Global Hydrological Cycle

Introduction

The thin spherical shell of thickness about 10 km at the surface of Planet Earth is unique in the universe because it has abundant amounts of water substance in all three phases: solid, liquid and vapor. Water substance has played a critical role in the evolution of the earth’s atmosphere and surface environment, including the development of biological organisms.

The importance of water, particularly fresh water (as opposed to saline ocean water), in human activities cannot be overstated. Freshwater scarcity already is taking its toll on the African continent and likely will be a limiting factor in future sustainable economic development. Present full commitments of river and lake water for human consumption coupled with water shortages in many regions and expanding future water needs for industrialization, agriculture, and a growing population will put new strains on water supplies. The Director General of the United Nations Educational, Scientific, and Cultural Organization (UNESCO), in a recent visit to Iowa State University warned that water use by humans could well be a source of significant social unrest — even wars — in the 21^st century.

Characteristics of Water

Phase diagram

Water substance exists in all three phases in abundant quantities in the range of temperatures occurring near the earth's surface. The accompanying phase diagram (Figure 1) for water substance gives the set of pressures, temperatures, and volumes that define the conditions separating solid from liquid, solid from vapor, and liquid from vapor. The relation between pressure (e_s) and temperature (T) of Figure 2 can be used to explain many commonly observed features of water substance. The convergence point of the three lines on this graph gives the triple point of water (T = 273.1675 K or 0.0075 ° C and e_s = 611.21 Pa or 6.1121 mb) at which vapor, solid, and liquid all exist in equilibrium. The critical temperature for H₂O of 647 K (compared, for instance, to 126 K for nitrogen) is quite high compared to earth surface temperatures and allows for abundant water and ice on earth. The negative slope of the line separating the solid and liquid phases in the plot of pressure vs. temperature means that raising the pressure on ice at constant temperature will cause it to melt. This is the reason that glaciers can slip down a mountain: high pressure from the glacier at the earth’s surface melts a thin layer that reduces friction and allows downward movement. Also, volume increases as water goes isothermally from liquid to solid. This means that ice is less dense than water, explaining the why ice floats on water rather than sinking to the bottom. This seemingly insignificant fact has a profound influence on the behavior of water bodies in cold climates. What would happen in a Minnesota lake if ice formed at the surface and sank to the bottom?

Latent heats

When substances change phase there usually is a release or absorption of energy (known as latent heat) in the process. Latent heats for the phase changes of water have the following values:

Latent heat of condensation (vapor to liquid, L₃₂) or vaporization (liquid to vapor, L₂₃):

L₃₂ = L₂₃ = 2.500 x 10⁶ J kg^—1at 0°C

= 2.25 x 10⁶ J kg^—1 at 100°C

Latent heat of fusion (liquid to solid , L₂₁) or melting (solid to liquid, L₁₂):

L₂₁ = L₁₂ = 3.34 x 10⁵ J kg^—10°C

Latent heat of deposition (vapor to solid, L₃₁) or sublimation (solid to vapor, L₁₃):

L₃₁ = L₁₃ = 2.834 x 10⁶ J kg^—1at 0°C

Latents heats are needed to calculate the equilibrium (saturation) pressures shown in the previous figure, as is the specific gas constant for water vapor, R_v = 4.61 J/(K kg).

Water vapor as a greenhouse gas

Water vapor belongs to a class of gases whose molecular structure (Figure 3) is such that it absorbs infrared radiation emitted by the earth. Details of this topic are covered in the unit on the Global Energy Balance. Water vapor is the most abundant greenhouse gas, and its global abundance in the atmosphere is considered to be constant. However, the prospect of global warming raises the possibility that water vapor amounts in the atmosphere might increase, thereby further accelerating the global warming (this is called positive feedback, and will be discussed in the unit on Climate Models.

