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Water Demand: Environmental Flows

The aquatic environment should always be considered as a bona fide consumer of water, whose requirements must be met alongside basic human requirements, and ahead of any other demand. In the case of water projects involving impoundment, this translates to maintenance of flow in the reaches of the river downstream of the impounding structure, dam, or diversion. Environmental flows are required to:

  • Maintain the riverine ecology
  • Recharge riverine aquifers
  • Maintain the river channel

Excessive abstraction or damming of rivers affects the flow, which in turn affects water chemistry, sediment transport and average temperatures. This has an impact on aquatic biota and the human beings that rely on the water and biota for their livelihoods and well-being.

International laws and regional agreements have been put in place to reduce these impacts, giving countries that share rivers a platform for discussion whenever a development could affect river flow.

Water requirements of freshwater-dependent ecosystems are often referred to as environmental flow requirements (EFR). A river basin is referred to as “closed” when all its river flow is allocated to different uses.The legal framework regarding environmental flow requirements (EFR) varies by basin country (LBPTC 2010).

Preliminary EFR Estimates in the Limpopo Basin

During the Joint Limpopo River Basin Study, preliminary estimates for EFRs were calculated for the main tributaries in the Limpopo River basin based on the South African Desktop Model (LBPTC 2010). EMCs in South Africa were taken from a desktop study, while in the other 3 basin countries they were assumed based on the level of development in the river and have a very low confidence level (LBPTC 2010).

The figure below shows a preliminary estimate of the ecolological status in each catchment, while the table provides estimated EFRs.

Estimated Ecological Management Classes for the Limpopo River sub-basins.
Source: LBPTC 2010
( click to enlarge )

Estimated EFRs in the Limpopo sub-catchments.

Sub-catchment EMC Estimated EFR (% of MAR)
Bonwapitse B 28
Bubi B 28
Changane A/B 35
Crocodile C 20
Lephalala D 13
Letaba B 28
Levuvhu B 28
Lotsane B 28
Lower Limpopo C 15
Lower Middle Limpopo C 16
Lower Olifants C 20
Mahalapswe    B 28
Marico    C 20
Matalabas   C 20
Mogalakwena    C 20
Mokolo   C 20
Mwenezi    B 28
Mwingwani    B/C 24
Notwane    B/C 24
Nzhelele D 13
Sand B 28
Shashe B 28
Shingwedzi A 40
Upper Olifants D 13
Source: Adapted from LBPTC 2010

It is recognised that establishing EFRs plays a major role in future water management to ensure sustainable development in the basin. The major challenge will be to implement different EFR study methodologies based on data availability and varying national legal requirements and implementation strategies.

Botswana

Although the Botswana Water Master Plan (1991) provides for the framework for managing water resources until 2020, no studies on EFRs in the Limpopo basin are currently available (LBPTC 2010).

Mozambique

In Mozambique, priority of water use is given to domestic use followed by environmental flows in the Water Law (1991) and Water Policy (2007). EFRs were studied in 2007 at two sites in the Limpopo basin. The table below outlines the results of the studies. For both locations an Ecological Management Class (EMC) was assumed using the South Africa desktop study model. Class A is close to natural conditions while Class D has been highly modified and anthropogenically influenced (LBPTC 2010).

EFR Studies in Mozambique in 2007.

Location EFR as a percentage of Mean Annual Runoff (MAR)
Ecological Management Class (EMC)
10 km downstream of Massingir Dam 9-14 % C/D
30 km downstream of the confluence of the Limpopo and Olifants Rivers 9-14 % C/D
Source: LBPTC 2010

South Africa

Under South Africa's National Water Act (NWA) all water resource developments must be supported by an Ecological Reserve (ER, synonymous with EFR) study of the appropriate confidence level (LBPTC 2010). There are various methods which are used in South Africa and internationally, however, as they can have financial and time implications, the South Africa Desktop Model was developed and it is routinely used. As noted above, the EMC is a required input as is natural flow data.

Current provisional assessments indicate that, as a national average, about 20% of the total river flow is required as ER which needs to remain in the rivers to maintain a healthy biophysical environment (DWAF 2003a-d).

Zimbabwe

Under Zimbabwe's Water Act (1998) and Environmental Management Act, water is made available for primary purposes and the needs of aquatic life and ecosystems (LBPTC 2010). The default EFR for Zimbabwe is approximately 5 % of the natural flow (LBPTC 2010).