How this calculation works
Increase depletion for crop ET, reduce it for entered effective rain and cap the storage model. At or above the entered trigger, an end-of-day refill brings depletion back to the selected target.
ETc = ET0 × Kc; depletion before refill = previous depletion + ETc − effective rain; gross refill = net refill / efficiencyA worked example
Starting at 20 mm depletion with 5 mm ETc per day reaches a 40 mm trigger on day four. Refilling to 10 mm supplies 30 mm net, or 37.5 mm gross at 80% efficiency.
Good to know
Entered bookkeeping scenario, not crop-specific scheduling advice. No stress coefficient, salinity, rooting change, rainfall forecast, pump timing or crop-yield prediction.
Reference: FAO 56: root-zone water balance (opens in a new tab) ↗
Depletion increases when water is consumed
Depletion measures water missing from the available storage basis. It rises with ET and falls with effective rainfall or irrigation, unlike a tank-storage amount that rises when water arrives.
ET0 and Kc are separate inputs. The page multiplies them but does not calculate reference ET from weather or select a crop coefficient for your field.
The rainwater daily tank balance calculator adds simulate daily harvest, overflow, delivery and unmet demand across candidate tank sizes using an explicit harvest-before-demand timing convention.
Effective rain is already an adjusted input
Enter rain that actually contributes to the root-zone balance under your chosen convention, not an automatically interchangeable roof-harvest or weather-station total.
If rain exceeds the modeled depletion after ET, the excess is shown as drainage. Demand beyond the entered storage is exposed as unserved modeled ET before the end-of-day refill.
A trigger is an entered policy
The selected threshold and refill target are user assumptions. Refill target must be below trigger and all depletion values must fit the entered storage capacity.
Gross irrigation divides the net refill by efficiency. That arithmetic does not establish whether a crop avoids stress, whether water can infiltrate in time or whether a pump can deliver the schedule.
Use the result with the next measurement
The drip irrigation emitter runtime calculator converts an entered depth target to active emitter time. It does not independently choose the depletion trigger or validate this crop-water scenario.
Entered bookkeeping scenario, not crop-specific scheduling advice. No stress coefficient, salinity, rooting change, rainfall forecast, pump timing or crop-yield prediction.
A few questions, answered
Does the page choose Kc?
No, coefficients are entered per row.
Is raw rainfall automatically effective rain?
No, effective rainfall is supplied directly.
When does irrigation occur?
After each row’s ET/rain balance and storage cap.