Irrigation Runtime Calculator

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Landscape preset:

Start with the closest hydrozone, then replace defaults with audit or field measurements.

System type:

Choose the delivery model used by this controller valve.

Water need basis:

Use weather demand for peak-week planning or a fixed value when an agronomic target is already known.

Target depth per event:

Changing the display unit preserves the physical target depth.

Weekly plant demand:

Changing the unit preserves the weekly volume.

Schedule mode:

Soil reserve is available only with ETo-based demand.

Watering days per week:

Use a whole number from 1 through 7 that matches local watering rules.

days/week

Irrigated area:

Changing the unit preserves the physical area.

Efficiency × uniformity:

Use field-audit values when available; lower percentages increase runtime.

DU%

Effective rain:

Changing the unit preserves weekly effective rainfall.

Soil profile:

Replace the seeded infiltration rate when an on-site test is available.

Root depth:

Changing the unit preserves the physical root depth.

Allowable depletion:

Use 15–80% as a planning range; sensitive plants typically use a lower value.

Infiltration rate:

Changing the unit preserves the physical intake rate.

Soak between cycles:

Use the controller's practical pause interval for this soil and slope.

minutes

Seasonal controller adjustment:

{{ params.seasonal_percent }}%

Leave at 100% for the calculated peak week; lower values model cooler periods.

Adjust seasonal percentage

Controller measure	Plan value	Basis	Copy
Correct the inputs to restore the controller plan.
{{ row.measure }}	{{ row.value }}	{{ row.basis }}

Controller action

Runoff and soak

Field verification

Model boundary

This is a controller starting plan, not a soil-moisture measurement. Verify pressure, catch-can distribution or emitter flow, wetting depth, slope runoff, weather restrictions, and plant response before relying on it.

An irrigation controller needs time, but plants use water as a depth or volume. Converting one into the other requires an estimate of plant demand and a field measurement of how fast the zone applies water. A schedule based only on minutes can be badly wrong when nozzles, pressure, emitter counts, weather, or wetted area change.

Reference evapotranspiration, written ETo, describes weather-driven water loss from a reference surface. Multiplying ETo by a crop coefficient, Kc, adjusts that demand for the selected turf, crop, or planting. Effective rainfall then reduces the amount that irrigation must replace. The result is a planning demand, not a sensor reading from the root zone.

Factors that change irrigation runtime
Factor	What it represents	Typical consequence
Weather and Kc	Atmospheric demand adjusted for the planting	Higher net demand increases required water.
Application rate or flow	Water delivered by the whole active zone	A faster verified rate shortens runtime for the same target.
Efficiency and uniformity	Losses and uneven distribution	Lower factors increase the gross water applied.
Soil intake	How quickly the surface can absorb water	A slow intake may require several shorter cycles.

Sprinkler zones are naturally described by precipitation depth per hour. Drip zones are usually easier to model from emitter discharge and outlet count, or from inline-drip spacing across an area. Keeping those delivery models separate prevents a common error: treating a point-source drip flow as though it were a uniform sprinkler depth.

Runtime remains an initial controller setting until the landscape is checked. Catch cans can reveal sprinkler precipitation and distribution differences, while a timed container test can check emitter flow. Runoff, pooling, dry edges, blocked emitters, wind, slope, root depth, and local watering restrictions can all justify a different schedule from the calculated value.

How to Use This Tool:

Choose the delivery model first, then use measured field values wherever they are available.

Select the closest Landscape preset and confirm System type. Replace preset values for crop coefficient, efficiency, soil, and root depth when local measurements or an agronomic plan are available.
Choose the water-demand path. For weather scheduling, enter peak-week ETo, Kc, effective rain, and either manual watering days or the soil-reserve schedule. For a fixed target, enter sprinkler depth per event or drip volume per week.
Describe delivery. Use a measured sprinkler precipitation rate when possible, or derive it from nozzle flow, arc, and spacing. For drip, enter emitter flow plus the counted outlets or the inline-drip spacing.
Enter application efficiency and distribution uniformity. These percentages multiply, so 75% and 75% produce a combined factor of 56.25%, not 75%.
Review Runtime per event, Runtime per week, and Cycle plan. If cycles are recommended, program the stated run minutes with the soak interval between starts, then verify the wetted depth in the field.

Interpreting Results:

Runtime per event is the controller time for one watering day after delivery losses and the seasonal adjustment. Runtime per week multiplies that value by the selected or soil-derived watering frequency. Neither value includes soak pauses.

