HYDROLOGY

Hydrology  

The assessment of major impacts of invading alien plants on hydrological processes and water yields.

A national review of potential stream-flow reduction by invading alien plants, conducted by the CSIR and funded by the Water Research Commission, found in 1997 that a total area of about 10.1 million hectares (6.8%) of South Africa and Lesotho had already become invaded to varying degrees of density. These invasions were estimated to be reducing the national mean annual runoff by about 3300 million m³ (6.7% of national runoff). Given the exponential spread of invading alien plants, it is of the utmost importance that Working for Water builds on this research to monitor this state of affairs and evaluate the clearing operation’s effectiveness in reversing this trend.

Most of the evidence for excessive water use by invading alien trees comes from long-term catchment experiments in South Africa and elsewhere. These experiments have been established in high-rainfall areas where shrublands or grasslands were afforested with pines or eucalypts. The results of these experiments have been used, together with crude estimates of the extent of invasion, to make a preliminary estimate of the size and significance of the problem in South Africa (outlined above). While this knowledge has been sufficient to put forward a convincing case for the clearing of invasive alien trees, there are several important gaps in our knowledge which need to be filled to allow us to make better estimates of the impact of invading trees across the landscape. These include a need to expand our understanding of impacts to areas receiving lower rainfall, and to species other than the pines and eucalypts that were used in the original catchment experiments. The current hydrological research programme is small, and focuses on a number of case studies aimed at filling some of those gaps. These include studies into water use by black wattle (Acacia mearnsii), a widespread weed in high rainfall areas, and several case studies aimed at quantifying the effects of clearing infestations of alien plants on water runoff.

A study was commissioned to establish the research priorities in the field of hydrology. The results of this study have provided some idea of the extent of available knowledge on the hydrological impacts of invasive alien plants. Some of these include the following:

• Tall alien trees, such as pines and eucalypts, generally cause greater streamflow reductions than the indigenous vegetation they replace.

• Impacts on the water resources observed after indigenous vegetation has been replaced by tall alien trees, include reductions of mean annual streamflow , mean annual "low flow", and changes in flow duration.
• Proportionally, the reduction in dry season flows due to tall alien trees is generally more severe than the reduction in wet season flows.
• After clearing of dense and extensive stands of alien trees, it may take a few years before streamflow recovery approaches pre-invasion levels; i.e. there may be buffer storages in the sub-soil that need replenishment.
• Invasion of riparian zones by alien woody plants cause greater streamflow reductions (up to 200%) than invasion of upland plant communities.
• Clearing of invasives along rivers, during relatively low-flow conditions, typically results in streamflow increases of 8 000 to 12 000 litres/ha per day in the winter rainfall region and up to 34 000 litres/ha per day in the summer rainfall region, regardless of location and species. This is only applicable to the first few months after clearing.
• Evapotranspiration and streamflow reductions due to different alien plant categories vary markedly.
• Streamflow reductions are dependent on age and biomass.
• Streamflow reduction is dependent on geo-climatic site conditions.
• Evapotranspiration by riparian alien plant communities is markedly greater than evapotranspiration by riparian indigenous plant communities.
• Evapotranspiration depends on age, biomass, and geo-climatic site conditions.
• Groundwater storage in certain shallow coastal sand dune aquifers appears to increase in response to the clearing of woody alien vegetation.
• Shallow and deep groundwater aquifers in the Karoo and the Kalahari can be accessed by woody invasive trees, e.g. Prosopis species.

This assessment provides a benchmark for current understanding, but it is clear that more work needs to be done to fully understand the hydrological implications of invasions.

Future work on the hydrological impacts associated with invading alien plants will focus on a number of questions that were identified in the study referred to above. These include:

• Which characteristics of a water resource are changed by alien vegetation invasion, or by the clearing of invaded areas?
• By how much does invasion by alien vegetation, or the clearing of invading plants, change the characteristics of a water resource?
• At what rate does invasion by alien vegetation, and its clearing, change the characteristics of water resources?
• Which areas in any specific catchment are prone to invasion?
• What are the environmental, social and economic implications of the changes to the characteristics of the water resource caused by alien plant invasions, or by the clearing of invaded areas?
• How do alien vegetation management decisions influence water resource economics and planning?

A second phase of developing a coherent hydrology research strategy will be commissioned, and it is envisaged that this study will be completed by March 2002.