Subtopic: Steady and unsteady flow in confined and unconfined aquifers
Question Statement:
A watershed of 100 km² is underlain by an unconfined aquifer having hydraulic conductivity of 15 m day⁻¹ and specific yield of 0.20. If 30 million m³ of water is pumped from this aquifer through uniformly distributed wells, the average drop of water table over the watershed in meter will be
Options:
(A) 1.50
(B) 0.75
(C) 0.06
(D) 6.00
Official Answer Key:
A
Step-by-Step Solution & Explanation
• For an unconfined aquifer, the volume withdrawn relates to the average water-table drop through the specific yield: Volume = Area × Sy × Δh.
• Area = 100 km² = 100 × 10⁶ m². Sy = 0.20. Volume = 30 × 10⁶ m³.
• Δh = Volume/(Area×Sy) = 30×10⁶/(100×10⁶×0.20) = 30×10⁶/20×10⁶ = 1.5 m.
• The hydraulic conductivity value given is a distractor — it governs flow rate/drawdown distribution, not the average storage-based drop.
• Final Answer: (A) 1.50
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