A lake with a surface area of 1050 acres was monitored over a period of time. During a one-month period, the inflow was 33 cfs, the outflow was 27 cfs, and a 1.5-in. seepage loss was measured. During the same month, the total precipitation was 4.5 in. Evaporation loss was estimated as 6.0 in. Estimate the storage change for this lake during the month.
Solution:
We are given the following values:
Area, ATime, tInflow, IOutflow, OGround seepage, GPrecipitation, PEvaporation, E=1050 acres=1 month=33 cfs=27 cfs=1.5 in=4.5 in=6.0 in
The formula that we are going to use is:
∑Inflows−∑Outflows=Change in Storage,ΔS∑I−∑Q=ΔS
In this case, the inflows are Inflow I and Precipitation, P, while the others are outflows. Our formula now becomes
∑Inflow−∑Outflow=ΔS(I+P)−(O+G+E)=ΔS
Before substituting, we need to convert all the given to inches. More specifically the outflow O and inflow I.
The inflow and outflow, in cfs, will be divided by the given are to come up with units of inches.
The inflow is
InflowInflow=1050acres33sft3⋅43560ft21acre⋅1ft12in⋅1hr3600s⋅1day24hr⋅1month30days⋅1month=22.4416 in
The outflow is
OutflowOutflow=1050acres27sft3⋅43560ft21acre⋅1ft12in⋅1hr3600s⋅1day24hr⋅1month30days⋅1month=18.3613 in
Now that everything is in inches, we can now substitute the values in the formula
ΔSΔSΔS=(I+P)−(O+G+E)=(22.4416 in+4.5 in)−(18.3613 in+1.5 in+6 in)=1.0803 in (Answer)
We can also state the change in storage in terms of volume by multiplying the given area
ΔS in volumeΔS in volume=1.0803 in×1050 acres×12 in1 ft=94.5263 (Answer)
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