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What is the soil like? Is it rock you are going to build the tank in? If so this could be difficult to excavate? Are you planning to put a roof on it? If not then the deeper the tank the better - reduces the evaporation from both the reduced surface area and, if the water level is some way below the ground level then evaporation from the wind blowing over it is reduced as well.

If the soil can stand it a circular shape is best as it is most stable but if you have to slope the sides, it is often easier to dig a square or rectangular shape.

In terms of the rainwater catchment - is this the only source of clean water for the community, have you been able to get rainfall records for the last 20 or 30 years? How are they to regulate their use of the water to make sure it lasts as long as possible.

I know of two technical briefs from Practical Action on rainwater catchment in general and on underground tanks. One has a lot of further resources to get more info.

Regards Toby Gould