JT Vahrmeijer
Head of Grains, Research & Development
Food production is fundamentally linked to the availability of fresh water and therefore global food security depends on irrigated agriculture, which contributes 40% of the world’s food, but occupies less than 20% of all cropped areas. Of the estimated 301 million ha irrigated worldwide, 38% is irrigated with groundwater and represents 43% of the total consumptive use.
An accurate estimate of the groundwater resources in South Africa is difficult to obtain with the early pioneers of hydrogeology estimating the sustainable groundwater yield to be between 2 500 x 106 m3a-1 and 5 400 m3a-1 x 106 m3a-1. In 2005 the Department of Water Affairs and Forestry completed an investigation to quantify the groundwater resource of South Africa at a national scale. From this study, 21 hydrogeological maps were compiled (1:500 000 scale) and the average groundwater resource potential was estimated to be 49 250 x 106 m3 a-1 under normal rainfall conditions, which decreases to 41 550 x 106 m3 a-1 during drought periods. These values are regarded as the maximum volumes that could be sustainably abstracted, if and only if production boreholes could be developed evenly over the entire aquifer system.
Due to the physical and economic constraints to fulfil these pre-conditions, the average groundwater utilisation potential was revised and estimated to be 19 000 x 106 m3 a-1 under normal rainfall conditions, which decrease to 16 250 x 106 m3 a-1 during drought conditions.
Figure 2: Examples of sinkholes on a karst aquifer.
A) A large cover-collapse sinkhole that developed in a catchment dam to collect stormwater runoff.
B) A sinkhole formed during the previous planting season, which the owner backfilled with soil.
Although groundwater is limited due to the geology, a variety of aquifer types can be found in South Africa of which karst aquifers (Figure 1), especially in dolomites, are the single most important type of aquifer. Groundwater utilisation occurs mainly in the Western Cape and eastern and north-eastern parts of South Africa. Accurate groundwater use figures are difficult to obtain and official figures are based on groundwater licenses, which may differ considerably from actual use. Early estimation put the annual increase in surface area, irrigated with groundwater, at 4% since the 1950s.
During the early 1980s South Africa experienced one of the worst droughts in 200 years, which resulted in a widespread emergence of private groundwater exploitation, with estimates of up to 45 000 boreholes drilled annually. Groundwater withdrawal is currently estimated to be 11% or 1 770 x 106 m3 of South Africa’s total (15 500 x 106 m3) with drawal, with irrigation comprising 64% of the groundwater use.
A prominent feature of aquifer systems is their complexity which includes both technical and socio-economic dimensions. Urbanisation, industrialisation, agriculture and forestry are the four main human activities that pose the highest risk for groundwater contamination, with agriculture having direct and indirect influences on the hydro-biogeochemistry of aquifers.
Direct influences are caused by water percolating through the soil profile that carries agrochemical substances, such as fertilisers, pesticides, organic and microbiological constituents to the water table. Indirect effects are associated with the water-rock and watersoil reactions in the soil and aquifer, due to changes in the weathering rates and ion-exchange equilibrium, when agrochemicals are introduced to soils.
When the concentration of these agrochemicals are higher than it would be in the absence of agricultural activities, they are termed agricultural contaminants, although they might not have been directly applied as an artificial substance.
Irrigated agriculture is by far the largest user of fresh water and will considerably increase further over the next decades, due to population growth and planned increases in irrigation and as surface water becomes more limited, groundwater sources will increasingly be exploited, especially in rural and arid areas.In karst aquifers (dolomites) preferential flow paths (e.g. macropores or fractures) acts as conduits for agrochemical substances from the surface to the groundwater. Aggressive solution of the dolomite occurs through preferential flow paths that may lead to the formation of sinkholes (Figure 2).
It is therefore of utmost importance that groundwater withdrawal should be monitored and managed for long term socio-economic stability.
References
- Foster S, Chilton 2003. Groundwater: The processes and global significance of aquifer degradation. Philosophical Transactions of the Royal Society B: Biological Sciences, 358: 1957-1972.
- Gutiérrez F, Parise M, De Waele J, Jourde 2014. A review on natural and human-induced geohazards and impacts in karst. Earth-Science Reviews, 138: 61-88.
- Middleton BJ, Bailey 2005. Water resources of South Africa. Water Research Commission, Report No. TT 381/08. Pretoria: WRC.
- Siebert S, Burke J, Frenken K, Hoogeveen J, Döll P, Portmann 2010. Groundwater use for irrigation – A global inventory. Hydrology and Earth System Sciences, 14: 1863-1880.
- Zektser I, Everett 2004. Groundwater resources of the world and their use. IHP-VI, Series on groundwater, No. 6. Paris: UNESCO.
