Retention Pond Water Budget Calculator
Average-year water budget of a retention pond: runoff in, evaporation and infiltration out, and the net volume.
Pond and watershed inputs
The fields open with the values of FHWA HEC-22 (FHWA-HIF-24-006) Example 10.13. Replace them with your own.
Annual water budget
Net volume
Runoff (in)
Evaporation (out)
Infiltration (out)
Formulas with your numbers (43,560 ft²/acre)
The formulas appear once all seven fields hold valid values.
Basis and limits
Positive net is a surplus, negative net is a deficit. Average annual conditions only: infiltration acts on the bottom area for 24 h × 365 days and evaporation on the average pool surface area, as in the source example. No basin outflow, no change in storage; dry or wet years are not covered.
Source: FHWA-HIF-24-006, Hydraulic Engineering Circular No. 22, Urban Drainage Design, Fourth Edition, February 2024, Chapter 10, Example 10.13, printed pp. 223–224.
For preliminary estimation only, not a substitute for review by a qualified/licensed engineer.
What a retention pond actually keeps in an average year
A retention basin is sized against storms, but whether it holds a permanent pool is decided by the long-term balance. Runoff from the contributing watershed fills it; evaporation off the pool surface and seepage through the bottom drain it. If the two losses outrun the inflow, the pool shrinks no matter how well the outlet works.
This calculator gives stormwater and highway drainage engineers the quick average-year balance used in HEC-22 Example 10.13: how much of the runoff from a 100-acre watershed the pond retains once evaporation and infiltration are taken out. Runoff, evaporation, infiltration and the net volume are each reported in cubic feet per year.
Use it as a screening check before committing to a full routing model. It covers average annual conditions only, with no basin outflow and no change in storage, so treat the net figure as a first estimate and have a licensed engineer review the design.
How to use it
- Enter the average pool surface area in acres and the pond bottom area in acres.
- Enter the contributing watershed area in acres and its runoff coefficient C as a decimal between 0 and 1.
- Enter the infiltration rate in inches per hour.
- Enter the average annual rainfall and mean annual evaporation in inches per year.
- Read the runoff, evaporation and infiltration volumes and the net ft³/yr figure with its surplus/deficit tag.
FAQ
How is a retention pond water budget calculated?
Runoff in minus evaporation and infiltration out: runoff = C × (rain ÷ 12) × watershed area × 43,560; evaporation = (evaporation ÷ 12) × surface area × 43,560; infiltration = (rate × 24 × 365 ÷ 12) × bottom area × 43,560; net = runoff − evaporation − infiltration, all in ft³ per year.
What does a negative net volume mean for my pond?
A negative net is a deficit — evaporation plus infiltration remove more water in an average year than runoff delivers; a positive net is a surplus.
Why does infiltration use the bottom area and evaporation the pool surface area?
In the HEC-22 procedure the infiltration rate acts on the pond bottom area for 24 hours × 365 days, while evaporation acts on the average pool surface area — Example 10.13 in FHWA-HIF-24-006 uses exactly that split.
Can you show a worked check of the numbers?
With surface area 3 acres, bottom 2 acres, watershed 100 acres, C = 0.3, infiltration 0.1 in/h, rainfall 50 in and evaporation 35 in, the tool returns runoff 5,445,000, evaporation 381,150, infiltration 6,359,760 and net −1,295,910 ft³ per year — the same figures printed in FHWA-HIF-24-006, Example 10.13.