Groundwater is generally a more dependable source of
irrigation than surface water and is free from seeds and plant organisms. The
first cost of installation is however, high. The best water bearing stratum or
aquifer is coarse gravel free from sand but such formation are rare to find. An
aquifer is a saturated formation which creates ground water reservoir and
yields sufficient quantity of water to wells or springs. These are made of
unconsolidated formations like sand, gravel, fractured rocks.
It is termed as sub â€'surface irrigation, because in
this type of irrigation, water does not we the soil surface. The underground
water nourishes the plant roots by capillarity. It may be divided into the
following two types:
Natural sub â€'irrigation; and
Artificial sub â€'irrigation.
sub â€'irrigation: leakage water from channels, etc., goes
underground, and during passage through the sub â€'soil, it may irrigate
crops, sown on lower lands, by capillarity. Sometimes, leakage causes the water
â€'table to rise up, which helps in irrigation of crops by capillarity. When
underground irrigation is achieved, simply by natural processes, without any
additional extra efforts, it is called natural sub â€'irrigation.
sub â€'irrigation: when a system of open jointed drains is
artificially laid below the soil, so as to supply water to the crops by
capillarity, then it is known as artificial sub â€'irrigation. It is a very
costly process and hence, adopted in India on a very small scale. It may be
recommended only in some special cases with favourable soil conditions and for
cash crops of very high return. Sometimes, irrigation water may be
intentionally collected in some ditches near the fields, the percolation water
may then come up to the roots through capillarity.