请教句中which在从句所做的成份~

请教句中which在从句所做的成份~
The proportion of empty space in a rock is known as its porosity.But note that porosity is not the same as permeability,which measures the ease with which water can flow through a material; this depends on the sizes of the individual cavities and the crevices linking them.
请问《 which measures the ease with which water can flow through a material; 》中的《 the ease with which water can flow through a material》which的先行词是什么,以及which在从句中的成分,