC Joint Limpopo River Basin Study Scoping Phase has identified large volumes of national and regional information relevant to the development of a water balance. However, the development of a basin-wide water balance will likely require integrated and normalised inflow and outflow information to be assembled if an accurate water balance is to be calculated for the entire river basin.

The Motloutse River bed at low-flow.
Source: DiPerna 2009
( click to enlarge )

Inflows

Precipitation

As stated in the Climate and Weather Chapter, rainfall is relatively low in the northern  left bank tributaries of the river basin and higher in the southern right bank tributaries, specifically in the Marico, Olifants, Crocodile and Steelpoort rivers.  The map below shows the distribution of mean annual rainfall across the Limpopo River basin.  The higher rainfall regions are shown in green and blue.

Mean annual precipitation in the Limpopo River basin.
Source: LBPTC 2010
( click to enlarge )

Run-off

A map of estimated natural mean annual run-off is shown below, created from national water resource management activities in South Africa, run-off assessments in Zimbabwe and observed river run-off in Botswana and Mozambique.  Run-off in eastern Botswana and southern Zimbabwe is very low, often as low as 5 mm/yr.

Mean annual run-off (mm/yr) - an estimate of natural water resources in the Limpopo River basin.
Source: LBPTC 2010
( click to enlarge )

Groundwater inflow

Groundwater inflow is an important part of water balance.  However, there is currently no information available on the scale of groundwater inflow (and outflow) from the Limpopo River basin.

Outflows

Evapotranspiration

The map below illustrates Evapotranspiration from the Limpopo River basin.  Average evapotranspiration ranges from 1 000 mm/yr to 2 000 mm/yr.

Evapotranspiration across the Limpopo River basin.
Source: FAO
( click to enlarge )

Water demand

Water demand as a percentage of Mean Annual Runoff is shown in the map below.  As can been seen from this map, demands are highest in the Crocodile, Sand, Mzingwani, Mwenezi and Lower Limpopo sub-basins.  Even though the Crocodile has relatively high rainfall compared to the rest of the basin, the demands for water in Pretoria and Johannesburg are high.

Water demand, as a percentage of Mean Annual Run-off (MAR) for the Limpopo River basin.
Source: LBPTC 2010
( click to enlarge )

Water Balance

The map below shows the estimated general water balances for the Limpopo River basin, calculated as a ratio of run-off against water use per sub-basin.  Only four of the sub-basins are 'in balance' and one sub-basin, the Changane, is in surplus.  The rest of the sub-basins are in deficit, with 11 sub-basins falling into the 'very stressed' category.

Estimated general water balance for the Limpopo River basin.
Source: LBPTC 2010
( click to enlarge )