Water bodies and vegetation in the water cycle

Evapotranspiration is the set of processes that return water from the Earth’s surface back into the atmosphere. So you can see we’re starting to build the components of the water cycle. We have water that’s delivered from the atmosphere to the Earth’s surface, and then a return of water from the Earth’s surface back into the atmosphere. But precipitation does not occur evenly across the globe. We have precipitation directly over the ocean, and then we have evaporation directly from the ocean. And there is an exchange process that occurs there. We also have precipitation over land. And when we have precipitation over land, that water is not necessarily contributing directly to a big body of water, but instead starts interacting with the land surface.

So some of the water that lands at the Earth’s surface, as I mentioned, infiltrates into the soil. And some of it can actually flow over the land surface. Water that flows over the land surface can ultimately be concentrated into rivers and streams, and can drain into lakes, and ultimately into oceans. Some of the water that infiltrates into the soil will drain very slowly into deeper and deeper soil layers, until it reaches the point where the conditions are completely saturated, where you have a body of water at some depth that you run into. That is called the groundwater. So if I dig a well in any place on the land, if I go deep enough, I will run into the groundwater.

And the depth of that groundwater from the surface depends on the climate conditions of a particular region. It tends to be the case that in very dry areas, the water table or the top of the groundwater is much deeper than it is in humid regions, where there’s much more rain contributing to that groundwater source. The groundwater exists within rocks and sediment way below the surface of the Earth. In areas that have shallow water tables, sometimes the water table is entirely at the surface, and is connected with lakes and rivers. And the water table within any region is always fluctuating up and down, depending on the season and depending on what sort of pressures that water table is under.

And the pressures I’m talking about are mostly human beings that are pumping water out for other purposes, like agriculture and industry and water supply for human beings. When you pump water away from that groundwater source, it tends to lower the water table within a region. So everything I’ve been talking about is really how the water cycle is expressed at one location. But we’re part of a global climate system, where air masses and water are transported across the globe through winds and clouds. So each region has a completely different expression of climate that controls the water cycle on a regional or local basis. That’s why we have dry regions and wet regions, areas with tropical forest and deserts.

So the end result of the hydrologic cycle at any given moment is a balance of water between different stores over the planet. In this day and age, we keep careful track of what’s happening within the hydrologic cycle over planet Earth. We have incredible datasets that allow us to understand how deep the water table is, how much water is available at the surface of the Earth, how much it rains, and over what seasons that rain occurs. This enables us to predict and plan and manage our water resources. Of course, we don’t have a perfect set of knowledge about these processes. There are many uncertainties that challenge us to manage our water resources.

When we’re faced with extreme conditions like droughts, we often struggle to provide adequate water supplies to human societies, and ecology tends to suffer as well.