Hydraulic Gradient

When following groundwater flow paths from a hill to an adjacent stream, water discharges into the stream from all possible directions, including straight up from the bottom of the channel. This curving path can be explained as a compromise between the force of gravity and the tendency of water to flow laterally in the direction of the slope of the water table. The tendency toward lateral flow is actually the result of the movement of water toward an area of lower pressure, the stream channel. The resulting movement is neither directly downward nor directly toward the channel but is, rather, along curving paths to the stream. Groundwater can actually move upward or downward. Groundwater can move upward against gravity because the hydraulic head at any point is a combination of both elevation and pressure. Hydraulic head is the level to which groundwater will rise in a well. Groundwater flows from high hydraulic head to low hydraulic head.

The curving path depicts the theoretical path of groundwater flow. The actual flow paths in fractured rocks will follow the gradient as the hydraulic head dictates, but only as the physical presence of the fractures allows.