Characteristic Curve. Calculation of the coefficient taking into account the aquifer head loss (B), the coefficient taking into account the well head loss (C), and provides indications on the productivity of the well.
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The step-drawdown test is used to define the hydraulic characteristics of the aquifer-well system and therefore to verify the productivity of the well. Field measurements (flow rates and drawdowns) are plotted on a diagram to construct the well’s characteristic curve.
The diagram has an initial straight section which, in unconfined aquifers, ends with a downward inflection corresponding to the point that identifies the “critical flow rate of the well”.
The straight section corresponds to the flow rate values for which the motion of water entering the well is laminar. At the critical flow rate, the turbulent flow regime takes over, causing quadratic head losses. The curve is described by the following equation:
s = BQ + CQ2
The parameters B and C of the characteristic curve can be obtained by interpolating the measured values plotted on the diagram (Flow rates – Specific drawdowns) using the least squares method. Parameter B is obtained from the intersection of the interpolating line with the specific drawdowns axis, while C is equal to the slope of the representative line. Through the estimation of parameter C, information about the condition of the well can be obtained (cf. W. C. Walton, 1962).