8.1: The Hydrologic Cycle

Water is constantly on the move. It is evaporated by solar energy from the oceans, lakes, streams, the surface of the land, and from plants (transpiration) (Figure \(\PageIndex{1}\)). It is moved through the atmosphere by winds and condenses to form clouds of water droplets or ice crystals. In response to the pull of gravity it comes back down as rain or snow and then flows through streams, into lakes, and eventually back to the ocean. Water on the surface and in streams and lakes infiltrates the ground to become groundwater. Groundwater slowly moves through the surface materials and underlying bedrock. Some groundwater returns to streams and lakes, and some goes directly back to the oceans.

Figure \(\PageIndex{1}\): The Various Components of the Hydrologic Cycle. Black or white text indicates the movement or transfer of water from one reservoir to another. Yellow text indicates the storage of water.

Figure \(\PageIndex{2}\): Representation of the Volumes of Earth’s Water Reservoirs. The 1-liter jug is filled with salty sea water (97%). The ice-cube is glacial ice (2%). The 2 teaspoons represent groundwater (1%), and the three drops represent all the fresh water in lakes, streams, and wetlands, plus all of the water in the atmosphere.

Even while it is moving around, water is stored in various reservoirs. The largest, by far, is the ocean, accounting for 97% of the total water volume at Earth’s surface. That water is salty. The remaining 3% is fresh water. Two-thirds of our fresh water is stored in ice and one-third is stored in the ground. The remaining fresh water—about 0.03% of the total—is stored in lakes, streams, vegetation, and the atmosphere. To put that in perspective, imagine putting all of Earth’s water into a 1-liter jug (Figure \(\PageIndex{2}\)). We start with sea water, by almost filling the jug with 970 mL of water and 2 tablespoons of salt. Then we add one regular-sized (roughly 20 mL) ice cube (representing glacial ice) and two teaspoons (roughly 10 mL) of groundwater. All the fresh water that we see around us in lakes and streams and up in the sky can be represented by adding just three more drops from an eyedropper (about 0.33 mL).

The volumes of the reservoirs are listed in Table \(\PageIndex{1}\), along with the average residence time of water in each reservoir. A molecule of water that falls into or flows into the ocean will stay there for an average of about 3,100 years. During that time, it might get moved all around the Earth via surface currents, and even to the deep ocean by thermohaline circulation. It will eventually be returned to the atmosphere via evaporation. A molecule of water that gets frozen into snow and then falls on a glacier, stays there for as long as 16,000 years on average (although some deep Antarctic ice is over a million years old) before it flows out as meltwater or calves into the ocean.

Groundwater stays in aquifers for an average of 300 years, although some very deep groundwater is hundreds of thousands or even millions of years old. In freshwater lakes with flows in and out via rivers, the average residence time is 1 to 100 years, while in salt lakes, from which the only exit is likely to be by evaporation, the average residence time is up to 1000 years. Moisture in the soil typically stays there for less than a year. It is cycled out via plants, or seeps down to become groundwater. Water remains in a river for an average of 12 to 20 days before flowing into a lake or the ocean. Water is held in the atmosphere there for an average of only 8 days.

Table \(\PageIndex{1}\): The Volumes and Average Residence Times of the Earth’s Various Water Reservoirs

Reservoir	Volume (thousands of km^3)	% of total	Average residence time (of a water molecule)
Ocean	1,370,000	97.1	3,100 years (1.1 million days)
Glaciers	29,000	2.05	16,000 years (5.8 million days)
Groundwater	12,000	0.85	300 years (110,000 days)
Freshwater lakes	125	0.009	1 to 100 years (365 to 36,500 days)
Salt lakes	104	0.008	10 to 1000 years (3,650 to 365,000 days)
Soil moisture	67	0.005	280 days
Rivers	1.2	0.00009	12 to 20 days
Atmosphere	13	0.0009	8 days

Although the proportion of Earth’s water that is in the atmosphere is tiny, the actual volume is significant. At any given time, there is the equivalent of approximately 13,000 cubic kilometers (km³) of water in the atmosphere in the form of water vapor and water droplets in clouds. Water is evapotranspired from vegetation and evaporated from the oceans and lakes at a rate of 1,580 km ³ per day, and just about the same volume falls as rain and snow every day—over both the oceans and land. The precipitation that falls on land goes back to the ocean in the form of stream flow (117 km³/day) and groundwater flow (6 km³/day). Most of the rest of this chapter is about that 123 km³/day of stream and groundwater flow.

Surface water exists within lakes and ponds, as snow and ice, and within flowing streams. If we want to use surface water as a resource, we generally must restrict ourselves to streams, and we must be careful to leave enough of the stream water for the ecosystems (and other people) that depend on it. It is true that we can extract water from lakes, and many municipalities do that, but that can only be done to the extent that the lake is part of a stream system, and we cannot take more water from the lake than is being added by streams. If we do that on a long-term basis (more than 1 year) the lake level will start to drop and before too long the lake will not be viable as a water source.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY SA 4.0, after Water Cycle by Ingwik, CC BY SA 3.0, via Wikimedia Commons, https://commons.wikimedia.org/wiki/F...ycle_blank.svg
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Modified by Steven Earle, from Mackenzie River with Heavy Sediment Load by Pierre Markuse, 2018, CC BY 2.0 via Flickr, https://www.flickr.com/photos/pierre...QFWJa5-2goyBLg