The Soil Bucket & Know the Day to Irrigate

It treats the crop's root zone as a reservoir, or bucket. Field capacity is a full bucket; rain and irrigation are deposits; crop water use (ETc) is a daily withdrawal. You keep a running balance of how much the bucket has dropped below full — the depletion. When depletion reaches a set limit you irrigate to refill it. It is the FAO-56 Chapter 8 way to schedule irrigation by accounting rather than guesswork.

TAW is how much water the bucket can hold for the crop: TAW = (field capacity − wilting point) × rooting depth, which this tool computes as the soil's available-water-holding capacity per metre times the rooting depth, in mm. A loam at 170 mm/m under a 1.0 m root zone holds 170 mm of usable water. It is the full size of the reservoir.

RAW is the share of TAW the crop can use without stress: RAW = p × TAW, where p is the soil-water depletion fraction for the crop. Once depletion passes RAW the crop starts to feel drought, so RAW is the trigger level — the Management Allowed Depletion. The tool flags the day depletion first crosses it.

p is the fraction of TAW a crop can lose before water stress begins, from FAO-56 Table 22 — around 0.3 for shallow-rooted vegetables and up to 0.65 for cotton or sugarcane. The tool adjusts it for the day's evaporative demand with p_adj = p + 0.04·(5 − ETc), bounded 0.1–0.8, because crops stress sooner on high-ET days.

Each day the depletion changes by Dr = Dr_previous + ETc − rain − irrigation. If a day's rain or irrigation pushes depletion below zero, the bucket has overflowed past field capacity and the excess is lost as deep percolation. Depletion is also capped at TAW, the wilting point. The tool runs this every day across your window.

When depletion reaches RAW (the MAD). The tool reports the first day that happens as the trigger day. Irrigating at that point refills readily available water before the crop stresses, without applying so often that you waste water or drain nutrients. Irrigating much earlier wastes water; much later costs yield.

It is the depletion at the moment you choose to irrigate — the net depth of water needed to bring the root zone back to field capacity, in mm. Divide it by your system's application efficiency to get the gross depth to apply. The tool reports the current refill so you can size the irrigation set.

Texture sets the available-water-holding capacity. Sand holds only about 70 mm of usable water per metre and drains fast, so it needs little-and-often irrigation. A silt loam holds around 200 mm/m and buffers the crop for longer between irrigations. The table on this page lists the AWC the tool uses for each texture.

Deep percolation is water that drains below the root zone when the bucket overfills — from heavy rain or over-irrigation. It is lost to the crop and can leach nitrate. The tool totals it across the window so you can see when an irrigation or a rain event was wasted, which is a sign to apply less or split the application.

Use the crop's typical maximum rooting depth, which the tool fills in from FAO-56 Table 22, or override it for the current growth stage — a young crop has shallow roots and a smaller bucket. A deeper root zone increases TAW and stretches the interval between irrigations, so getting it right matters for scheduling.

Yes — enter the effective rain for each day alongside any irrigation. The balance credits rain the same way it credits irrigation, and any amount that overfills the bucket is counted as drainage rather than stored. That lets you see how a rain event resets the schedule and pushes the next irrigation back.

It is a sound planning estimate built on the standard FAO-56 equations and published soil and crop values. Real fields vary in texture, rooting and ET, and capillary rise and runoff are simplified, so treat the trigger day and refill depth as a guide. Confirm with a soil-moisture probe or feel-and-appearance check and adjust the inputs as the season unfolds.