What is the fundamental difference between specific yield and specific retention in an unconfined aquifer regarding the water released from storage by gravity?

In an unconfined aquifer, which is an aquifer where the upper water surface, called the water table, is free to rise and fall and is in direct contact with the atmosphere through overlying unsaturated material, water is stored within the pore spaces of the aquifer material. When the water table drops, water is released from storage due to the force of gravity. The fundamental difference between specific yield and specific retention lies in what portion of this stored water they represent regarding its drainability by gravity.

Specific Yield (Sy) represents the volume of water that *drainsby gravity from a saturated unit volume of an unconfined aquifer material. This is the water that is actually released from storage and becomes available for pumping or natural discharge as the water table declines. It quantifies the portion of the total pore space from which water can overcome the adhesive and surface tension forces (capillary forces) and move downwards under the influence of gravity. For example, if a sand aquifer has a specific yield of 0.20, it means that for every cubic meter of aquifer material where the water table drops, 0.20 cubic meters of water will be released.

Specific Retention (Sr), on the other hand, represents the volume of water that is *retainedwithin a saturated unit volume of the unconfined aquifer material after gravity drainage has ceased. This water is held against the force of gravity by capillary forces. Capillary forces occur due to the surface tension of water and its adhesion to the surfaces of the aquifer grains, particularly in smaller pore spaces. These forces are stronger than gravity in very small pores, preventing the water from draining out. This retained water is not available for pumping or gravity-driven flow. For example, in the same sand aquifer, if it has a specific retention of 0.15, it means 0.15 cubic meters of water remain in the pore spaces, held by capillary forces, for every cubic meter of aquifer material after gravity drainage.

Therefore, the fundamental difference is that specific yield quantifies the drainable water that is released from storage by gravity and can be extracted, while specific retention quantifies the undrainable water that remains within the aquifer due to capillary forces, despite the influence of gravity. Together, specific yield and specific retention sum up to the total porosity (n) of the aquifer material, which is the total volume of void space per unit volume of material (n = Sy + Sr). This illustrates that all the water initially present in the pore spaces of a saturated unconfined aquifer is either released by gravity (specific yield) or held by capillary forces (specific retention).