VAAL RIVER: Climate

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Map of the VAAL River

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TEMPERATURE
Upper Vaal Area (Upstream of the Vaal Barrage at Vereeninging): The mean annual temperature ranges between 16°C in the west to 12°C in the east, with an average of about 15°C for the catchment as a whole. Maximum temperatures are experienced in January and minimum temperatures usually occur in July. (Ref.1.)
Middel Vaal Area (between Rietspruit River and Bloemhof Dam): Climatic conditions can vary considerably from west to east across the area. The mean annual temperature ranges between 18 °C in the west to 14 °C in the east, with an average of about 16 °C for the catchment as a whole. Maximum temperatures are experienced in January and minimum temperatures usually occur in July. (Ref.2.)
Lower Vaal Area (between the Bloemhof Dam and the Orange River):  Climatic conditions are fairly uniform from east to west across the area. The mean annual temperature ranges between 18.3ºC in the east to 17.4ºC in the west. Maximum temperatures are experienced in January and minimum temperatures usually occur in July. (Ref.3.)
    RAINFALL                                    
Upper Vaal Area (Upstream of the Vaal Barrage at Vereeninging): Rainfall is strongly seasonal with most rain occurring in the summer period (October to April). The peak rainfall months are December and January. Rainfall occurs generally as convective thunderstorms and is sometimes accompanied by hail. Frost occurs in winter and there is occasional light snow on high lying areas. The overall feature of mean annual rainfall over the area is that it decreases fairly uniformly westwards from the eastern escarpment regions across the central plateau area. The mean annual precipitation (MAP) for the watershed ranges from a high of 1 000 mm in the east to a low of 500 mm in the west with an average of about 700 mm. (Ref.1.)
Middel Vaal Area (between Rietspruit River and Bloemhof Dam):  Rainfall is strongly seasonal with most rain occurring in the summer period (October to April). The peak rainfall months are December and January. Rainfall occurs generally as thunderstorms and is sometimes accompanied by hail.  The mean annual precipitation (MAP) franges from a high of 700 mm in the east to a low of 500 mm in the west with an average of about 550 mm. Frost occurs throughout the area in winter, typically over the period mid-May to late August.   (Ref.2.)
Lower Vaal Area (between the Bloemhof Dam and the Orange River): Rainfall is strongly seasonal with most rain occurring in the summer period (October to April). The peak rainfall months are December and January. Rainfall occurs generally as convective thunderstorms and is sometimes accompanied by hail. The overall range of the mean annual precipitation (MAP) for the entire area is 100 mm to 500 mm. Frost occurs throughout the study area in winter, typically over the period mid-May to late August. (Ref.3.).
  References                                      

Ref 1. Department of Water Affairs and Forestry, South Africa. 2004. Upper Vaal Water Management Area: Internal Strategic Perspective. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: National Water Resource Planning. DWAF Report No P WMA 08/000/00/0304.

Ref 2. Department of Water Affairs and Forestry, South Africa, Middle Vaal Water Management Area: Internal Strategic Perspective, 2004. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: Water Resource Planning. DWAF Report No P WMA 09/000/00/0304.

Ref 3. Department of Water Affairs and Forestry, South Africa. 2004. Lower Vaal Water Management Area: Internal Strategic Perspective. Prepared by PDNA, WRP Consulting Engineers (Pty) Ltd, WMB and Kwezi-V3 on behalf of the Directorate: National Water Resource Planning. DWAF Report No P WMA 10/000/00/0304.

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