8.2: The Global Water Cycle

To understand the nature of salt in the ocean, we must talk about water, the substance that dissolves the salt. The global water cycle represents a model of the movement of water between various reservoirs on Earth’s surface. Over days to decades, the global water cycle drives variations in salinity throughout the world ocean. Over longer periods, other processes account for salinity variations.

The water cycle describes where water resides, the processes that move water, and the rate at which these exchanges occur. The water cycle serves as the main vehicle by which the hydrosphere interacts with the atmosphere, cryosphere, geosphere, and biosphere. It represents water’s journey from the ocean to the atmosphere to the land and back to the ocean again. It also serves as the means by which salts (and a variety of other things) move from land to sea. Fundamentally, it explains why some parts of the ocean are saltier than others.

Earth’s Water Reservoirs

Seven important reservoirs for water exist on our planet. The largest reservoir exists in Earth’s interior, the geosphere—perhaps as much as 18 oceans’ worth (Peslier et al. 2017). But most of Earth’s interior water is chemically bound to rock and cycles over scales of millions of years (e.g., Bodnar et al. 2013). So it doesn’t factor into our discussion here. The water we consider here can be found just beneath, upon, and above Earth’s surface, Earth’s surface reservoirs.

As you undoubtedly know, the ocean basins hold the most water on Earth’s surface. This includes the parts of the world ocean that freeze over in winter as sea ice. It also means that most of the water on our planet is salty. Solid water constitutes the second-largest surface reservoir. Snow, ice, and glaciers are the most familiar forms. Beneath our feet we find the third-largest reservoir of water on our planet’s surface—groundwater. This reservoir proves most critical to humans as it provides much of our drinking water. The land surface water reservoir ranks fourth. Freshwater lakes, rivers, streams, and ponds make up this reservoir.

While people don’t usually think about it as a place with water, the atmosphere is the fifth most plentiful reservoir. Water vapor, an invisible gas, surrounds us and makes our air humid. Clouds, a visible form of suspended liquid or solid water (i.e., ice crystals), provide an endless form of entertainment on a summer day. My favorite clouds look like whales. What are your favorites?

And we can’t forget about one other important reservoir: the water contained in living matter—the biosphere. The amount is small—0.0003 percent, according to one estimate (Bodnar et al. 2013)—but life manipulates water in ways that the physical world doesn’t. Life influences rates of evaporation, the formation of clouds, the severity of floods, and the impacts of waves and tides on coastal erosion. Of course, life cannot exist without water, and the availability of water determines when and where life may flourish. The intersection of water and life figures prominently in discussions of the biology of the ocean and climate change, so we’ll place the biosphere sixth on our reservoir list.

Rank	Reservoir	Volume (cubic miles)	Percentage	Residence Time
1	Ocean	328,680,477	96.89	3,100 years
2	Glaciers/Ice Caps	7,965,104	2.35	1,900 years
3	Groundwater	2,519,084	0.74	690 years
4	Land Water	49,662	0.15	2 years
5	Atmosphere	3,119	0.001	9.5 days
6	Biosphere	1,128	0.0003	248 days
Total	All	339,218,574

Pathways of Exchange between Reservoirs

These reservoirs all connect to each other, so each represents a potential source or sink to the others. Movement of water in or out of a given reservoir defines the pathways and rates of flow, or fluxes, between reservoirs. But let’s make one thing clear from the outset: the global water cycle is primarily driven by interactions between the ocean and the atmosphere. More water evaporates from the ocean and more rain falls into the ocean than anywhere else on Earth. In fact, the amount of rain that falls on the ocean equals about 71 percent of the rain that falls on Earth (e.g., Boldnar 2013). This should come as no surprise. The ocean covers 71 percent of the planet’s surface after all. So naturally the ocean will be more involved in the water cycle than the land. I stress this point because most illustrations of the water cycle emphasize the land. Now you know it’s the ocean where most of the global water cycle takes place.

Let’s consider the ways water enters and leaves the ocean. Precipitation, the gravity-driven descent of liquid or solid water out of the sky, moves water from the atmosphere to the ocean. Gravity drives the flow of liquid water from rivers and streams (and everything they carry) into the ocean. Gravity also causes water to flow through soil and porous rocks, a process called infiltration. This water then flows underground through aquifers, porous subsurface sediments that serve as reservoirs of water. These ultimately drain to the ocean. Gravity makes glaciers flow downhill, where they may break off directly into the ocean, a process called calving. Seaborne chunks of glaciers—icebergs—eventually melt. Glaciers also release meltwater—water that orignates from frozen sources—to the ocean, a contribution that is rapidly increasing due to global warming. And each day humans discharge hundreds of billions of gallons of wastewater into the ocean—water into which human waste and chemicals from homes and businesses have been discharged. Globally, at least half of this wastewater enters the ocean untreated (Jones 2021). Surface runoff—water that flows over the land surface—originates from rain, snowmelt, and even human activities, such as landscape irrigation. This water often carries nutrients and chemicals that enter the ocean.

The main pathway by which water leaves the ocean is evaporation, the conversion of liquid water into water vapor. Though hard to observe (water vapor is invisible to our eyes), you may see water vapor from a warm surface (a lake or a field) condense when colder air passes over it, a phenomenon known as evaporation fog, or steam fog. Water also returns to the atmosphere via the process of transpiration, the uptake of water by roots and its subsequent evaporation through the stems, leaves, and flowers of plants. Globally, trees release an average of 39 percent of the precipitation they receive back to the atmosphere (Schlesinger and Jasechko 2014). Finally, small amounts of ocean water may be temporarily stored in sea ice, glaciers, coastal groundwater, and even Earth’s crust. The water cycle has more twists and turns than a Los Angeles freeway.

The Water Cycle Is Solar- and Gravity- Powered

In the global view, heat from the Sun and gravity supply the energy that drives the water cycle. The transformation of liquid water to water vapor and its subsequent transport by winds (also solar- powered) distribute water across the planet. Precipitation of water onto elevated land surfaces supply the gravity-driven flows.

The transformations of water between its three physical states prove important for understanding how heat moves around our planet. An understanding of these transformations can help you understand a lot about the world ocean and your everyday life.