It is virtually impossible to provide a general cost-bereft analysis of the different types of reservoirs and storage capacity because of the many unknown factors. However, the following general indicators for decisionmaking can be given.

1. The value of rainwater rises with increased distance to or inaccessibility of other water sources. This means that if rainwater becomes the only source, its value is extremely high. Thus the high investment in a large reservoir becomes cheaper in relation to the value. of water.

2. If rainwater remains the only source of water, rainfall pattems must be studied carefully. If the pattern shows a more equal distribution over a long observation period, it is possible to choose the size of a reservoir according to the precipitation, even on a semi-annual basis. Where the rainfall is extremely unevenly distributed with frequent drought periods, a reservoir should be as large as possible, based on the maximum rainfall. This is expensive but still economic after taking all other factors into account.

3. It is imperative to analyse the purpose of water use and the volume of consumption in advance. Only rough indicators can be given since the consumption will vary from case to case. Rural households in Africa often manage with 40-60 litres of water per day. As mentioned earlier, since easy access does not encourage saving, but on the contrary consumption increases, arrangements should be made to provide additional amounts. Water consumption rates for cultivation cannot be given since this depends largely on the type of crop and soil conditions. If the planned reservoir is expected to serve as a stand-by facility because of frequent breakdowns of a centralized supply, the size can be smaller and the capacity limited to the consumption of a few weeks, depending also on the rainfall pattern.

4. Access to construction materials is another factor to be considered. For instance corrugated iron tanks which are usually very economical might be available only hundreds of kilometres away and therefore become too expensive. Or if reinforcement mesh is not available, a ferro-cement tank cannot be built (see Table 4, indicating the material needed for different types of reservoirs.)

5. Life expectancy and maintenance demands are other factors to be considered. As the example of rainwater catchment at public buildings in Lobatse (Chapter 1.2) shows, under certain circumstances high construction costs combined with long service life expectancy can pay off. Maintenance, usually a weak point in developing countries, has to be taken into consideration. A decision on the capacity and type of structure to be chosen should take all these factors into consideration.

Attention should also be given to the following questions:

-For which purpose is the rainwater to be used?

-What is the likely monthly consumption of water?

-What amount of water can be harvested?

-What is the rainfall pattern, and how is rainfall distributed during the year?

-Which construction materials are available, which are unavailable?

-How high is the financial amount to be invested?

-By how much can costs of construction be reduced by self-help?

Answers to these questions will lead to a decision on the size and type of reservoir. Compromises must be made on the basis of the answers.

The following Table 3 offers assistance in decision-making. Costs are based on 1985 prices in Botswana where all materials are available.