Question

The plan area of a reservoir is $1k{m}^2$. The water level in the reservoir is observed to decline by 20 cm in a certain period. During this period, the reservoir receives a surface inflow of 10 ha-m and 20 ha-m are abstracted from the reservoir for irrigation and power. The pan evaporation and rainfall recorded during the same period at a nearby meteorological station are 12 cm and 3 cm respectively. The calibrated pan factor is 0.7. The seepage loss from the reservoir during this period in hectare-m is

• A. 0.0
• B. 1.0
• C. 2.4
• D. 4.6

Answer 1

The correct option is D 4.6

Inflow to reservoir,
I = 10 ha-m
Outflow from reservoir,
O = 20 ha-m
Loss due to evaporation,
$E=12cm\times \text{pan coefficient}\times \text{Area}$
$=12\times {10}^{-2}\times 0.7\times A$
$=12\times {10}^{-2}\times 0.7\times {10}^6$
$=8.4\times {10}^4{m}^3$
$=8.4ha-m$

Rainfall, P = 3 cm
$=0.03\times {10}^6{m}^3$
$=3\times {10}^4{m}^3$
$=3ha-m$

Change in storage,
$\Delta S=-20cm$
$=-0.20\times {10}^6{m}^3$
$=-20ha-m$

inflow - outflow = change in storage
(I + P) - (O + E + seepage) = - 20
(10 + 3) - (20 + 8.4 + seepage) = - 20
Seepage = 4.6 ha-m