A natural outflow of ground water at the earth surface is said to form a spring a pervious layer sandwiched between two impervious layers gives rise to a natural spring. The springs are generally capable of supplying very small quantities (amounts) of water and therefore generally not regarded as sources of water supply.
FORMATION AND TYPES OF SPRINGS
Springs are usually formed under 3 general conditions of geological formations They are
i. Gravity springs
ii. Surface springs
iii. Artesian springs.
YIELD AND SPECIFIC YIELD
The volume of ground water extracted by gravity drainage from the saturated water bearing material is known as YIELD and when it is expressed as the ratio of the volume of water that can be drained by the gravity to the gross volume of the soil then it is known as SPECIFIC YIELDA Therefore
= volume of water obtained by gravity / drainage gross volume of the soil.
Values of specific yield are dependent on soil' particle size, shape and distribution of pores and degree of compaction of the soil.
SPECFIC RETENTION OR FIELD CAPACITY
The quantity of water retained by the materials against the pull of gravity is termed as specific retention or field capacity. This is also expressed as the percentage of total volume of materials drained.