Step 1: Recall what Darcy's Law describes. Darcy's Law relates the discharge through a porous medium to the hydraulic gradient using a simple proportional relationship. Step 2: Recognise the assumption behind that simplicity. Such a straightforward linear relationship only holds when the fluid moves slowly and smoothly through the pore spaces, without the chaotic mixing seen in turbulent flow. Step 3: State the required condition. Groundwater generally moves slowly enough through soil pores for this smooth, orderly laminar flow to hold, which is exactly why Darcy's Law applies here. \[ \boxed{\text{Laminar flow}} \]