A zero runtime means effective rain equals or exceeds the modeled plant demand for the period. It is a reason to pause the modeled irrigation, not a forecast that the soil cannot become dry later.
A multi-cycle plan means the modeled application rate exceeds the entered soil intake rate. The elapsed event time is longer than valve runtime because soak intervals are added between cycles.
The seasonal comparison scales the peak-week runtime to 60%, 80%, 100%, and 110%. It does not recalculate weather, rainfall, or soil reserve for those seasons.
Confirm the delivery rate and the driest part of the zone before increasing minutes to correct a field problem. Poor uniformity or a blocked outlet may need repair instead.

Technical Details:

The water-balance path first expresses demand as a weekly depth in inches. It then converts that net plant requirement into gross applied depth or volume, because efficiency losses and uneven distribution require more delivered water than the roots are expected to use.

Formula Core

Weekly crop demand is ETo multiplied by Kc, less effective rainfall. Negative demand is clamped to zero.

D_{net} = \max (0, {ET}_{o} \times K_{c} - R_{eff})

Application efficiency E and distribution uniformity U form one combined delivery factor. Net water is divided by that factor before runtime is calculated.

F = E \times U, W_{gross} = \frac{W_{net}}{F}

Sprinkler runtime uses gross event depth d and precipitation rate P. Drip runtime uses gross event volume V and total zone flow Q. The factor of 60 converts hours to minutes.

t_{sprinkler} = \frac{d}{P} \times 60, t_{drip} = \frac{V}{Q} \times 60

Irrigation formula variables and units
Symbol	Meaning	Canonical unit
`D_net`	Weekly net plant demand	in/week
`E`, `U`, `F`	Efficiency, uniformity, and their product	ratio from 0 to 1
`d`, `P`	Gross event depth and sprinkler precipitation rate	in and in/hr
`V`, `Q`	Gross event volume and drip zone flow	gal and gal/hr
`t`	Peak runtime per event before seasonal scaling	min

Delivery and Scheduling Rules

A derived sprinkler rate uses nozzle flow in gallons per minute, arc angle in degrees, and head and row spacing in feet:

P = \frac{34650 \times q}{θ \times s_{head} \times s_{row}}

For counted emitters, total drip flow is outlet flow multiplied by the number of emitters. Inline drip estimates the emitter count by dividing area by emitter spacing times lateral spacing, then rounds up to a whole outlet count.

Soil-reserve scheduling is available only for ETo-based demand. Available reserve equals available water-holding capacity times root depth in feet times the management-allowed depletion fraction. The interval is reserve divided by daily net demand, with a minimum of 0.5 day; starts per week are ceil(7 / interval), limited to 1 through 7.

Cycle-soak planning compares delivery rate with soil intake. When delivery is faster, the maximum cycle is 85% of the time needed to apply one hour's intake at that delivery rate, never less than one minute. The number of cycles is rounded up, then runtime is divided evenly among them. Seasonal scaling is applied before this split, and displayed values are rounded only for presentation.

Accuracy Notes:

The equations are deterministic, but the inputs describe a changing outdoor system. A precise-looking runtime can still be a poor schedule when field conditions are uncertain.

Preset Kc, efficiency, uniformity, root depth, soil reserve, and infiltration values are planning defaults. Local measurements and extension guidance take precedence.
Effective rainfall should represent water that reaches and remains useful in the root zone, not simply total rainfall.
The single-coefficient Kc method averages crop transpiration and soil evaporation. High-frequency microirrigation or detailed daily scheduling may need a dual-coefficient water balance.
Slopes, compacted soil, pressure variation, wind, mixed nozzles, leaks, and clogged emitters can invalidate the assumed application rate.

Worked Examples:

Peak-week turf with cycle-soak

With ETo of 5.6 mm/day, Kc 0.8, no effective rain, three watering days, a measured rate of 35 mm/hr, and 75% efficiency plus 75% uniformity, the plan gives about 31.9 valve minutes per event. Because delivery exceeds the entered 12.7 mm/hr soil intake, it divides the event into two cycles of about 15.9 minutes with the selected soak pause between them.

Fixed-volume shrub drip zone

A weekly target of 80 L split across three days, with 24 emitters at 3.8 L/hr and an 81% combined delivery factor, becomes about 21.7 minutes per event. The result stays a single start because this drip model's area-equivalent delivery rate does not exceed the entered intake rate.

References:

FAO-56 Chapter 6: Crop evapotranspiration using a single crop coefficient, Food and Agriculture Organization.
Evapotranspiration-based irrigation scheduling or water-balance method, University of Minnesota Extension.
Irrigation water: How it is delivered and measured, Oregon State University Extension.
Weather-based irrigation controllers, U.S. Environmental Protection Agency.