Water by the Pot: A Pan-Evaporation Scheduling Rule for Hand-Watered Plots That Stays Safe Across the Pan-Coefficient Band

A smallholder who waters by hand usually has no weather station, no soil-moisture meter, and no extension visit scheduled. What such a farmer does have is a pot, a ruler, and a plot. This paper turns those three objects into a complete irrigation schedule. A plain open pan — a cooking pot or a cut drum set beside the plot — is read each morning; its evaporation, multiplied by a pan coefficient of 0.70 (uncertainty band 0.60 to 0.80) and by crop coefficients of 0.60, 0.90, 1.15, and 0.80 across four growth stages, gives crop water use of 2.94 to 5.64 mm per day at a mean pan reading of 7.0 mm per day. Readily available water in a 0.40 m root zone, at half depletion of the stored water, is 12.0 mm in sand, 30.0 mm in loam, and 32.0 mm in clay, which sets nominal intervals between waterings from 2.13 days (sand, mid-season) to 10.88 days (clay, initial stage). The catch is the pan coefficient itself: if its true value sits at the top of the band while the farmer schedules by the middle, every cycle ends late — by 0.67 days per cycle on loam at mid-season — and the crop spends 11.25 days of a 90-day season past its readily-available-water limit, 12.5 percent of the season, on every soil. The originated rule removes that exposure with one fraction: water when the cumulative pan readings since the last watering reach a tabulated trigger — 32.61 mm of pan evaporation for loam at mid-season — and apply seven-eighths (0.875) of the readily available water, 26.25 mm net or 30.88 mm gross on loam at mid-season. Across the whole coefficient band this rule never lets depletion pass the limit; its worst cost, when the coefficient is actually at the bottom of the band, is 3.75 mm of water draining below the roots per watering on loam and about two extra waterings in a season. The study is simulated, not measured: every figure derives from the stated assumptions by the accompanying computation, and no field trial is claimed.