Evapotranspiration for Irrigation: Convert Reference ET into a Field Water Budget

To use evapotranspiration for irrigation, match local reference ET to an appropriate crop coefficient, calculate crop water use, and reconcile it with rainfall and root-zone storage. Convert the resulting net water requirement into a gross delivery volume and runtime using documented field performance. A weather estimate alone cannot tell a controller when a particular soil profile can safely accept that water.

An agricultural weather station used to collect data for water-management models. Photo: Scott Bauer, USDA Agricultural Research Service. Public domain.

Start with the reference surface, place, and reporting day

Reference evapotranspiration, written ETo here, describes atmospheric demand over a defined, well-watered reference surface. Crop evapotranspiration, ETc, describes a particular crop under stated conditions. They are different inputs in a scheduling calculation. FAO's introduction to evapotranspiration explains the grass reference and the distinction between standard and constrained crop conditions.

A useful weather record includes the station or grid location, elevation if supplied, reference surface, calculation method, accumulation period, time zone, and data-quality flag. An alfalfa-reference value cannot simply replace a grass-reference value while keeping a coefficient calibrated for grass. Likewise, a monthly average cannot stand in for yesterday's observed conditions without clearly changing the calculation's purpose.

For a field worksheet, keep forecast and observed columns separate. A forecast helps reserve pumping time; the completed day's observation updates the balance. Replacing the forecast with the observation should revise that day's entry, not add a second day of water use. Missing weather data must remain identifiable as missing. A zero inserted for convenience falsely reports no atmospheric demand.

This article uses invented teaching data for a hypothetical open-field drip-irrigated block, with grass-reference ETo and a constant crop stage over a short interval. No location, weather station, crop recommendation, or IrriNex field trial is claimed for the numbers. A farm must substitute its own verified weather and agronomic inputs.

Convert ETo to crop ET without treating Kc as permanent

The basic single-coefficient relationship is ETc = Kc × ETo. Kc is dimensionless; ETo and ETc must use the same depth and time units. The FAO crop-coefficient method incorporates crop development and average soil evaporation, with adjustments for local conditions. A coefficient selected for one growth stage is not automatically valid after canopy development, harvest, or a change in wetting pattern.

For the example, assume a suitably selected Kc of 0.8 and daily ETo values of 5, 6, and 4 mm. The corresponding ETc values are 4, 4.8, and 3.2 mm per day. Their three-day total is 12 mm. These are calculated standard-condition estimates, not measured withdrawals from the soil and not a universal crop coefficient.

A constant single coefficient makes this short arithmetic example transparent. Detailed daily scheduling may need separate crop-transpiration and soil-evaporation coefficients to represent wetting events. Stress, salinity, sparse growth, or disease can also change actual ET. A lower estimated actual ET under stress does not establish that the crop's water supply is adequate or that healthy-crop demand has disappeared.

Record the coefficient's source, reference basis, crop stage, and review date beside the value. If the stage changes partway through the reporting period, calculate the relevant days with their appropriate coefficients. Do not hide that change by averaging unrelated coefficients and presenting the result as a locally validated irrigation prescription.

Define rainfall credits and soil depletion consistently

A rain gauge measures precipitation at its location. It does not directly measure the contribution retained for the crop. FAO's explanation of effective rainfall distinguishes useful rainfall from water lost before it serves crop needs. Event conditions matter: runoff, infiltration, existing storage, and drainage can change the useful contribution. A monthly estimation formula should not be applied as a daily storm rule.

For this worksheet, Dr means root-zone depletion relative to field capacity: a larger value means a larger water deficit. The reduced accounting equation is Dr,end = Dr,start + ETc − Pe − Inet, where Pe is the credited rainfall contribution and Inet is irrigation retained for the root-zone budget, all in mm over the same field area and interval. These effective credits must use a consistent accounting boundary, with any losses already removed from them excluded from additional loss deductions.

The reduced equation is appropriate here only because the example assumes no capillary contribution and no additional drainage term. The FAO root-zone water balance explicitly includes runoff, infiltrated irrigation, capillary rise, and deep percolation. Use that fuller treatment when these flows matter; its infiltrated irrigation input must not be confused with an already retention-adjusted credit.

For example, do not subtract the same evaporation once from a rainfall credit and again through an ET term that already includes it. Similarly, counting retained irrigation after drainage and then subtracting that drainage again understates storage. A negative calculated depletion flags water beyond the assumed storage boundary or inconsistent inputs; it is not a bank of negative irrigation demand to carry indefinitely into dry weeks.

Work through a three-day field water budget

Assume initial depletion of 8 mm. Credited rainfall is 0, 3, and 1 mm over the three days. Add one retained irrigation depth of 8 mm early on day 2. For this teaching calculation, assume the active root volume, infiltration, and agronomic depletion limit permit the stated sequence without stress or additional drainage. Rainfall credits and irrigation occur early in their respective days, followed by the daily ET debit.

