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Reservoir Yield & Storage Estimator
Enter monthly inflows and demands for a dry spell and the calculator applies the Rippl mass-curve method: it tracks the cumulative deficit month by month and reports the active storage needed to meet demand without shortage. See Topic 18 for the theory.
Required storage = largest cumulative (demand − inflow) drawn during the dry spell
| Rippl rule | accumulate each month’s deficit (demand − inflow); surpluses refill the reservoir but never below zero; the peak accumulation is the storage needed |
| Firm yield | the largest constant demand the reservoir can sustain through the driest period on record, given its storage |
For study and checking only. These calculators run entirely in your browser — nothing is sent anywhere. They are meant to verify your hand calculations while you study; on the exam you will work by hand with the NCEES reference handbook.
Worked example: Rippl storage check
A reservoir serves a constant demand of 8 million m³/month. Six-month inflows (million m³): 12, 9, 6, 4, 5, 10. Find the required active storage.
- Run the Rippl balance (demand − inflow each month; surpluses spill, so the drawn-down amount never goes below zero)
- Month 1: 8 − 12 = −4 → full Month 2: 8 − 9 = −1 → full Month 3: 2 drawn Month 4: 6 drawn Month 5: 9 drawn Month 6: 7 drawn
- Required active storage ≈ 9 million m³ — the deepest drawdown, reached at the end of month 5
Exam tip: Rippl counts only the maximum drawdown — pre-drought surpluses spill and don’t bank storage.
Go deeper
- Read the full flood frequency & reservoir operations theory with worked examples →
- Test yourself: free 7-question flood & reservoir quiz →
- See every FE Civil formula in one index →
Preparing for exam day? The 110-question FE Civil practice exam is a full 5-hour-20-minute rehearsal with detailed solutions.