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Irrigation interval: FC 32%, PWP 13%

Short answer

Assuming the printed depth 130 m is 130 cm, root depth = 1300 mm. Available moisture = (32–13)/100 × 1.3 × 1300 = 321.1 mm. Interval = 321.1 / 14 = 22.94 days; irrigate every 22 days.

Find the irrigation interval in which the field capacity is 32%, permanent wilting point is 13%, dry density of soil is 1.3 gm/cc, effective root zone depth is 130 m [as printed] and daily consumptive use of water for the given crop is 14 mm.

Given: FC = 32%, PWP = 13%, γd=1.3\gamma_d = 1.3 g/cc, root zone depth Dr=130D_r = 130 cm =1300= 1300 mm, ETc=14ET_c = 14 mm/day. The depth "130 m" is a misprint and is taken as 130 cm.

Step 1: Available moisture depth

d=FC−PWP100×γdγw×Dr=32−13100×1.3×1300=321.10 mm\begin{aligned} d &= \frac{FC-PWP}{100}\times\frac{\gamma_d}{\gamma_w}\times D_r\\ &= \frac{32-13}{100}\times 1.3\times 1300 = 321.10\ \text{mm} \end{aligned}

Step 2: Irrigation interval

I=dETc=321.1014=22.94 daysI = \frac{d}{ET_c} = \frac{321.10}{14} = 22.94\ \text{days}

Answer: irrigate after about 22.9 days, i.e. every 22 days (rounded down so the crop does not suffer stress).

✓ Answer checked by Claude Opus 5.5 (high effort)

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