Estimated reading time: 3 minutes
Hydrologic Cycle
The water cycle, also known as the hydrologic cycle, transfers water between land, oceans, and the atmosphere. Water undergoes evaporation from both oceans and land surfaces, transitioning from a liquid to vapor state and entering the atmosphere. Oceans contribute significantly more to evaporation, approximately six times greater than land, due to their extensive coverage and the varying wetness of land surfaces. Then, Water vapor in the atmosphere condenses to form clouds and precipitates in the form of rain, snow, or hail on the Earth’s surface.
| Hydrologic Cycle Meaning The hydrologic cycle describes the global flow of water to and from oceans, land, and atmosphere. Water moves by evaporation, precipitation, and runoff. |
Precipitation over oceans is nearly four times greater than precipitation over land. Once precipitation reaches land, it follows three potential courses. Firstly, it can evaporate back into the atmosphere as water vapor. Secondly, it may infiltrate the soil, moving into cracks and crevices in rock layers to become groundwater. This subsurface water later emerges to contribute to rivers, lakes, and ocean margins. Thirdly, precipitation can run off the land, accumulating in streams and rivers that eventually transport it to the ocean or lakes, known as runoff.
Maintaining a global water balance among land, oceans, and the atmosphere is crucial since the Earth contains a fixed amount of water. Evaporation from the ocean is approximately 420 km³/yr (101 mi³/yr), while precipitation entering the ocean is 380 km³/yr (91 mi³/yr). To compensate for the imbalance, 40 km³/yr (10 mi³/yr) flows from the land back to the ocean. Similarly, on land surfaces, of the 110 km³/yr (27 mi³/yr) of water received, 70 km³/yr (17 mi³/yr) is re-evaporated into the atmosphere, and the remaining 40 km³/yr (10 mi³/yr) remains as liquid water, eventually returning to the ocean.
Among these pathways, our primary focus will be on the flow of water from the atmosphere to the surface in the form of precipitation. To comprehend this process, we first need to explore how water vapor in the atmosphere transforms into clouds and, subsequently, precipitates.
