Question

Mumbai needs $1.4 \times 10^{12}\, L$ of water annually. Its effective surface area is $600 \,km^2$ and it receives an average rainfall of $2.4\, m$ annually. If 10% of this rain water is conserved, it will meet approximately

• A. $1\%$ of Mumbai’s water needs
• B. $10\%$ of Mumbai’s water needs
• C. $50\%$ of Mumbai’s water needs
• D. $100\%$ of Mumbai’s water needs

Answer 1

Answer: (B)

Surface area over which rain is received,
$A = 600 \,km^2$
$= 600 \times (10^3 )^2\,m^2$
$= 6 \times 10^8\, m^2$
Average rainfall, $h = 2.4 \,m$
Volume of water received by rain, $V$
$= A \times h = 6 \times 10^8 \times 2.4 \,m^3$
Water conserved $= 10\%$ of volume received by rain
$ = 6\times 10^8 \times \frac{10}{100} \times 2.4 \,m^3$
$ = 1.44 \times 10^8 \,m^3$
$ = 1.4 \times 10^8 \times 10^3 \,L$
$= 1.4 \times 10^{11} \,L$
Percentage of total water consumption received by rain is
$ = \frac{1.4\times 10^{11} \times 100}{1.4]\times 10^{12}} = 10\,\%$