When water drops are fallen on the earth there may be a conditions accord to identify the water is soft or hard if the rainfall is taking place in very large Industrial areas, near to volcano and any polluted very large polluted area the rain may be acidic and the the Harvested Rainwater may be Hard due to the water is contacting with pollution,if the rainfall is at less polluted as well as near to forest area, village area the water water is 100% soft because Areation takes place, water is contacting with AIR/Oxygen due to that Quality of water increased. So for any design related to Rainwater Harvesting please consider above information
Water that contains dissolved minerals like magnesium and calcium is considered to be hard while water that contains only sodium ions is considered to be soft.
Since water harvested from rainfall does not contain any dissolved minerals, it can be considered as soft water.
However, if the rainwater comes in contact with different types of materials from which it may absorb different minerals before getting harvested, it may have to be considered as hard water. This harvested water might also need to undergo softening treatment before being used for any applications.