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Irrigation Scheduling Calculator
Soil-aware watering schedule: allowable depletion depth, refill volume in gallons and days between waterings at your management deficit.
Irrigation Scheduling Calculator
Chlorova · updated for 2026
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This is a budget, not a controller: the schedule assumes steady daily crop use and no rain, so adjust after storms and during heat waves when use doubles. Available-water figures are mid-range textbook values per texture, and organic matter, compaction and true field root depth all move them. Water at a rate the soil absorbs rather than the rate the emitter delivers, and confirm the verdict with a screwdriver or probe check of actual moisture at root depth before committing to a calendar.
Overwatering and underwatering are really the same mistake wearing different gloves: watering on the calendar instead of the soil's account balance. This calculator reads that balance from soil texture, root depth and how much depletion you allow before refilling, then returns the inches of safe drawdown, the gallons that restore it across your area, and the honest interval between waterings. It is the same balance-sheet logic crop consultants use, reduced to five inputs, one verdict on your watering rhythm, and no guessing at the spigot.
Building the Schedule in Steps
- Pick your soil texture, from the jar test if you have one.
- Enter the active root zone depth in feet, honestly short for seedlings.
- Set the allowed depletion, 50 percent being the sound default.
- Fruiting crops and herbs want tighter and looser settings respectively.
- Enter the area you are watering in square feet.
- Add the crop's daily use in inches of water per day.
- Read the allowable depletion in inches and the refill gallons.
- Check the days-between-waterings figure and the rhythm verdict band.
- Adjust after rain or heat waves and re-run the whole budget.
The Soil Water Balance Sheet
Each foot of a texture stores a known slice of plant-available water, roughly a foot of silt loam holding 1.6 inches you can drink from and sandy loam only 1.0. Multiply that by your root depth, then by the depletion you allow, and the product is the drawdown in inches before stress: 1.4 times 1.5 feet times 50 percent is 1.05 inches in a loam tomato bed.
The refill converts depth to volume the way irrigation does everywhere else, one inch over a square foot being 0.623 gallons, so depletion times area times that constant gives gallons in a single soak. Dividing depletion by daily crop use reports the days the reservoir lasts, and the rhythm band translates that number into a watering habit. Blanks and zeros return dashes rather than an infinite interval.
Worked Example: A 100 Sq Ft Tomato Bed in Loam
- Loam stores 1.4 inches of plant-available water per foot.
- Tomato roots reach about 1.5 feet deep in the bed.
- You allow 50 percent depletion, the sound default.
- Drawdown is 1.4 times 1.5 times 0.5, so 1.05 inches.
- Each inch of water over a square foot weighs 0.623 gallons.
- Refill comes to 100 times 1.05 times 0.623 gallons.
- That is roughly 65 gallons for one thorough soaking.
- Divide the same depletion by area for the inches view.
- At 0.12 inches of daily use the reservoir lasts 1.05 divided by 0.12.
- That is about 8.8 days, so a weekly rhythm fits.
- In a heat wave pushing use to 0.25 inches, the same bed wants water every four days.
Available Water by Texture
Plant-available water per foot of roots for the textures this picker applies, mid-range values the formulas inject directly. The depletion you allow matters as much as the storage: sandy soils empty fast, and clay keeps a sizeable slice of its water out of the plant's reach.
| Texture | Available in/ft | Refill at 50% of 1.5 ft | Feels like |
|---|---|---|---|
| Sandy loam | 1.0 | 0.75 in | Little and often |
| Loam | 1.4 | 1.05 in | The weekly baseline |
| Silt loam | 1.6 | 1.2 in | Longest comfort |
| Clay loam | 1.3 | 0.98 in | Slow, deep soaks |
| Clay | 1.0 | 0.75 in | Wet outside, dry inside |
Key Terms
- Available water
- The inches of water per foot of soil that plant roots can actually extract.
