Question Bank • QB_IDE_DRN_003 NAT • 2 Marks

Section 5: Irrigation and Drainage Engineering

Agricultural Drainage

Subtopic: Leaching requirement and salinity control

Question Statement:

An agricultural crop with an electrical conductivity threshold in the saturated soil extract of $EC_e = 3.0\text{ dS/m}$ is irrigated with canal water having $EC_{iw} = 1.5\text{ dS/m}$. Using the USDA formula for leaching requirement $LR = \frac{EC_{iw}}{5 EC_e - EC_{iw}}$, the percentage leaching requirement ($LR \times 100$) is ________ % (round off to 1 decimal place).

Answer Key:

11.1

Step-by-Step Solution & Explanation

From the USDA leaching requirement equation: $LR = \frac{EC_{iw}}{5 EC_e - EC_{iw}}$ Given: • $EC_{iw} = 1.5\text{ dS/m}$ • $EC_e = 3.0\text{ dS/m}$ $LR = \frac{1.5}{5(3.0) - 1.5} = \frac{1.5}{15.0 - 1.5} = \frac{1.5}{13.5} = \frac{1}{9} \approx 0.1111$ $\text{Percentage } LR = 0.1111 \times 100 = 11.11\% \approx 11.1\%$
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