Cistern / Storage Reserve Days and Required Volume

days = usable storage / daily demand, and required volume = daily demand x target days. A 2,500-gal usable cistern at 150 gpd carries 16.7 days; banking a 30-day dry spell needs 4,500 gal. Use USABLE storage (above the pump intake and below overflow, past any fire or first-flush reserve) and a realistic peak demand, not the annual average. For a rain-fed cistern the reserve must bridge the longest dry spell between refills; for a hauled or well-fed tank it sets the refill interval. A planning estimate; local rainfall or the source yield, the storage detail, and the AHJ (for potable use) govern.

reserve_days = usable_storage_gal / daily_demand_gpd; required_gal_for_target = daily_demand_gpd x target_days.

Rainwater-harvesting / off-grid storage mass balance (ARCSA / TWDB rainwater manuals; water-storage practice), by name; local rainfall or the source yield and the AHJ govern.

The reserve relations are a public first-principles mass balance (storage over demand; demand times days); the usable storage, demand, and target come from the tank detail and the water budget.

Estimate. Operator of record and primacy agency govern.

Field names used by the API: usable_storage_gal, daily_demand_gpd, target_days, reserve_days, required_gal_for_target

• Usable storage above the pump-intake dead volume and below overflow, past any fire / first-flush reservestorage-tank detail
• Peak demand use a realistic peak daily demand, not the annual average; the reserve must bridge the worst stretchwater-budget practice