Storing water is legally defined as diverting water from a stream and holding it in a tank or pond use later in the year when streamflow is low. In coastal California, storing water is generally a very good idea for rural landowners, for several reasons:
• It provides a more secure supply of water during the dry season, when water sources become unreliable
• It protects fish and wildlife, which need water most when streamflows are lowest
• CDFW may place legal restrictions on diversion during the dry season under an LSA Agreement
• In drought years, the Water Board may place emergency restrictions on diversion during the dry season by issuing a curtailment order
Unfortunately, storing water is one thing you cannot do with a riparian water right (the water right that comes automatically with the ownership of land along a stream). To store water legally, you need an appropriative right from the State Water Board. The most common way to do that is to file a Small Domestic Use Registration (SDU) with the Water Board.
Note that just because you have a water tank does not mean you are technically storing water. If the tank holds a relatively small amount relative to your water use – generally, less than a month’s supply – it may be considered to serve the purpose of regulation (evening out short-term fluctuations in streamflow) rather than storage (saving water for the dry season). But again, if your tanks are too small to meet your needs during the dry season, you should strongly consider adding capacity so that you are truly storing water. The security of your water supply is worth the trouble.