1 That proportion of the total pore space in a rock which is capable of releasing its contained water. Clay, for example, may have a total porosity of 50% or more, but little if any of the water contained in these pores may be released, because of the retentive forces (e.g. surface tension) that hold it within the rock.
2 The proportion of the pore space through which groundwater flow occurs. For example, in fractured rocks the majority of flow occurs in the fractures, and intergranular pore water may be almost static. In porous rocks, some pores may have only one connection with the general pore space (‘blind’ pores) and so contain only static water.
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