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Flow Net Analysis
Contours of the watertable elevation as interpreted from a piezometer network, can be used in flow net analysis to estimate groundwater seepage rates. This is a hydrometric method that can be applied to water level data either in the horizontal or vertical plane. As a first step, if the network is of sufficient density, particularly near the stream or lake, contours of the potentiometric surface can indicate the relationship between the aquifer and surface water feature. Contours that curve in towards the upstream end of the stream indicate a gaining stream (Figure 1a) and contours that curve towards the downstream end indicate a losing stream (Figure 1b).
Figure 1: Watertable contour patterns around streams (a) contours pointing upstream for gaining streams (b) contours pointing downstream for losing streams (Winter et al, 1998)
A flow net is generated by the combination of the potential contours and groundwater flow lines that by definition are perpendicular to these contours. The objective in constructing the flow net is to generate cells that are equidimensional (Figure 2). A flow tube is defined as being bounded by the groundwater flow lines.
In the example given in Figure 2, total groundwater discharge to the stream (Q = q1 + q2 +q3 + q4) is derived from Darcy’s Law as:
(Equation 1)
where K is the average hydraulic conductivity, p is the number of flow tubes (=4), Δh is the difference in head between the two bounding potentiometric contours (=6-0), b is the aquifer thickness and n is the number of head drops in the flow net ( n=3, 6 to 4, 4 to 2 and 2 to 0) (Loaiciga & Zekster, 2003).
Figure 2: Plan view of example groundwater flow net towards a gaining surface water feature (after Loaiciga & Zekster, 2002)
References
- Loaiciga HA and Zekster IS, 2002. Estimation of submarine groundwater discharge. Water Resources 30(5): 473-479.
- Winter, TC, Judson, WH, Franke, OL and Alley WM. 1998. Groundwater and surface water a single resource. Circular 1139, U.S. Geological Survey, Denver.