I was wondering whether there is any equation for viscous liquids (probably derived from energy and mass conservation principles) relating the pressure, rate of volume flow, area of cross section and  height of the liquid (or any other parameters if required).

NOTE : I am aware of [Poiseuille's law](http://hyperphysics.phy-astr.gsu.edu/hbase/ppois.html), but that is not what I am looking for as it relates the pressure *gradient* to the rate of flow.