CE 304 Water Resources Engineering
Test 1 Precipitation, Water Balance, Watersheds
Solutions
Problem #1
During the month of July, a lake had a mean inflow of 88 cfs. The outflow was 78 cfs. The increase in stored
lake volume was 530 acre-ft. Precipitation during the same month was 9.8 in, while evaporation was 16.5 in.
a)
Draw a schematic of the lake’s water balance.
b)
What is the surface area of the lake?
State any assumptions. Clearly write out full unit conversions. Answer with appropriate significant digits.
A detailed step-by-step procedure on how to think through the solution is posted under a separate file.
∆
S/
∆
t = P + Q
in
– Q
out
– E – G
Assume groundwater, G = 0
Let A = lake surface area, acre
∆
S = (P – E) + (Q
in
– Q
out
)
∆
t
P, E already in units for the whole month
∆
t = 31 days
(530 acre-ft) = (9.8-16.5 in)(ft/12 in) (A) + (88-78)(ft
3
/s)(86400s/d)(31 d)(acre/43560ft
2
)
530 = -0.6A + 615
0.6A = 85
A = 142 acres
Surface
Inflow, Q
in
Surface Outflow, Q
out
Precipitation, P
Evaporation, E
G (assumed negligible)

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