A reservoir can be large enough to fill one bench and still overflow when several benches drain back. It can also contain plenty of water while the pump runs below its required operating level. Tank capacity is not a single fill-volume calculation.
For an ebb-and-flow system, account for water sent to active benches, water returning later, the pump's minimum operating level, pipe contents and operating reserve. Define which zones may run together before selecting the reservoir and controls.
Start with the bench's water volume
As a first geometric estimate, multiply flooded plan area by water depth. In a hypothetical empty bench measuring 10 m by 1.5 m, a 0.02 m layer is 0.30 cubic meters, or 300 L. Those dimensions are an arithmetic example, not a recommended flood depth.
The actual operating volume changes with channels, slopes, containers, substrate uptake and pipework. Measure a representative loaded bench during commissioning. A calculation using an empty rectangular tray should not be presented as its final crop-cycle demand.
University of Arkansas bench-system guidance describes ebb-and-flow operation as a sequence of filling and draining. The relevant design question is therefore not just how much water a bench holds, but how it fills uniformly and returns water within the intended sequence.
Separate the three reservoir checks
| Check | Worst credible operating state | Design question |
|---|---|---|
| Available drawdown | All permitted simultaneous zones have filled | Is the remaining level safely above the pump's minimum? |
| Return capacity | Filled zones drain back, including after a power interruption | Is there sufficient empty volume without overflow? |
| Cycle recovery | The next irrigation demand arrives before the previous return finishes | Do storage, pump flow and controls prevent an incomplete cycle? |
Nameplate tank volume includes space that may not be usable between the permitted low and high operating levels. Specify those levels and the control behavior, including how automatic make-up water interacts with delayed return flow.
Check concurrency before paying for a larger tank
If two example benches each require 300 L, simultaneous filling starts with a 600 L bench demand before other allowances. Sequential operation may reduce that drawdown, but only if the available irrigation window and return timing permit it. A software change that allows more valves to open together can invalidate the original storage calculation.
Ask for a zone sequence showing fill time, contact time, drainage time and the next permitted start. These should come from the crop and actual hydraulic system, not a timer setting copied from a different bench.
The same cycle may not suit every container
Different pot formats and substrates can take up different amounts during the same flood event. Keep incompatible watering requirements in separately manageable zones. Inspect edge and central containers after the trial cycle rather than accepting a wet bench surface as proof that every root plug received the intended water.
The container-height guide explains why changing pot geometry can change the root-zone water balance. Reservoir sizing cannot compensate for an unsuitable crop-and-container irrigation protocol.
Test the failure states with the installer
Have qualified personnel verify low-level protection, high-level response, drainage paths and the controlled power-loss sequence without risking crop or electrical equipment. Record actual fill and return volumes. Do not bypass alarms to force a test to pass.
For a CFGET bench-system quotation, send a zone plan, container dimensions, proposed crop groups and available water-treatment information. Request both the hydraulic calculation and the operating-level drawing. The useful acceptance evidence is a complete, repeatable loaded-bench cycle with safe storage margins, not merely a tank labeled with a large number of liters.