residents have been utilizing their shallow aquifer fordecades.Prior to developm

Question

residents have been utilizing their shallow aquifer fordecades.Prior to development,the sandand gravel aquifer had anaverage saturated thickness of about 100feet.The average porosity of the
Group is about 22%.The average precipitationin Mountains is about10-25 inches per
year,but on average,only a small percentage(10to30%)of this precipitation recharges
the aquifer.assume that thewater level in the aquifer is about 3 feet lower than the river water level,onaverage.

the agricultural drains are 8 feet lower than thewater table,so that the aquifer is losing water to the drains.Assuming upstream subsurface aquifer inflows are balanced by downstream outflows we can write the water budget for

the Group of aquifers of the Basin asfollows:

ATh

t

= QMR +QRS +QIR QDR QET QPUMP

whereQMR ismountainfrontrechargeinft^3/year,QDR is drain seepage losses inft^3/year,
QRS is river seepage inflows in ft3/year, QIR isirrigation see page that recharges the aquifer
in ft^3/year, QPUMP is groundwater pumping inft^3/year, QET is riparian zone forest
evapotranspiration losses in ft^3/year, AT is area ofthe Basinaquifer system in
ft^2, is aquiferporosity,h is average aquifer water levelin feet,and t is time in years.One
acre-foot equals 43,560ft^3.

You take two very careful flow measurements in the River one mile apart from
each other. At the upstream cross section you measure a discharge of 991 cubic feet per
second (cfs). At the downstream point you measure 981 cfs. Assume that your
measurements are representative of the whole river as well as the whole year. If the river loses 10 ft3 of water every second over each mile of flow distance, how much water seeps through its bed into the aquifer over the full 96 mile length that flows over the Basin over an entire year?

Explanation / Answer

The river loses 10 ft3 of water per mile per second.

So, in entire 96-mile length, it will lose 960 ft3 of water per second.

Now, a year= 60*60*24*365= seconds.

So, over 31536000 seconds the will lose = 31536000*960= 30274560000 ft3 of water. And the whole of this water will seep through the bed into the aquifer.

N.B. for the student, this is a simple question with broad explanation. Don't be fooled by the big question. Remember here the questioner did not ask for solving of the water-budget equation. Lots of things are given here but only ask to calculate the water seepages through the river.

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