Derive a $3-$D groundwater unsteady flow equation for a saturated, confined aquifer in a non$-$homogeneous, anisotropic porous media. From the equation so derived, write down continuity equations under the following cases $-$

$($i$)$ When the media is homogeneous but anisotropic $($flow is unsteady$).$

$($ii$)$ When the media is isotropic but non$-$homogeneous $($flow is unsteady$).$

$($iii$)$ When the media is isotropic and homogeneous but the flow is steady.

$($iv$)$ When the media is homogeneous and isotropic but the flow is unsteady.