Introduction: Once the water reaches the ground, one of two processes may occur;
1) Some of the water may evaporate back into the atmosphere or
2) The water may penetrate the surface and become groundwater. Groundwater either seeps its way to into the oceans, rivers, and streams, or is released back into the atmosphere through transpiration. The balance of water that remains on the earth's surface is runoff, which empties into lakes, rivers and streams and is carried back to the oceans, where the cycle begins again.
- Evaporation: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evaporation amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans.
- Precipitation: Condensed water vapor that falls to the Earth's surface .Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans.
- Condensation: The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.
- Transpiration:The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.
- Snowmelt: The runoff produced by melting snow.
- Runoff: The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
- Infiltration: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.
- Subsurface Flow: The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or Water cycle eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.
OVER UTILIZATION OF SURFACE AND GROUND WATER: With increasing human population and rapid development, the world water withdrawal demands have increased many folds and a large proportion of the water withdrawn is polluted due to anthropogenic activities.Out of the total water reserves of the world, about 97% is salty water and only 3% is fresh water
Even this small fraction of fresh water is not available to us as most of it is locked up in polar ice caps and just 0.003% is readily available to us in the form of ground water and surface water.
FLOODS AND DROUGHT:
- Heavy rainfall often causes floods in the low-lying coastal areas.
- Prolonged downpour can also cause the over-flowing of lakes and rivers resulting into floods.
- When annual rainfall is below normal and less than evaporation, drought conditions are created.
Causes of flood and drought:
- Deforestation, overgrazing, mining, rapid industrialization, global warming etc., have contributed largely to a sharp rise in the incidence of floods.
- Deforestation leads to desertification and drought too. When the trees are cut, the soil is subject to erosion by heavy rains, winds and sun.
- The removal of thin top layer of soil takes away the nutrients and the soil becomes useless.
- The eroded soils exhibit droughty tendency.
CONFLICTS OVER WATER: Indispensability of water and its unequal distribution has often led to inter-state or international disputes. Issues related to sharing of river water have been largely affecting our farmers and also shaking our governments. Many countries are engaged in bitter rivalries over this precious resource.
For instance,Argentina and Brazil, dispute each other's claims to the La Plata river,India and Pakistan fight over the rights to water from the Indus,Mexico and USA have come in conflict over the Colorado river,India and Bangladesh are fighting for Brahmaputra river, and Iran and Iraq contest for the water from Shatt-Al- Arab River.
Effects of over exploitation of water:
- Subsidence: When ground water withdrawal is more than its recharge rate, the sediments in the aquifer (a layer of rock that is highly permeable and contains water) get compacted, a phenomenon known as ground subsidence. It results in sinking of overlying land surface. Due to this structural damage in buildings, fracture in pipes etc., occurs.
- Lowering of water table: Mining of groundwater is done extensively for irrigating crop fields. However, excessive mining would cause lowering of water table.