The four steps of an irrigation schedule
A complete irrigation schedule answers four questions in sequence, following the standard soil-water-budget approach used in irrigation water management planning: how much water to apply, how often, how much to actually run the system to deliver that, and how long to run it.
| Step | Answers | Depends on |
|---|---|---|
| 1. Net depth | How much water refills the allowed deficit | Total available water (TAW) × allowed depletion (MAD) |
| 2. Interval | How many days that lasts | Net depth ÷ crop water use (ETc) |
| 3. Gross depth | How much to actually apply, accounting for losses | Net depth ÷ system efficiency |
| 4. Run time | How long to run the system | Gross depth ÷ application rate |
If you only need the interval on its own, the irrigation interval calculator covers just that step. For a day-by-day running water balance that tracks actual rainfall and changing ET over time, see the soil water balance calculator instead; this calculator produces one fixed schedule rather than a daily ledger.
How this irrigation scheduling calculator works
1. Total available water (TAW)
Available water capacity (AWC) is how much water a soil holds per foot of depth for plants to draw on. University of California Agriculture and Natural Resources gives typical ranges of about 0.5-1.0 in/ft for sand, 1.0-1.5 in/ft for sandy loam, 1.5-2.0 in/ft for silt or clay loam, and 2.0-2.5 in/ft for clay. This calculator uses the middle of the range for the texture you choose, or your own entered value.
TAW
TAW = Root zone depth × (AWC ÷ 12)
2. Net depth per irrigation
North Dakota State University Extension's Checkbook Method of irrigation scheduling starts irrigation once the soil-moisture deficit reaches a set share of TAW, commonly 50%; that share is the management allowed depletion (MAD).
Net depth
Net depth = TAW × MAD
3. Interval, from ETc
Days between irrigations
Interval = Net depth ÷ ETc
Rounded down to a whole number of days, minimum 1, so the schedule doesn't exceed the allowed depletion.
4. Gross depth, using efficiency
No system delivers all its water to the root zone. California's Model Water Efficient Landscape Ordinance worksheet uses 0.75 for spray-head irrigation and 0.81 for drip as typical application efficiencies.
Gross depth
Gross depth = Net depth ÷ Efficiency
5. Run or set time
Run time
Minutes = Gross depth ÷ Application rate × 60
When this calculator isn't the right tool
Use a different approach for:
- Just the interval, without the full net/gross depth and run-time chain. See the irrigation interval calculator.
- A day-by-day water balance that tracks actual rainfall and ET over time rather than a fixed repeating schedule. See the soil water balance calculator.
- A specific crop's water use figure, if you don't already have an ETc value. See the vegetable garden water calculator or the evapotranspiration calculator.
Irrigation scheduling calculation example
Sandy loam soil (AWC 1.25 in/ft), a 12 in root zone, 50% MAD, ETc of 0.2 in/day, a spray system at 75% efficiency, applying 1.5 in/hr.
- TAW: 12 × (1.25 ÷ 12) = 1.25 in.
- Net depth: 1.25 × 50% = 0.625 in.
- Interval: 0.625 ÷ 0.2 = 3.125 → every 3 days.
- Gross depth: 0.625 ÷ 0.75 = 0.833 in.
- Run time: 0.833 ÷ 1.5 × 60 ≈ about 33 min.
The same calculation in metric
Sandy loam soil, a 30 cm root zone, 50% MAD, ETc of 5 mm/day, a drip system at 81% efficiency, applying 8 mm/hr:
- TAW: 30 × (1.25 ÷ 12) = 3.125 cm.
- Net depth: 3.125 × 50% = 1.5625 cm = 15.625 mm.
- Interval: 15.625 ÷ 5 = 3.125 → every 3 days, matching the US example.
- Gross depth: 15.625 ÷ 0.81 ≈ 19.29 mm.
- Run time: 19.29 ÷ 8 × 60 ≈ about 2 hr 25 min.
Both examples use the same formulas the calculator runs; enter your own site's numbers above for your schedule.
Practical guidance
Measure your system's real application rate rather than assuming a manufacturer figure. For sprinklers, run a catch-can test: set several straight-sided containers across the irrigated area, run the system for a set time, and measure the average depth collected to get inches or millimeters per hour. For drip systems, use the manufacturer's rated emitter flow and spacing, then check it against an actual measurement where practical.
If the calculated interval comes out under one day, the net depth this soil and root zone can hold depletes faster than once-daily watering can sensibly deliver at that ETc. Consider a shallower target depth, splitting the gross depth into shorter cycles with soak time between them, or improving distribution uniformity so less water is needed to reach the driest spots.
