Question

In a subsurface drainage network of an agricultural watershed with 10 lateral drains laid at 40 m spacing, each 150 m long, joining a collector drain with an average discharge of 10 L/s, and the water table dropping 40 cm below the ground in 3 days, the average drainable porosity of soil is:

• A. 5.8\%
• B. 7.8\%
• C. 10.8\%
• D. 15.8\%

Hint:

Drainable porosity represents the percentage of soil volume that can be drained under gravity.

Answer 1

The Correct Option is B

Step 1: Understanding Drainable Porosity Formula
\[ n_d = \frac{\text{Total Water Discharge} \times \text{Time}}{\text{Total Area} \times \text{Water Table Drop}} \]
Step 2: Substituting Values
\[ n_d = \frac{10 \times 86400 \times 3}{10 \times 40 \times 150 \times 0.4 \times 10000} = 7.8\% \]