In this essay we will discuss about the water cycle with the help of a diagram.
Importance of Water:
Water is indispensable for life. It is an essential need of both plants and animals. Every organism contains water about 60-90% of body weight. It is required for the working of macro- molecules, as a good ionizer, transport of materials, solubilization of chemicals and for several chemical reactions like hydrolytic digestion of polymeric nutrients, photosynthesis, etc. It also acts as a habitat for hydrophytic and many aquatic animals. It is also an agent of geological change.
It is an agent of energy transfer and use. The large amounts of energy involving in converting ice to water (80 g cal/g), of raising water temperature (1 g cal/g/ degree) and vaporizing it (536 g cal/g) make water a tremendous factor of neutralizing heat radiations of sunlight, so acts as a temperature buffer.
Two types of water cycles:
These are two overlapping water cycles in nature i.e:
1. Global water cycle not involving life, and
2. Biological water cycle involving the life.
1. Global Water Cycle:
Water evaporates into the atmosphere from sea, lakes, rivers etc. It is estimated that about 4.46 x 1020 grams of water gets evaporated and precipitated annually. More than 75% of total evaporation occurs from the ocean.
It is further estimated that about 95% of earth water is bound in the lithosphere and sedimentary rocks, and only about 5% is actually or potentially free and in circulation out of which 99% is in the ocean. But the atmosphere can hold only 0.13 x 1020 grams of water vapours at any time so the atmosphere must be reple-nished with water vapours 34 times in a year (approximately after every 11 days).
The clouds thus formed are blown over to land. Here they are cooled to form rain, hail, snow etc. Part of rain falls into oceans (Fig. 14.28). Some water falls into ground. The runoff water is finally collected in ocean making its way through ponds, rivers and sea.
Water from these water bodies may percolate in the soil reaching the permanent zones (gravitational water). Water is also made available due to melting snow. A large amount of water remains underground as well as in the form of perennial snow in the Polar Regions and mountain peaks above snow line.
The evaporation from these water bodies return the water to the atmosphere. The energy for the global water cycle is provided by sunlight. So global water cycle involves the interchange, between the earth’s surface and the atmosphere via precipitation and evaporation. The cycle is a steady state one because total precipitation is balanced by total evaporation.
2. Biological Water Cycle:
In this cycle, underground water acts as a starting point. Aquatic organisms take water from their surroundings. The plants may take water from these water bodies and also from the soil while the animals may take water from the plants which they eat, or consume water from different water bodies, or the gravitational water directly. After the death and decay of these organisms, the water is again made available.
Animals release the water into air by respiration in the form of vapours or to soil as fluid excretion. The plants lose most of the water by the process of transpiration. Water vapours thus formed enter the global water cycle and form clouds. Rain adds water to the soil and reservoirs. From this water is again made available to plants. So there is exchange of water between the global water cycle and biological water cycle.