Hydrological Cycle

The hydrologic cycle (water cycle) is a global, sun-driven process. On Earth, water travels in a cyclical process, transported from water bodies to the atmosphere, then to land, and subsequently to water bodies in the hydrologic cycle. Water is evaporated by the sun, incorporated into clouds as water vapor, falls to the land and water bodies as rain, and from land it goes back to the water bodies through several hydrologic processes, such as runoff and infiltration. The hydrologic cycle is a closed system for Earth, as the amount of water remains fixed throughout, but it may vary with its distribution geographically, temporally, and seasonally. 

Six major components of the hydrologic cycle are precipitation, infiltration, evaporation, transpiration, surface runoff, and groundwater flow. Evaporation and transpiration together are known as evapotranspiration. The volumes of different components of hydrologic cycle are: 110,000 cu. km of precipitation onto land and 458,000 cu. km of precipitation on the ocean surfaces, with 502,800 cu. km of evaporation from oceans and 65,200 cu. km of evaporation from land, of which 42,600 cu. km is from river runoff, and 2,200 cu. km is from underground runoff.

Increasing atmospheric concentrations of greenhouse gases warmed the Earth’s surface by nearly 1 degree F (0.6 degree C) during the 20th century. It may continue in this century, leading to a higher sea-surface temperature. One consequence of a warmer ocean surface is a larger vapor pressure difference between the sea surface and the adjacent atmosphere. Therefore, there would be an increased evaporation rate and subsequent increase in the other components of the hydrologic cycle. 

Computer simulation models found that a global warming by 7.2 degrees F (4 degrees C) is expected to increase global precipitation by about 10 percent, and that rainfall intensity will be greater than at present. Scientists, through models, found that the upper tropospheric water vapor amount will increase by 15 percent with each degree of atmospheric temperature rise. The global water vapor amount will increase by 7 percent with each degree of atmospheric temperature rise.

Conversely, to make matters worse, water vapor acts as a prominent greenhouse gas. Increased water vapor alters the climate feedback loop. With a rise in surface temperature, the water vapor amount in the atmosphere increases. The additional water vapor absorbs additional radiated energy, which would normally escape from Earth’s surface to outer space, and it makes the Earth’s surface even warmer. This somber picture is further complicated by important interactions between water vapor, clouds, atmospheric motion, and radiation from both the sun and the Earth’s surface.