File:Relation between heads flowing.svg

Summary
This diagram illustrates the three different heads (hydraulic, pressure and elevation) for downward vertical flow. The yellow rectangle represents a sand and water filled tube or column with its top open to the atmosphere and its bottom drained by a pipe or tube (blue line).

Hydraulic head (red line) gradients actually cause groundwater to flow (from high head to low head, down in this case).

Pressure head (blue line) is zero at the top of the column, as designated by the inverted triangle and horizontal lines (showing the water table).

Elevation head (green line) always increases 1:1 with elevation.