Earth is sometimes referred to as the Blue Planet. Seen from space, our planet is a most striking blue color due to the abundance of water. Our planet is unique in the solar system because we have water in all forms; solid, liquid and gas. Most of the water on earth (97%) is stored in the oceans. Other stores are lakes, rivers, soil, snowfields, glaciers, groundwater and the atmosphere. Water is constantly changing state, and moving around between these reservoirs. We call this the water cycle or hydrological cycle.
Some of the water that evaporated from the oceans will precipitate over land. The movement of water vapor through the atmosphere is called advection. The rain or snow that falls over land will take one or multiple longer routes back to the ocean.
As the water cycles through all the water stores, they get replenished. For example, the water in the atmosphere gets completely renewed every 8 days. Other stores take much longer to get completely recycled; a river can take a few months, and a glacier can take 100 years. Groundwater can take hundreds or even thousands of years to renew.
Humans impact the water cycle in several ways, from building reservoirs and dams, to pumping of groundwater, and changing landscapes altogether through urbanization and deforestation.
We have spent a lot of time and money to divert water from places where we did not want it to places where we did want it. We’ve dug canals, built dams and reservoirs, and regulated river flows, all to control the movement of water. By diverting water upstream, the downstream areas are impacted in the form of erosion, changes in ecological systems, and land inundation.
About a third of the world population uses groundwater as a source of drinking water every day, but most groundwater is used for irrigation and industrial use. Some of that water is relatively young water found in the first few hundred meters under the surface. Overall, we do not pump more water from the ground than the daily natural recharge. However, the groundwater is not equally distributed. In places like California people are already using non-renewable water that is thousands of years old. In Egypt, people are now tapping into aquifers that were last replenished a million years ago. Overusing groundwater can result in lower subsurface water and reduced stream and river flow. Many areas of the U.S. already show significant depletion of ground-water storage. Pumping water out of the ground faster than it can be naturally replenished can also cause land subsidence, where the ground literally lowers over time.
Urbanization increases demand for water, which causes more water diversions and groundwater mining. Also, cities increase water run-off, because impervious surfaces like roads and buildings keep water from penetrating the soil replenishing the groundwater. Instead, this water gets transported away through drains and sewers, but the flow is often very high following a rainfall event, leading to flash flooding in some cases.
Deforestation causes drying of the soil and increased surface run-off. Besides a reduction of through flow, evapotranspiration is also reduced or absent. Without trees releasing water vapor, weather patterns above the land will change. There will be less precipitation, for example.
As the climate gets warmer, the water cycle intensifies. More water evaporates from the surface of the oceans and more water comes from vegetation through evapotranspiration. Higher temperatures also mean that the atmosphere is able to hold more water before it condenses in the troposphere. This results in more rainfall in coastal areas, causing floods and landslides. It can also produce more frequent strong tropical storms.
In other areas, the soil dries out through evaporation. When it does rain, the water cannot penetrate the baked soil and will turn into surface run-off. Groundwater does not replenish and the soil gets drier and drier, causing severe droughts.