Available Water and MAD Irrigation Trigger
When to irrigate and how much, from the soil water reservoir (FAO-56 / NRCS). 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 irrigation-requirement). 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 and source
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.
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