Available Water and MAD Irrigation Trigger
When to irrigate and how much, from the soil water reservoir (FAO-56 / NRCS).
Example
You enter
- Field capacity (in/in, e.g. 0.30) 0.3
- Wilting point (in/in, e.g. 0.12) 0.12
- Root depth (in) 24
- MAD fraction (0-1, ~0.5) 0.5
- Crop use ETc (in/day) 0.25
You get
- Total available water (TAW) 4.32
- Readily available / net refill (RAW) 2.16 in
- Irrigation interval 8.6 days
Checked against published worked examples from: Oregon State University Extension.
Details, formula, and sources
ScopeTotal available water TAW = (field capacity - wilting point) x root depth (water contents in in/in); readily available water RAW = MAD x TAW is what you let deplete before the crop stresses, MAD ~0.5 (0.3-0.6 by crop). A silt loam at FC 0.30, PWP 0.12, 24 in root zone holds 4.32 in; at MAD 0.5 you irrigate after 2.16 in is used, an 8.6-day interval at ETc 0.25 in/day, and the net refill is that 2.16 in (gross adds the application efficiency -- see the Irrigation Requirement calculator). Hotter weather (higher ETc) shortens the interval, a deeper/finer soil lengthens it. A scheduling aid; the field-measured soil moisture, the actual root depth and ETc, and the agronomist govern.
Formula or tableTAW = (field_capacity - wilting_point) x root_depth_in (in); RAW = mad_fraction x TAW (in); irrigation_interval_days = RAW / etc_in_day. Water contents in in/in; MAD ~0.5 (0.3-0.6 by crop).
Source and editionSoil-water-reservoir irrigation scheduling (FAO Irrigation and Drainage Paper 56, Allen et al. 1998; USDA NRCS Irrigation Guide), by name; the field-measured soil moisture, root depth, and ETc govern.
Free accessThe available-water and depletion relations are public (FAO-56 / NRCS); the field capacity, wilting point, root depth, MAD, and ETc are soil, crop, and weather values the user supplies.
What governsEstimate. AHJ and licensed professional govern.
API field namesfield_capacity, wilting_point, root_depth_in, mad_fraction, etc_in_day, taw_in, raw_in, irrigation_interval_days
- Soil water reservoir TAW = (FC - PWP) x root depth; RAW = MAD x TAW (net refill and trigger); interval = RAW / ETcFAO-56 / NRCS Irrigation Guide
- MAD by crop MAD ~0.5 (0.3 shallow/sensitive to 0.6 deep-rooted grains); gross depth adds application efficiency (irrigation-requirement)FAO-56