Day 2 is a useful sign check: 12 + 4.8 − 3 − 8 = 5.8 mm. In the assumed early wetting sequence, its combined 11 mm credit fits within the starting 12 mm depletion. The subsequent ET debit leaves the stated end-of-day deficit. Changing when the water arrives can change within-day storage and stress even when daily totals stay the same.

The period closes at its starting depletion because 8 + 12 − 4 − 8 = 8 mm. With the same hypothetical ET and rainfall but no irrigation, the bookkeeping would end at 16 mm depletion. Whether that alternative would cause stress requires actual root-zone capacity and crop-specific limits; the arithmetic alone cannot answer it.

If the desired end depletion equals 8 mm, the required retained irrigation is 8 + 12 − 4 − 8 = 8 mm: starting depletion plus ET, minus credited rain, minus the target depletion. This equality with ET minus rain occurs because the initial and target depletion are equal. Refilling a previously depleted profile or intentionally ending with a different deficit changes the result.

The table demonstrates a water account, not a recommended three-day watering interval. A field may require smaller events or an earlier start. Do not copy its event timing into shallow soils, young plantings, restricted rooting zones, or another crop without checking those conditions.

Translate retained depth into volume and runtime

Use one consistent area for the ET depth, rainfall credits, irrigation depth, and delivery volume. Assume this example covers one 4 ha zone and that a separately established overall application factor is 0.85, defined here as the fraction of metered zone delivery retained for the stated water-budget purpose. This is a hypothetical performance factor, not an efficiency rating assigned to drip irrigation or to any product.

The volume conversion uses 10 m³ per mm per hectare. Here the assumed operating flow is 80 m³/h for this zone. Preserve full precision in the worksheet and round the final instruction appropriately. The displayed runtime is decimal hours, not hours and minutes separated by a decimal point. A pump serving other zones simultaneously may have a different total flow; its totalizer cannot be assigned entirely to this block.

Confirm that flow is measured over representative operation using a suitable installation, such as the checks in the irrigation flow-meter installation guide. If flow varies, reconcile delivered volume by integration or totalizer difference rather than multiplying a single snapshot by the whole event duration. Exclude water that the chosen accounting boundary assigns elsewhere.

Because these depths already refer to the entire field, do not multiply the volume by the drip-wetted fraction again. That would apply a second area reduction without redefining the ET model. Do not automatically divide again by distribution uniformity either. The drip-emitter uniformity field test evaluates spatial delivery, which is different from the retained-delivery factor used here.

Use field observations to challenge the calculation

Reconcile the planned volume with the metered event, then compare the predicted soil-water direction with representative observations. Rising depletion after a supposedly sufficient event may reflect a delivery shortfall, an unrepresentative sensor, underestimated demand, or an incorrect storage assumption. A wet sensor beside one emitter cannot establish that the whole root zone received its intended share.

The soil-moisture sensor placement guide helps choose monitoring positions that can challenge a field-average estimate. Keep the weather, meter, and soil records on the same clock. A midnight totalizer reading and a weather service's different reporting day can create an apparent imbalance without a real change in field water use.

Change a documented input when evidence supports the change. Keeping a short reason beside the revision makes it possible to distinguish an updated weather observation from a coefficient change or an equipment fault. Overwriting the calculated balance merely to match one sensor hides the disagreement instead of resolving it.

Keep the budget separate from event authorization

A calculated water volume is an agronomic planning input. Pump capacity, permitted zone combinations, infiltration limits, equipment availability, and existing safety protections still constrain the event. The 4.71 h calculation does not authorize a continuous run, a particular valve sequence, or a fertilizer injection. Those decisions require the corresponding field and equipment checks.

For remote scheduling, record the last accepted weather update and the source of any fallback instruction. The guide to irrigation control with unreliable internet addresses continuity of local control. A delayed weather upload should not silently duplicate an event already completed locally or erase its delivered volume.

Close each event with its actual start and stop, zone identity, measured delivery, relevant alarms, and the resulting budget revision. If an event stops early, carry its verified delivered amount into the calculation before planning the remainder. Reissuing the original full volume after a partial completion would count water already applied as still outstanding.

What should the completed worksheet let an operator verify?

The operator should be able to trace every proposed event back to a dated weather record, a justified crop coefficient, an initial soil-water estimate, a rainfall credit, and a target end condition. Each has a different source of uncertainty. Recording them separately makes a later adjustment understandable instead of hiding all uncertainty inside a single runtime multiplier.

For this example, the trace is explicit: 12 mm of calculated ET, 4 mm of credited rain, unchanged starting and ending depletion, 8 mm retained irrigation, 376.47 m³ gross delivery, and approximately 4.71 h at the assumed 80 m³/h. These numbers form a reproducible calculation. They do not establish crop suitability, acceptable stress, distribution quality, or a guaranteed water saving.

Before releasing a field instruction, verify the accounting area, unit conversions, current crop stage, observed rainfall update, root-zone acceptance, and achievable zone delivery. After the event, retain the measured outcome with the assumptions used. That record makes the next scheduling decision testable against what happened in the field.