Evolutionary Biology Topics allow us to trace the history of life on Earth.
How the Hydrological Cycle Works
Earth differs from other planets in this solar system as it has large amounts of water mostly in liquid form, which supports all life on Earth. There is a constant exchange of water between the air, land and sea and between the living organisms and their environments. Thus, there are two overlapping water cycles in nature: the larger global water cycle which does not involve living organisms, and the smaller biological water cycle which involves living systems.
Global Water Cycle
Water evaporates from the hydrosphere (oceans, seas, rivers, streams, lakes, ponds, moist soil) under the sun’s heat and forms clouds. The wind blows the clouds over to the land, where after getting cooled enough, the water in clouds falls on Earth as rain, hail, and sleet (called precipitation). Rain may fall directly into the oceans also. Some water from rain and melting snow soaks into the ground, but most of it flows in rivers and returns directly to the seas.
The groundwater comes to the surface again with the help of springs and pumps. It is used in agriculture, industry, and homes. This water finally evaporates into the atmosphere. Water also evaporates from the snow, reservoirs, and ground. Water vapors, added by evaporation to the atmosphere, again form clouds. This completes the global water cycle. The energy to run this cycle comes from the sunlight. The ocean forms the largest global reservoir of water and the atmosphere forms the smallest global reservoir of water.
Sizes of Reservoirs of Global Water
| Reservoir | Quantity (in teratons, i.e, 1018 g) |
| 1. Oceans | 1,380,000 |
| 2. Polar Ice, Glacier | 29,000 |
| 3. Ground Water (Actively Exchanged) | 4,000 |
| 4. Freshwater Lakes | 125 |
| 5. Saline Lakes | 104 |
| 6. Soil Moisture | 67 |
| 7. Rivers | 1.32 |
| 8. Atmosphere Water Vapour | 14 |
Therefore, rapid evaporation from land and sea is essential to replenish the atmosphere’s water vapour. It should be noted here that the second largest global reservoir of water is polar ice (glacier) and the third largest global reservoir of water is groundwater. Thus, a large amount of water is locked underground and as perennial snow in the polar regions and on mountain peaks. This is called locked water.
Biological Water Cycle
The organisms get water from and return it to the global water cycle. Plants absorb water from the soil or water reservoir and add it to the air in vapour form by a process called transpiration. Water transpired by trees cools the surrounding air and plays an important role in determining the microclimate around them. Animals take water from the water reservoir or with food (plants or other animals or their products). They return it to the air as vapours by respiration or to the soil as fluid by excretion.
Mammals, also excrete water as sweat, which evaporates from their bodies. Water is also added to the environment by the death and decay of organisms. Water vapour formed by transpiration and respiration form clouds and enter the global water cycle. Rain adds water to soil and reservoir for reuse by plants and animals. This completes the water cycle.
The surface water of Earth represents the most extensive habitat for organisms. A variety of animals and plants live in water. They possess special adaptations for aquatic life. Water is a universal solvent, so, is capable of dissolving a large number of substances. As the water flows through or over the rocks containing soluble minerals, various soluble minerals get dissolved in it and get transported to distant places. Thus, many nutrients are carried by water from the parent rock to the river and then to the seas. Here, these minerals are used by the marine organisms.
Two aspects of the water cycle need special emphasis:
- More water evaporates from the sea than returns to it by rainfall, and vice versa for the land. In other words, a considerable part of the rainfall that supports land ecosystems and food production comes from water evaporated over the sea.
- Human activities tend to increase the rate of runoff. Practices such as paving over the Earth, ditching and diking rivers, compacting agricultural soils, and deforestation, all prevent the recharge of the very important groundwater compartment. Groundwater holds about 13 times more water than all freshwater lakes, rivers, and soils).
The role of vegetation in promoting transpiration, evaporation, and hence rain is especially evident in tropical rain forests. When people cut down rainforests in one area, rainwater drains off and eventually reaches the sea instead of evaporating. The ongoing massive destruction of the Earth’s tropical rainforests is gradually changing the environmental conditions.