Water Affairs and Forestry Minister Buyelwa Sonjica says one of South Africa's challenges in managing water resources lies in the supervision of the spread of invasive alien-tree species.
She told the World Water Forum in Mexico that South Africa had about 1.35 million hectares of plantation forests, of almost entirely alien species.
"The nature of these invasions are exponential, so there will be an increase of invasions, given the foothold that has already been gained by these species.
"Many of the large trees that are invasive have a far greater impact on water than the plants they displace, and the estimate is that around seven percent of mean annual run-off is being lost to invasive alien plants."
She said the impact of these invasive trees on water security would increasingly be problematic, especially in mountain catchment areas and other difficult-to-reach areas.
"The problem is that alien invasive trees tend to go to the very areas that responsible forestry industry avoids - riparian areas, steep slopes, wetlands and other areas where the impacts are more severe.
"Due to this problem on our water security we have established a major job creation programme, the Working for Water Programme, to clear alien invasive species from our catchment and riparian areas," said Ms Sonjica.
She urged delegates to debate the role that forests played in water related risk management.
"In our discussions we should not focus only on the technical understanding of the role of forests in water related risk management but we must also focus on the people who are at risk.
"In South Africa, as in most parts of the world, people who are most at risk are poor women and men," she said
Ms Sonjica said not only did people at risk often live in areas vulnerable to natural hazards, such as urban flood plains where they crowded together in shanty towns, but they had little or no access to emergency services, to emergency transport, to insurance to enable them to rebuild their lives after a disaster.
"They are the most vulnerable members of our societies, wherever we live. It is in their interests in particular that I am speaking here.
She said South Africa was a relatively dry country that received around half the world's average rainfall.
"Within that dryness, we are vulnerable to both floods and droughts. Over the past several years we have been in the grip of a dreadful drought. We have put in emergency water schemes, tankered water to communities where water supplies have dried up, cut water supplies to farmers and watched crops and animals die," she said.
However the minister said the past two months had brought heavy rains that filled the dams, replenishing groundwater in many areas but also brought flooding to some areas.
She said the benefits and costs of further commercial forestry expansion [water and others] must be weighed against the benefits and opportunity costs that competing water uses presented, and "we need to have the flexibility to allocate and re-allocate according to the outcome of such evaluations".