YielixIrrigation Scheduling: When and How Much to Water
Schedule from root-zone water, not a calendar alone
An irrigation schedule links measured or independently justified soil-water limits, effective root depth, crop ET and allowable depletion. The Soil Moisture Depletion Calculator makes that water-balance step explicit.
Use the output to prompt field checks—not to replace soil moisture observations, rainfall records, system capacity or crop-condition monitoring.
Calculate a soil-moisture depletion threshold and irrigation interval
Soil Moisture Depletion Calculator
Calculate irrigation scheduling from soil properties, root depth, and daily crop water use
Define the usable root-zone reservoir
Field capacity (FC) and permanent wilting point (PWP) are volumetric water contents. Their difference across effective root depth is total available water (TAW). Management allowable depletion (MAD) selects the portion of TAW that may be used before irrigation; readily available water (RAW) is the chosen depletion threshold.
A texture name is not a field water-retention measurement. The Soil Texture Triangle Calculator classifies a fine-earth sample, but FC, PWP, coarse fragments, rooting depth and compaction still need local measurement or justification.
Formula and units
TAW [mm] = (FC [%] − PWP [%]) ÷ 100 × root depth [m] × 1,000
RAW [mm] = TAW [mm] × MAD [%] ÷ 100
interval [days] = RAW [mm] ÷ crop ET [mm/day]
gross application [mm] = RAW [mm] ÷ efficiency [fraction]
Use a root depth that represents the effective water-extracting zone, not simply the deepest root seen in a profile.
Worked example: the retained soil-moisture fixture
Use the calculator's verified fixture: FC 27%, PWP 13%, MAD 55%, 1.0 m effective root depth and crop ET 5 mm/day. The live calculator's default efficiency is 85%.
TAW = (27 − 13) ÷ 100 × 1.0 × 1,000 = 140 mm.RAW = 140 × 0.55 = 77 mm.interval = 77 ÷ 5 = 15.4 days.gross application = 77 ÷ 0.85 = 90.6 mm.
The result means that, under these stated steady inputs, irrigation should be considered at about 77 mm of root-zone depletion. Rain, changing ET or measured soil water can move that date; 90.6 mm is the gross depth implied by entered efficiency.
Calculate a depletion threshold, interval and application depth from your root-zone inputs
Soil Moisture Depletion Calculator
Calculate irrigation scheduling from soil properties, root depth, and daily crop water use
Update the interval as conditions change
The calculated interval is not a fixed recurring calendar. Recalculate when ET, effective root depth, crop stage, rainfall, soil-water observations or efficiency assumptions change. Use the Irrigation Runtime Calculator only after choosing an application depth and independently checking system flow and distribution.
Assumptions and limitations
- FC, PWP, root depth, ET and MAD represent the managed field and period; the calculation treats ET as steady over the interval.
- The calculator does not infer rainfall effectiveness, salinity, variable rooting, compaction, groundwater contribution, runoff, infiltration rate or spatial soil variability.
- Efficiency is a stated conversion from net depletion to gross applied depth, not a measured uniformity result.
Common mistakes
- Using texture class as a measured FC/PWP pair: classify texture separately and use locally justified retention data.
- Using total available water as the trigger: schedule against the selected RAW/MAD threshold, not wilting point.
- Ignoring rainfall timing: credit only verified effective rainfall and recheck root-zone water after storms.
- Applying the gross depth without checking intake: split or adjust applications when infiltration or system capacity requires it.
Sources
- FAO Irrigation and Drainage Paper 56: Crop Evapotranspiration — primary reference for ET₀, crop ET and irrigation-scheduling methods.
- USDA NRCS National Engineering Handbook — primary U.S. engineering reference for irrigation planning and site-specific design.