In groundwater that is open to atmospheric pressure (unconfined aquifers), hydraulic gradient is the degree of inclination of the water table, usually described as the number of feet the water table drops per mile.
The diagrams above and below depict an unconfined aquifer where the water table is exposed to the atmospheric pressure through the pores in the soil. The datum level is a reference level from where elevations are measured — the mean sea level (msl) is most frequently used. The hydraulic head
is the elevation of the groundwater table, usually measured in
feet. For example, 180 feet above mean sea level (amsl).
The difference between “Head 1” and “Head 2” divided over the distance gives the gradient.
For example, if “Head 1” is 180 feet amsl and “Head 2” is 170 feet amsl, and the distance is 1,000 feet, then the Gradient is (180 – 170)/1000 = 0.01 foot per foot.
Gradient is slope. Standing on a steep incline, you can measure its slope in any direction, but usually you pay most attention to the greatest of slopes because this is where you’d fall the hardest
Same with groundwater, the greatest gradient tells us the direction where the water would essentially flow.
The diagram below depicts the gradient and its direction (west-southwest) in the water table under a leaking underground storage tank site. The parallel lines are lines of same elevation on the plane that represents the groundwater table. The line perpendicular to these lines indicated the greatest gradient and its direction. The amount of the gradient is named as a dimensionless rate of 0.0029 which can be translated to feet of elevation for every foot of distance, centimeters of elevation for every centimeter of distance or light years of elevation for every light year of distance. If you want to know the gradient in feet per mile, multiply the number by 5,280 (feet per mile) and you’ll get 15.31.