In collecting surface run-off, it is worth thinking in terms of micro-catchments - small areas that collect surface water for a specific area or specific purpose.

In India (ref cse) it has been calculated that every village has sufficient surface area to collect all of its needed water for cooking etc (15 litres per day per person) , given their average rainfall and assuming a collection efficiency of 50% - the calculation even suggests all villages could collect sufficient rainfall in a drought year in which only half the normal rainfall is received.  



ref cse