Check local watering rules. Many water utilities set watering days, hours or restrictions, especially during drought. Where those apply, they take priority over any calculated schedule.
What changes the schedule
| Factor | Effect | What to adjust |
|---|---|---|
| Soil texture / AWC | Coarser soils hold less water per foot, giving a shorter interval. | Use a soil test or Web Soil Survey value where possible. |
| Root zone depth | A deeper effective root zone stores more water and lasts longer. | Use the crop's real rooting depth, not a maximum potential depth. |
| MAD | A lower allowed depletion shortens the interval and net depth. | Lower it for drought-sensitive plantings; extension guidance commonly uses 50% as a general default. |
| ETc / weather | Hot, dry, windy weather raises ETc and shortens the interval. | Update ETc regularly rather than using one seasonal figure. |
| System efficiency | Poor maintenance, wind or mismatched heads lower real-world efficiency below typical values. | Audit and maintain the system, or enter a measured efficiency. |
| Rainfall | Meaningful rain can reset or delay the next scheduled irrigation. | Track rainfall separately and adjust the schedule manually, or use a day-by-day water balance. |
Frequently asked questions
What is total available water (TAW) and how is it calculated?
Total available water is the depth of water a soil can hold for plant use within a given root zone depth. It's calculated as available water capacity (AWC), typically expressed as inches of water per foot of soil, multiplied by the root zone depth. University of California Agriculture and Natural Resources gives typical AWC ranges of about 0.5-1.0 in/ft for sandy soils up to 2.0-2.5 in/ft for clay.
What is management allowed depletion (MAD)?
Management allowed depletion is the share of total available water that's allowed to be used up before the next irrigation. North Dakota State University Extension's Checkbook Method of irrigation scheduling keeps the soil-moisture deficit from exceeding 50% of available water capacity before starting irrigation again, which is a widely used default; more drought-sensitive plantings are often managed at a lower depletion.
How is the irrigation interval calculated from ETc?
The interval is the net depth per irrigation divided by the crop's daily water use (ETc), rounded down to a whole number of days. ETc itself follows the FAO-56 approach of multiplying reference evapotranspiration (ETo) by a crop coefficient (Kc) for the specific plant and growth stage.
Why is gross depth higher than net depth?
No irrigation system delivers 100% of the water it uses to the root zone; some is lost to evaporation, wind drift, deep percolation or uneven coverage. Dividing the net depth by the system's application efficiency accounts for that loss. California's Model Water Efficient Landscape Ordinance worksheet uses 0.75 for spray-head irrigation and 0.81 for drip as typical efficiency values.
How do I find my irrigation system's application rate?
Run the system for a measured time and collect the water in several straight-sided containers spread across the area, such as a catch-can test for sprinklers, then measure the depth collected and convert to a rate per hour. Manufacturer specifications for drip emitters and tubing can also give a starting figure to check against a real test.
What root zone depth should I use?
Use the effective rooting depth for what you're irrigating, not the full depth roots could theoretically reach. Shallow-rooted turf and most vegetables are commonly managed on a root zone of roughly 6-12 inches (15-30 cm), while shrubs and trees draw from considerably deeper and wider zones.
Does this calculator track soil moisture day by day?
No. This calculator produces one schedule from a fixed net depth, interval and gross depth, projected forward at a constant rate. For a day-by-day running water balance that adjusts for actual rainfall and changing ET, use the soil water balance calculator instead.
See the soil water balance calculator.
I only need the number of days between waterings. Is there a simpler tool?
Yes. The irrigation interval calculator focuses on just that single step, if you don't need the full net depth, gross depth and run time chain this calculator builds.
See the irrigation interval calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The TAW, net depth, interval, gross depth and run-time math were verified by hand, and both worked examples were traced against the code, including a US/metric cross-check.
- AWC, MAD and efficiency figures are checked against the extension and ordinance sources below.
- Every input is validated and every division guarded so no combination of entries can return a blank, negative or invalid schedule.
Sources
- North Dakota State University Extension. Scherer, T.F. & Steele, D.D. Irrigation Scheduling by the Checkbook Method (AE792).
- University of California Agriculture and Natural Resources. Why Is Soil Texture Important? (AWC by soil texture).
- California Department of Water Resources / City of Los Altos Hills. Water Efficient Landscape Worksheet (irrigation efficiency values).
- Food and Agriculture Organization of the United Nations. Allen, R.G. et al. FAO Irrigation and Drainage Paper No. 56: Crop Evapotranspiration (ETc = ETo × Kc methodology).
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide (standard TAW, MAD and net/gross irrigation depth terminology).