drawdown

drawdown - Geology 351: Geomath Computer lab -Organizing...

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Geology 351: Geomath Computer lab -Organizing Complex Computations in PsiPlot & Excel Estimating Maximum Sustained Yield from a Pump Test The probability of making errors increases in complex computations where numerous organizational and computational steps are required to arrive at the solution. The estimation of maximum sustained yield from pump test data is an excellent example of numerical analysis that involves several transforms of the raw field data, combined with logarithmic and regression line computations, and a mixture of different units in which the input and output data can be reported. An error at any stage in this complicated process will yield an incorrect answer. In this lab exercise computations are structured to help you minimize the chances of making simple errors in a long stream of computational steps. While the initial key to success is to thoroughly understand the background for the mathematical process that is being performed, Dr. Rauch’s hydrogeology course will introduce you to the underlying theory associated with this example. This exercise is taken from one of Dr. Rauch's class handouts. The brief discussions presented in our class provide a basic summary of the main ideas involved in this procedure, the measured and calculated quantities, and will hopefully, better prepare you to handle problems encountered in his or other hydrogeology classes should you take them in the future. Of equal importance, is the need to summarize the steps taken in the analysis, the definition of terms, a list of the equations you will be working with, and the set up and sequence of calculations to be performed. Q - In a pump test, water is removed from a well at a constant rate Q, the pumping rate. This rate is usually reported in gallons per minute (gpm). Q is also used later to represent the maximum sustained yield, which will be one of your last computations. You will often encounter situations where the same symbol is used to represent different variables, so you just have to keep the application clearly in mind to avoid confusion . t – The time since well pumping was initiated and t ' - the time since pumping ceased. s u - The basic data collected in a pump test are the depth to the water surface (d) in the well and the duration time (t) of pumping. The value d, however, needs to be transformed into a number related to the depth below the undisturbed depth of the water table in the well prior to pumping. This depth is referred to as the static water depth (d s ) and d s is subtracted from d to obtain the drawdown, s u . ( s u = d-d s ) m – the initial saturated thickness of the aquifer. s a – The drawdown is further transformed into a quantity referred to as the equivalent (or corrected) drawdown (s a ). s a =s u -s u 2 /2m.
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drawdown - Geology 351: Geomath Computer lab -Organizing...

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