Reading through the responses ...

Reading through the responses (most are very good and helpful), I noticed there was only one mention of evaporation and whether evaporation exceeds transpiration throughout the year.  In particularly, how many months in a year evaporation exceeds transpiration at a proposed storage location.  This is a key design criteria and often determines whether a storage should be built at all.  In a situation were evaporation exceeds precipitation and the storage is to be a shallow one, evaporation could remove 2-3 metres from the storage each year to evaporation with this increasing as storage reservoir reduces its volume.  If 90% of the storage capacity is held in the first few metres from the surface, the losses from such a poorly designed storage would render such a structure obsolete with no net gain whatsoever to farmers, the people and state and the cost of building this structure would never be written off against benefit derived.  The only way to help offset this situation is to have good evaporation and precipitation records for extended periods and only select potential structure sites that are suitable.  Regards, Kevin.