Exercise 2) Lower Ratio of Streamflow to Pumping
The Base Case analysis of Exercise 1 assumed that the streamflow was much greater than the well pumpage. What if the pumping rate was higher than in Exercise 1 and the ratio of streamflow to pumping (withdrawal) rate was much lower? In the Base Case, river inflow is about ten times greater than the well pumpage. Consider a case in which the well pumpage is increased by a factor of three (two more wells are drilled close to the original well to form a well field, and each well has the same pumping capacity, so the total Q in that cell of the model grid is -6,078 m3/d) and the river inflow is reduced by a factor of three (Qin = 6,667 m3/d instead of 20,000 m3/d). The ratio of streamflow entering the system to pumping out of the aquifer would then be about 1.1. How would that affect (1) the streamflow in space and time, (2) the drawdown in the aquifer, (3) the head distribution in the aquifer, and (4) the hydrograph for the pumping well? How does that affect (5) the water budget of the aquifer and (6) the fractional sources of water to the well? Is this pumping scenario sustainable?
Suggested approach: To determine the effect and importance of the relative strength of the pumping stress to the magnitude of streamflow, copy the Base Case input files into a new folder for Exercise 2 and modify the input parameters to match the assumptions of the above exercise. Specifically, you have to reduce the river’s inflow (this requires changing the values “2.000000000000E+004” to “0.666700000000E+004” in the “Base.Case.sfr” input file and increase the pumping rate (by changing the value “-2.026000000000E+003” to “-6.078000000000E+003” in the last line of the “Base.Case.wel” input file). Also, consider adding another stream gage to monitor changes in streamflow (by (1) changing the number on the first line of the “Base.Case.gag” file to 2, (2) adding a third line to the Base.Case.gag” file “1 46 20206 1”, and (3) adding a line to the “Base.Case.nam” file that says “DATA 20206 ..\Output.Files\Base.Case.sfrg2 REPLACE” after the similar line for the first gage).