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
Details, formula, and sources
Total 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.
TAW = (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).
Soil-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.
The 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.
Estimate. AHJ and licensed professional govern.
Field names used by the API: field_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