Introduction

By the year 2025 two-thirds of world population will have to cope with water stresses. South Africa, with its semi-arid conditions, variable climate and rapidly growing population is one of the countries that is effected. The region is already becoming increasingly prone to periodic water shortages that do threaten necessary developments and even the sustainability of existing industry and agriculture.

Improved resource conservation can make a significant contribution to countering this threat. However, at the same time there is a growing need to augment the country's water resources. The challenge of finding and developing new water sources to avert disaster in some rural areas, to meet rapidly growing needs in others, and also to satisfy the steadily growing water requirements of the urban and industrial sectors, is becoming greater all the time. Obtaining additional water either from areas of surplus or from the successful exploitation of unconventional water resources will be vital for the country's sustained growth and stability.

Atmospheric Water

It is calculated that only about a tenth of the moisture advected across the country in the atmosphere falls out as rainfall. The presence of suitable aerosol (dust) particles in the atmosphere plays a crucial role in cloud and precipitation formation. The activation of each droplet and ice crystal in a cloud is dependent on a suitable aerosol (dust) particle. It is for this reason that a distinction can be drawn between the properties and the rainfall production efficiency of clouds developing over maritime and continental areas. Maritime clouds are generally more efficient in producing rain as the lower aerosol concentrations (with a high sea salt content) activate less droplets that have to compete for a given quantity of water vapour. Over continental areas the aerosol and droplet concentrations are higher and the competition for the available moisture is intense.

A large portion of the rain over the interior of South Africa is produced by convective storms. These storms are not efficient in converting the available moisture in the atmosphere into rainfall as a large portion of the water is carried high into the troposphere to produce the characteristic anvil clouds. Processes of rain formation in summer convective (cumulus) clouds have been studied over a continuous period of approximately 15 years in South Africa. These studies were funded by the Weather Bureau and the Water Research Commission up to early 1997. During this period unique skills, facilities and infrastructure have been developed. These include skills in cloud physics, aviation, instrumentation, electronic hardware and software engineering, radar signal processing and data analysis. Facilities include instrumented aircraft, meteorological radars, and sophisticated radar storm tracking software. Of particular relevance is the development done to improve radar rainfall measurement. The high spatial and temporal resolution of radar data is a distinct advantage over rain-gauge measurements when area rainfall from convective storms needs to be determined.

Differences between clouds which are efficient and inefficient producers of rain provided the necessary direction to the research undertaken in pursuit of a viable rainfall enhancement technology. For example, such differences revealed the need to focus attention on promoting growth of cloud droplets in preference to growth of ice crystals in order to speed up the rain-production process.

A New Approach To Rainfall Enhancement

By 1991 the research had resulted in a novel, internationally patented, cloud seeding technique employing aircraft-borne flares which, when ignited at cloud base, release hygroscopic salt particles into the cloud. Such particles are most effective in promoting rapid growth of cloud droplets by coalescence. This has been confirmed both by measurements made in the clouds and by computer modelling.

Carefully controlled experiments between 1991 and 1997 have proved that storms seeded with the hygroscopic flares produce considerably greater amounts of rain than storms which were not seeded. These experiments were designed with the assistance of statisticians at UNISA and were conducted on a randomized basis in order to obtain comparable groups of seeded and non-seeded storms. The results of this experiment are shown in Figure 1 as a quartile analysis of average radar measured accumulated rain mass for the seeded and non-seeded groups.

Figure 1. Quartile analysis of South African randomised cloud seeding experiment.

Such seeding-induced rainfall increases from individual storms translate into meaningful increases in areal rainfall if all "seedable" and "reachable" storms within an area can be seeded. Scientific evaluation indicated that the areal rainfall increased by 7% if all these storms are seeded. Other studies, using a daily catchment model, found that this areal rainfall increase translated to a median increase of between 20% and 48% in the mean annual run-off in 13 different catchments over the eastern Highveld and escarpment. The model also indicated that, in addition, the average median timber yield increase due to seeding came to 22% in areas were timber production is important. Similar benefits to crops and grazing are also expected.

Results obtained from the seeding experiments with hygroscopic flares and from scientific evaluation have been complemented by valuable experience gained during an emergency operational seeding programme initiated in South Africa's Northern Province since 1995. The programme was undertaken at the request of the communities and the provincial government and aimed to assist in alleviating the devastating impacts of drought in the province. The nature of the experience gained was in the organisation, management and execution of a pilot operational programme for the seeding of summer convective storms using the hygroscopic seeding approach. Although the NPRP came to an end in early in 1997, the Departments of Environment Affairs and Tourism, Water Affairs and Forestry, and Agriculture decided to fund the new South African Rainfall Enhancement Programme (SAREP) towards the end of 1997. The focus of SAREP is to implement the rainfall enhancement technology developed by the NPRP on an operational basis using sound benefit-cost ratio principles. As a result of the operational seeding experiments, complex issues pertaining to how the benefits of seeding operations may be assessed are now being clarified.

As randomised experiments can not be conducted under operational conditions, obtaining a control group of storms is done by pairing each seeded cloud with a non-seeded storm that matches the seeded one best during the early part of its lifetime. It is during this early part of the storm lifetime that no effect of seeding is expected. The average accumulated rain mass of the seeded and control groups obtained in this manner is shown in Figure 2.

Figure 2. Analysis of South African operational cloud seeding experiment.

Conclusions

The South African achievement in rainfall enhancement is internationally recognised as a significant step forward in addressing the world's water related problems. The radar related developments done in South Africa to obtain the accurate measurement needed to verify the results of seeding experiments have been exported to six countries and the hygroscopic seeding technology is already used in France and Mexico. Requests for information and assistance are received continuously from other countries and regions of the world. The South African research has shown that it is possible to use the infrastructure needed for rainfall enhancement experiments in real-time systems to monitor area rainfall and issue flood warnings. As also stated by the World Meteorological Organisation there is a growing need to continue in the development, testing and implementation of successful rainfall enhancement technology as an integrated part of water resources management.