Darcy's Law is a fundamental principle in hydrogeology that describes the flow of fluid through porous media. It states that the flow rate of a fluid is proportional to the hydraulic gradient, which is the difference in pressure over a distance. This relationship helps in understanding how water moves through soil and rock, making it essential for groundwater studies. The law is mathematically expressed as Q = -kA(dh/dl), where Q is the flow rate, k is the hydraulic conductivity, A is the cross-sectional area, and dh/dl is the hydraulic gradient. Darcy's Law is widely used in fields like civil engineering, environmental science, and petroleum engineering.