The change in hydraulic head per unit distance in the direction of groundwater flow. It represents the driving force that causes water to move through porous media.
The hydraulic gradient is the rate of change of hydraulic head with distance measured along the direction of maximum decrease, and it represents the driving force for groundwater flow through porous media. It is calculated as the difference in hydraulic head between two points divided by the distance between them along the flow path. In Darcy's law, the hydraulic gradient is the key variable that, when multiplied by hydraulic conductivity and cross-sectional area, determines the volumetric flow rate of groundwater. The gradient is a dimensionless quantity, though it is sometimes expressed as meters per meter or feet per foot. In an unconfined aquifer, the hydraulic gradient can be visualized as the slope of the water table. In confined aquifers, it corresponds to the slope of the piezometric (potentiometric) surface. Accurate determination of the hydraulic gradient requires measurements from multiple observation wells or piezometers and is essential for determining groundwater flow direction and velocity, designing well fields, and predicting contaminant migration paths.