First, in mountain area, there is generally less groundwater observations, which makes it’s difficult to define the boundary condition. Without observations, the initial head can only be assumed or estimated using a steady state simulation. Second, groundwater recharge in mountain area is very low. This is because some mountain surface has no soil or a shallow soil layer. And the permeability of rock fractures is very low. As a result, infiltration from the surface/subsurface to groundwater systems is extremely low. Third, the hydraulic priorities of the fractured and unfractured rock material are also very low, which means the groundwater system is very sensitive. Beside, topography effects from mountain area usually require high spatial resolution grid cell in the numerical simulation.