“Lake Waiau was never a very big lake. Hawaii’s only alpine lake measured just 6
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Question
“Lake Waiau was never a very big lake. Hawaii’s only alpine lake measured just 6,900 square meters at its maximum and 3 meters in depth. But it was considered sacred to native Hawaiians. According to myth, the lake was bottomless and a portal to the spirit world. But in early 2010, the lake started to shrink, and by September 2013, the lake was little more than a pond, covering a mere 115 square meters and reaching a depth of less than 30 centimeters. Such a decline is “unprecedented in modern times,” the U.S. Geological Survey reported last year. The cause of the lake’s decline is currently unknown, but drought is among the suspects.”
a. Write out a water budget for Lake Waiau in words.
b. What was the original volume of Lake Waiau? What is the present volume? (in m3)
c. Given that the watershed is 13.5 hectares and receives 7.5 inches on average annually in precipitation, calculate the annual rainfall depth into Lake Waiau. Assume the lake is only losing water to evaporation, calculate total evaporation from the Lake over a year. State your results in m3/yr.
Explanation / Answer
a. The water budget of lake Waiau includes the water that is fed into the lake by its watershed which receives precipitaion, this water can be in the form of surface runoff and infiltration that finally leads to the lake Waiau, other parameter that comes into the water budget of the lake Waiau is evaporation from the lake which decreases the amount of water present in the lake percolation that takes place in the lake which also decreases the volume of water that is present in the lake.
b. original volume of Lake Waiau = 6,900 meters2 x 3 meters
= 20,700 meter3
present volume of Lake Waiau = 115 meters2 x 30 centimeters
= 115 meters2 x 30/100 meters (As there are 100 centimeter in one meter)
115 meters2 x 0.30 meter
= 34.5 meter3
c. The watershed is 13.5 hectares and receives 7.5 inches on average annually in precipitation
Therefore, the annual rainfall depth into Lake Waiau =(13.5 x 10000 meters2) x(7.5 x0.0254) {As there are 10000 meter2 in a hectare and 0.0254 meter in an inch}
=135,000 meters2 x 0.1905 meter
25,717.5 meter3
If the lake is only losing water to evaporation then evaporation during original time is
25,717.5 meter3 - 20,700 meter3
= 5,017.5 meter3
If the lake is only losing water to evaporation then evaporation during current time is:
= 25,717.5 meter3 - 34.5 meter3
= 25,683 meter3
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