Page 74 - Soil Degradation, Conservation and Remediation

60 2 Physical Deterioration of Soil

unsaturated and saturated zones. It represents the upper surface of the groundwater.
The water level found in unused wells is often the same level as the water table. Soil
pore spaces in soil become completely saturated with water up to the groundwater
table. The entry of water to the saturated zone beneath the ground is called the
groundwater recharge. Groundwater recharge is a hydrologic process where water
moves downward from surface water to groundwater. This process usually occurs in

the vadose zone below plant roots and is often expressed as a flux to the water table
surface. Recharge occurs both naturally and through anthropogenic processes. The
sources of recharge to a groundwater system include both natural and human-
induced phenomena. Natural sources include recharge from precipitation, wetlands,
lakes, ponds, and rivers (including perennial, seasonal, and ephemeral fl ows), and
from other aquifers. Human-induced sources of recharge include irrigation losses

from canals and fields, leaking water mains, sewers, septic tanks, and over-irrigation
of parks, gardens, and other public amenities. Recharge from these sources has been

classified as direct recharge from percolation of precipitation and indirect recharge
from runoff ponding. When groundwater recharge exceeds discharge, the ground-
water table rises upward. When groundwater pumping and discharge exceeds
recharge, there is the lowering of the groundwater table.
Removal of water from the groundwater is known as the groundwater discharge.

Some, and often a great deal, of the water flowing in rivers comes from seepage of
groundwater into the streambed. Groundwater contributes to streams in most phys-
iographic and climatic settings. The proportion of stream water that comes from
groundwater inflow varies according to a region’s geography, geology, and climate.

Movement of groundwater in these directions is known as groundwater discharge.
It is the removal of water from groundwater. A plenty of groundwater is also
withdrawn from shallow and deep tube wells in urban and agricultural areas. The
most severe consequence of excessive groundwater pumping is that the water table
is lowered. This poses a threat to the future shortage of water for urban and agricul-
tural use. A related effect of groundwater pumping is the lowering of groundwater
levels below the depth that streamside or wetland vegetation needs to survive. The
overall effect is a loss of riparian vegetation and wildlife habitat. The basic cause of
land subsidence is a loss of support below ground. When water is taken out of the
soil, the soil collapses, compacts, and drops. This depends on a number of factors,
such as the type of soil and rock below the surface. Thus, the water table should be
maintained at a desirable depth beneath the soil surface. A shallow water table
restricts root growth and elongation by limiting oxygen. Large-scale lowering of
water table has been considered as an important cause of desertifi cation.

2.5 Subsidence of Organic Soils

Subsidence is the fall of the land level due to compression, consolidation, compac-
tion, and oxidation of organic matter, particularly as a result of draining and cultiva-
tion of peat soils (Histosols). Subsidence has long constituted a serious problem in