GATE 2016 CE (CE2) – Question 25
Water table of an aquifer drops by 100 cm over an area of 1000 km2. The porosity and specific retention of the aquifer material are 25% and 5%, respectively. The amount of water (expressed in km3) drained out from the area is _________.
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Correct answer: 0.19 to 0.21
Explanation
Specific yield is porosity minus specific retention:0.25−0.05=0.20. Water-table fall100cm equals0.001km. Drained volume is area times fall times specific yield:1000·0.001·0.20=**0.20km³**. Porosity alone includes retained water that does not drain.