THUKELA RIVER: Hydrology and geohydrology

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Map of the THUKELA River

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HYDROLOGY
Due to the mountainous nature of the Thukela Water Management Area and its proximity to the Indian Ocean, the rainfall is high by South African standards, ranging from over 1 500 mm per annum in the mountains to about 650 mm per annum in the central parts of the catchment. As a result of the high rainfall, there is substantial runoff from the Thukela catchment, with the Mean Annual Runoff ,estimated at 3 799 million m3/a. Rainfall is however erratic and years of prolonged drought in the central and lower catchment alternate with very wet periods. Ref.1.
The Thukela is said to  have "terrible peak flood" characteristics , so that during the rainy season "it's not uncommon to see walls of muddy water 0,9m to 1,5m high advancing downstream, bearing trees, animals and detritus...  As the speed of the current increases 2 to 5 fold, wholesale movement of the river bed results, and the river often changes course within its bed.  (Ref 2)

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Streamflow: 
  • MAR (Mean Annual Runoff): 4000 x 106m3 (equivalent to 17% of the mean catchment MAP)
  • Varies: 50 – 250 mm.annum-1 between subcatchments
  • Coefficient of variation of annual flow: 40 – 50% in Drakensberg mountains to > 150% in central valleys, mostly 60 – 100%, i.e. 4 – 6 x CV of MAP
  • January and February flows account for  > 40% MAR, June to September together for < 8% of MAR     (Ref 3)
Irrigation water requirement:
  • 800 – 1200 mm.annum-1
  • 30 – 100 mm in January
  • 80 – 120 mm in July

Approximately 2% of agricultural land is irrigated                     (Ref 3)

Solar radiation:
  • 25 – 30 MJ.m2.day-1 in January (midsummer)
  • 13 – 17 MJ.m2.day-1 in July (midwinter)                        (Ref 3)
Potential evaporation:
  • 1600 – 2000 mm per annum
  • 180 – 220 mm in January
  • 100 – 110 mm in July                                                (Ref 3)
    GEOHYDROLOGY                                    
The Department of Water Affairs data indicates that present groundwater usage in the Thukela Water Management Area is about 2.75 million m³/a. This converts to a usage of only 100 m³/km²/a over the Water Management Area as a whole, which is only some 0.4 per cent of the mean annual recharge over the area. Usage is lowest in the areas of low-density population and highest in its more densely populated rural areas. It is evident that present groundwater usage in the Water Management Area is in the low to very low range in terms of the sustainability of the available resource. Groundwater usage in the Water Management Area can thus be safely increased considerably without detrimental impact on the resource.  Increased groundwater use should therefore not impact significantly on the surface water resource. (Ref. 1)
Groundwater harvest potential mostly in the range of 10 000 – 25 000 m3.km2.annum-1, restricted primarily by the volume of effective storage.  Average borehole yield over most of the Thukela catchment is 0.6 – 0.8 litres.s-1 with some areas 0.8 – 1.5 litres.s-1.  Groundwater quality, in terms of total dissolved solids (TDS) is < 200 mg.litre-1 in the west and 300 – 400 mg.litre-1 in the north. (Ref 3)

  References                                      

Ref 1:  Department of Water Affairs and Forestry, South Africa. 2004. Internal Strategic Perspective: Thukela Water Management Area : Prepared by Tlou & Matji (Pty) Ltd, WRP (Pty) Ltd, and DMM cc on behalf of the Directorate: National Water Resource Planning (East). DWAF Report No. P WMA 07/000/00/0304.

Ref 2: DWAF, 2002a. Department of Water Affairs and Forestry (2002a); Thukela Water Project Feasibility Study: Main Report. Report No. V000/00/9600. Department of Water Affairs and Forestry

Ref 3:  Professor R.E. Schulze University of Natal , Pietermaritzburg, South Africa School of Bioresources Engineering and Environmental Hydrology (BEEH)

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