An unconfined alluvial aquifer has a total porosity of $n = 0.38$ and a specific retention of $S_r = 0.14$. Over an irrigated watershed of area $A = 10.0\text{ km}^2$ ($10.0 \times 10^6\text{ m}^2$), intense seasonal pumping causes a uniform water table decline of $\Delta h = 2.50\text{ m}$. The total volume of groundwater drained and yielded from gravity storage in million cubic meters ($\text{million m}^3$) is ________ (round off to one decimal place).
Answer Key:
6.0
Step-by-Step Solution & Explanation
1. Specific yield ($S_y$):
$S_y = n - S_r = 0.38 - 0.14 = 0.24$
2. Volume of water released by gravity ($V$):
$V = S_y \times A \times \Delta h$
$V = 0.24 \times (10.0 \times 10^6\text{ m}^2) \times 2.50\text{ m} = 0.24 \times 25.0 \times 10^6 = 6.00 \times 10^6\text{ m}^3$
3. Expressed in million cubic meters:
$V = 6.0\text{ million m}^3$