In a well-drawn flow net, what does the drop in total head between two successive equipotential lines represent?

The drop in total head between two successive equipotential lines in a well-drawn flow net represents a constant reduction in the potential energy of the groundwater as it flows through the porous medium. An equipotential line is a line connecting points of equal total head. Total head, often denoted as 'h', is the sum of the elevation head, which is the vertical distance of a point above a chosen datum, and the pressure head, which is the height to which water would rise in a piezometer tube inserted at that point. Essentially, total head represents the total potential energy per unit weight of water at any given location. Groundwater naturally flows from regions of higher total head to regions of lower total head. Therefore, the drop in total head, often symbolized as Δh, between two adjacent equipotential lines indicates the specific amount of potential energy lost by the water as it travels from the upstream equipotential line to the downstream one. In a correctly constructed flow net, this head drop Δh is constant between any pair of successive equipotential lines. This constant head drop, when divided by the average length of the flow path between those equipotential lines, defines the hydraulic gradient, which is the rate of head loss per unit length of flow and is the direct driving force that propels the groundwater movement. For example, if the total head difference across an entire permeable layer is H and there are N_d such equipotential drops, then each drop represents H divided by N_d, meaning Δh = H / N_d.