What is Darcy's law and how is it applied to groundwater flow?

Darcy's law describes groundwater flow through porous media: Q = -KA(dh/dl), where Q = discharge, K = hydraulic conductivity, A = cross-sectional area, dh/dl = hydraulic gradient. Negative sign indicates flow from high to low head. Darcy velocity v = Q/A is apparent velocity; actual pore velocity = v/porosity. Law valid for laminar flow (Reynolds number < 1 based on grain diameter). Applications include well yield estimation, seepage under dams, and contaminant transport calculations.