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Depth and frequency of irrigation

Short answer

Readily available moisture (net depth) is 367.50 mm, gross depth is 459.38 mm. Irrigation frequency is about 92 days (91.9 days).

Following data are given below, calculate the depth and frequency of irrigation. Field capacity = 80%, Permanent Wilting Point (PWP) = 30%, Root depth = 70 cm, soil density = 1.5 gm/cc, Consumptive use = 4 mm/day, Application efficiency = 80%, Readily available moisture (RAM) = 70%.

Given: FC = 80%, PWP = 30%, Dr=70D_r = 70 cm =700= 700 mm, γd=1.5\gamma_d = 1.5, ETc=4ET_c = 4 mm/day, ηa=80%\eta_a = 80\%, RAM =70%= 70\% of available moisture.

Step 1: Available moisture

AM=80−30100×1.5×700=525.00 mmAM = \frac{80-30}{100}\times 1.5\times 700 = 525.00\ \text{mm}

Step 2: Readily available moisture (net depth of irrigation)

RAM=0.70×525.00=367.50 mmRAM = 0.70\times 525.00 = 367.50\ \text{mm}

Step 3: Gross depth of irrigation at the field

dgross=RAMηa=367.500.80=459.38 mmd_{gross} = \frac{RAM}{\eta_a} = \frac{367.50}{0.80} = 459.38\ \text{mm}

Step 4: Frequency of irrigation

f=RAMETc=367.504=91.88 daysf = \frac{RAM}{ET_c} = \frac{367.50}{4} = 91.88\ \text{days}

Answer: depth of irrigation = 367.50 mm net (459.38 mm gross); frequency ≈\approx 91 days. (The data are very high for a field soil, but the method is as shown.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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