Explain the Theis equation for unsteady well flow.

Theis equation for drawdown in confined aquifer under unsteady pumping: s = (Q/4piT) * W(u), where s = drawdown, Q = pumping rate, T = transmissivity, W(u) = well function, u = r^2S/(4Tt), r = distance from well, S = storativity, t = time since pumping started. Well function values from tables or software. Equation describes cone of depression expanding over time. Assumptions: homogeneous, isotropic, infinite aquifer, fully penetrating well, constant pumping. Used for aquifer testing and impact assessment.