Sucking the mountain dry?

Is it possible that by pumping out water for agriculture and use in towns and villages we are drying out the mountain? This is the question that Wietsche Roets, an Aquatic Scientist with Cape Nature in George put to a meeting of the Gouritz Initiative (GI) at Oudtshoorn Proefplaas in December 2006. The GI is a group of landowners and others interested in conservation-friendly and sustainable development in the Gouritz River catchment that extends roughly from Mossel Bay to Plettenberg Bay and from George to Prince Albert.


Wietsche explained that rainfall is far greater on the mountain tops than on the lower slopes or plains of the Karoo. Most of the rain that falls on the mountain seeps down into the Table Mountain sandstone layers and continually tops up the groundwater aquifer that underlies both the mountain and the plains. When the aquifer is full, the water that seeps into the mountain overflows, in hydrological terms “interflows” as small streams on the higher parts of the mountain (see sketch A). Lower down on the mountain, the larger rivers are fed by both interflow and discharge from the aquifer.

When large quantities of ground water are abstracted (or extracted) from the aquifer, a cone of depression forms around the borehole (see sketch B). Depending on the extraction rate and the rainfall, the level of the water table gradually drops, not only around the borehole but may extend throughout the aquifer. The rainfall on the sandstone mountain tops infiltrates, replenishing the ground water in the aquifer, however, according to Wietsche Roets, if the level is not sufficiently topped up because of extraction in the valleys, then water will not “overflow” as interflow in the mountains, and the mountain streams will dry up and perennial streams may become seasonal.

These changes may take many years, so while at first we may see no effect of ground water extraction, in the longer term as the depletion continues, the river flow and water availability will diminish. Once this has happened, the problem is difficult and slow to reverse.

Although the folding and faulting of the sediments make it difficult to predict exactly which stream will dry up when, there can be no doubt that all the underground water in this part of the Karoo and in the Little Karoo is linked with the mountain aquifer. In the Kammanassi, the Vermaaks River has already run dry as a result of water extraction for agriculture.

It is the mountain streams and rivers that give life to many kinds of plants such as the indigenous willows and kiepersols along the Dorps River near Eerstewater, and the diversity of aquatic animals found only in rivers that run through Fynbos vegetation. These include red fin minnows, dragonflies, and frogs of various kinds. Probably less well known is the fact that the larvae of the extraordinary long-tongued horse flies that are essential for pollinating some Fynbos bulb plants such as Afrikaners (Gladiolus species), malvas (Pelargonium species) and orchids, start off life as worm-like larvae filter-feeding on fragments of dead plants in the flowing water of mountain streams.

In making decisions about how we use our water in this beautiful and fertile valley of Prince Albert, we need to remember that the Karoo is in world terms a desert. The water we use does not come from our local rainfall, but from the rainfall on the Swartberg Mountain. The mountain water can sustain the village comfortably, so long as there are not too many people, and so long as our agricultural and industrial development does not use water extravagantly. Water is wasted in many ways – from open and leaking canals through flood irrigation and maintenance of gardens more suited to the European climate

Report by Sue Milton-Dean, Conservation Ecology, University of Stellenbosch and resident of Prince Albert