Water is a finite resource on this planet. Only 2% is fresh, clean water that is needed for human life. Water is precious, and we need to treat that way. Historically in California, it has been under-valued and under-priced, a factor leading to overuse. Things are changing, however, and water is becoming more expensive.
You should plan for water security and economy. Pumping more water than you need wastes money. Storing water during the wet season and conserving will help insure that you have water when you need it. If we all conserve, the health of the Eel River will improve, and so will community health and our quality of life.
Before you can understand how much storage you need, you need a water budget. Excess storage can lead to unneeded expense and diversion of more water than is necessary
The first step to achieving water security is installing a water meter. You should have one for your house and another for your garden so that you know your domestic and irrigation water use. Make sure that you purchase a system with a leak indicator to prevent unneeded loss and water waste.
Using Graywater and Stormwater to Enhance Local Water Supplies: An Assessement of Risks, Costs and Benefits. Committee for Beneficial Uses of Graywater and Stormwater. 420pg.
To achieve water security, you need to think about available resources and your potential need, and then calculate a water budget. Water planning needs to be done in advance, not when you are in a crisis.
People often over-estimate their water needs, which can lead to unnecessary expense of purchasing too much storage capacity. Water use varies tremendously with the season, as does water supply, so it may take a while to gather the data needed.
Water use data may also be required for some permitting processes. In order to obtain a State Water Resources Control Board permit, basic water use data must be collected and estimates of use based on those data.