- Allowable depletion
- The share of available water you remove before refilling, typically 35 to 55 percent.
- Root zone
- The depth of active drinking roots, the multiplier on the whole soil reservoir.
- Crop water use
- Daily inches consumed by evaporation and the plant, the withdrawal rate on the account.
- Leaching
- Watering past the root zone so the surplus carries nutrients below the plants, pure waste.
Irrigation Planners Think in Reservoirs
- Vegetables differ from turf mainly in root depth, so a tomato at two feet drinks from triple the reservoir of a strawberry at eight inches on the same soil.
- Depletion setting is a crop choice, not a soil one: fruiting crops like steady moisture near 30 to 40 percent drawn, while drought-tough Mediterranean herbs tolerate 60 percent and rot if you hover.
- The most common failure is not the calendar but the application rate, because loam accepts about half an inch per hour, so 65 gallons over a bed needs an hour of gentle soaks rather than ten hard minutes.
- Organic matter is the hidden multiplier in every row of the table, and a heavily composted bed may beat the textbook figure for its texture by a third.
- Heat waves double withdrawal, so the days-between figure from a mild week quietly halves in a July stretch: re-enter in summer rather than trusting the April schedule.
Common Mistakes & Fixes
- Watering a fixed gallon amount each week regardless of the soil's reservoir size.
- Using full-grown root depth for tiny seedlings and drowning the shallow roots.
- Reading the depletion band as permission to skip all in-ground moisture checks.
- Applying the whole refill at once, faster than the soil can honestly absorb.
- Ignoring rain as a free refill and watering right over a soaked bed.
- Setting allowed depletion at 90 percent on sandy soil where stress arrives far sooner.
When to Use This (and When Not To)
Reach for it when establishing a new bed's rhythm, cutting back a wasteful habit, or sizing how much a weekly soaking should deliver. It deliberately ignores rain, wind and shade, so in variable climates re-run after storms; pair the gallon figure with the sprinkler run-time or drip planners to convert the schedule into minutes at the spigot.
Irrigation Scheduling Calculator: Frequently Asked Questions
How often should I water a vegetable garden?
Most deep-rooted vegetables in loam want a good soak every five to ten days in mild weather, and the calculator pins your number from soil storage, root depth and crop use. Frequent light sprinkling is the alternative that fails, since it grows shallow roots that then need daily drinks.
What does allowable depletion mean in practice?
It is how much of the soil's plant-water bank you let drain before refilling, and 50 percent suits most vegetables. Deeper drawdown risks stress and blossom-end problems in fruiting crops, while shallower intervals cost you water and time for little benefit.
How many gallons is one inch of water over a garden?
About 0.62 gallons per square foot, so one inch over 100 square feet is roughly 62 gallons. That conversion is exactly what this tool multiplies through the depletion depth to report a single refill volume.
Do clay soils really hold less plant water?
Clay holds plenty of total water but grips much of it so tightly roots cannot pull it, which is why its available-water figure matches sandy loam despite the opposite feel. The practical result is that clay gardens often need watering at the first sign of leaf curl, earlier than the numbers imply.
Should I change the schedule after rain?
Yes, rain is a refill from a different source, so a half-inch storm buys about four days off the next watering in a loam bed. The budget logic still runs, you simply subtract what fell from the gallons owed.
Is drip or sprinkler better for this schedule?
The schedule defines the amount; either system can deliver it, and drip wins where foliage disease or wind losses matter. Sprinklers suit cool-season and establishment watering, while the refill gallons translate to run time through either system's flow rate.
Sources & Further Reading
- Irrigation Scheduling and Water Balance — University of California Agriculture & Natural Resources. Framework behind the depletion-based budget used here.
- Available Water Capacity by Soil Texture — USDA Natural Resources Conservation Service. Source of the inches-per-foot storage values.
- Watering Home Vegetables Efficiently — Iowa State University Extension. Supports root-depth and rhythm guidance for garden crops.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated