Food for thought when sizing tanks...

A comment on a previous blog post raised the questions of tank sizes for Rainwater Harvesting (RWH). How large would the tanks need to be to collect and store water for 6-8 months of the year? Well there is no simple answer. In this blog post I will be detailing some of the factors, as mentioned by Kahinda et al (2007), which should be considered when sizing the RWH tank.

There is obviously a limit to the amount of water that a tank can supply in a day or week. This depends on its size and location. It needs to be sized properly to get the full benefit from it so its use can be optimised for the location and climatic conditions it is in.

To find the optimum size the following conditions need to be considered: 

• How much water is available and when; if the rainfall is high intensity in a short period then things like seepage and high evaporation rates need to be minimised.
• The space available for the tank; some areas, such as peri- urban areas, may be too crowded. Any alternative water supplies; if there are alternatives the tank may not need to be as big. 
• Technical constraints such as lack of labour and rocks in the ground which make digging and underground storage difficult.  
• The roof type; rural houses usually have thatched roofs which allow less runoff than corrugated iron roofs  
• The soil type; above ground tanks should not be built on clay soils which expand.
• Socio- economic limiting factors such as the amount of labour available or regular upkeep of the tank which require financial input. For example in South Africa, 67% of rural households cannot afford the purchase of a tank, never mind about the maintenance (Kahinda et al 2007).

 

Therefore, we can see that RWH is not as simple as one may first think. With sizing there are many aspects which need to be considered and there needs to be extensive cooperation between governments, financers, communities, users, NGOs…the list can go on. It is not only physical aspects that need to be considered but socio- economic aspects too. 

The success needs to be evaluated in terms of locality and so in subsequent blogposts I will be looking at a particular country to see rainwater harvesting in practice.