Pellicular Water:
When a well has digged in a completly saturated soil formation voids in the soil is drained by under the action of gravity. But it is found that the entire water contained in this voids cannot be drained out. Some of the water is being retained by their molecular attraction. This retained water is known as pellicular water.
Specific Yield (Sy): The volume of ground-water extracted by gravity. drainage from a saturated water bearing material is known as the yield, and when it is expressed as the ratio of the volume of the total material drained then it is known as specific yield.
Specific Yield = (Volume of water obtained by gravity drainage/Total volume of the material drained or dewatered) × 100%
In order to give a higher specific yield, the aquifer material must be porous and should have high permeability. Porosity, being the percentage of voids pores), represents the maximum amount of water that a formation can store. However, porosity along cannot ensure higher specific yield (of ground water), since ground water cannot get stored (or withdrawn) from a formation with any greater amount of porosity, unless and until its permeability is also high, as to permit flow of water through it. Since a rock or a soil, which is porous, may or may not be permeable ; the higher porosity alone cannot give higher specific yield.
Specific Retention (Sr) or Field Capacity: The quantity of water retained by the material against the pull of gravity is termed as the specific retention or the field capacity, and it is expressed as percentage of the total volume of the material drained.
Specific Retention (Sr) or Field Capacity = (Volume of the water held against gravity drainage/Total volume of the material drained) x 100%
It is evident that the sum of the specific yield and the specific retention is equal to its porosity (n); i.e.
Sy + Sr = n