Coupling to the Global Energy Cycle

From the fact that energy is required to cause liquid water to change to a vapor, it is readily apparent that any consideration of global changes in water substance will become entangled with considerations of the global energy budget (Figure 4) of the earth. For example, a major portion of the intense solar radiation received by the earth in the tropics is used to evaporate water, using about 2.4 x 10⁶ J for each kg of water evaporated. This kg of water (1 liter, or approximately 1 quart) may move toward the mid-latitudes and be re-condensed to liquid in a cloud over California, for instance. During the condensation process, the 2.4 x 10⁶ J/kg is released as sensible heat, so the net effect is that the water vapor has served as a vehicle for moving heat from the point of evaporation to the point of condensation. To provide an appreciation for the magnitude of energy carried as latent heat by this one kg of water, 2.4 x 10⁶ J of heat energy is equivalent to the kinetic energy of a 1,000 kg object moving at 71 m/s (e.g., a 2,200 lb automobile moving at 158 miles per hour!) or the potential energy that would have to be supplied to raise this 1,000 kg object a vertical distance of 24.5 m (e.g., raising an automobile from the ground to the top of an 8 story building!).

Importance of Water for Global Processes

In addition to its critical role for human consumption, water also is a key element in many natural functions of the planet. For instance, a map of global vegetation patterns correlates very strongly with global land precipitation patterns: rainfall is a strong regulator of vegetation. As we have seen in the unit on Ocean Structure and Circulation, fresh water has a lower density than salt water of the same temperature. Fresh water from precipitation or snow/ice melt may ride on top of more salty water and thereby influence the vertical mixing in oceans and also global ocean circulation patterns.

Water in the form of hydropower also is a source of renewable energy as a byproduct of evaporation of sea and lake water and subsequent precipitation at high altitudes producing stream and river flow. Another critical aspect of water is its role as a medium for ecosystem functioning. Many chemical, physical, and biological processes take place in water bodies or in the presence of water in soil. These provide critical transformations that allow diversity of ecological systems.

Components of the Global Hydrological Cycle

The hydrological cycle consists of a collection of reservoirs (each having a particular mass of water substance) and movement (fluxes, measured in units of mass or volume per unit time) of water substance between these reservoirs. The following are reservoirs of the global hydrological cycle:

Global oceans

Ice masses

Antarctic ice sheet (Figure 5)
Greenland ice sheet (Figure 6)
Mountain glaciers
Arctic ice
Sea ice (Figure 6a*)

Continental seasonal snow

Surface fresh water

Lakes
Rivers
Marshes and wetlands

Subsurface water

Soil moisture
Permafrost
Ground water

Biospheric water

Atmospheric water vapor

Clouds

Liquid
Ice

Figure 7 gives the relative sizes of the land, ocean, and atmospheric reservoirs of the global hydrological cycle and the processes (evaporation, precipitation, and runoff) that lead to interchanges among these reservoirs. The ocean is obviously the largest reservoir, followed (by a factor 40 smaller) by land, and followed by an additional factor of 2500 by the atmosphere.

Precipitation over land is larger than evaporation, meaning that considerable water evaporated over oceans eventually falls on land and returns to the ocean by way of runoff through rivers.

By dividing the reservoir volume by its loss rate, we get an estimate of the residence time or timescale of H₂O in each reservoir. Figure 8 describes the largest reservoirs and the typical residence times, which range from thousands of years for oceans to about a week for the biosphere.

*Figure 6a is theb0481, from the The Ship Collection of the NOAA Photo Library.

Role of the Global Hydrological Cycle

1. Global Oceans

Global oceans participate in the global energy balance by transporting heat energy, which is preferentially received in the tropics by solar radiation, toward the polar regions. Over half of the solar energy reaching the earth is first absorbed by oceans and then transmitted to other regions. Global oceans take excess CO₂ from the atmosphere at a rate that depends on ocean temperature. The partial pressure of CO₂ in cold water is depressed by enhanced solubility and (in the spring bloom) phytoplankton growth (Figure 9), making cold water a sink for atmospheric CO₂ . Global oceans are rich media for biological growth and contribute substantially to the global carbon cycle as was presented in the unit on the Carbon Cycle.
These surface water biological processes support a full spectrum of the food chain from phytoplankton to whales. Global oceans play a significant role in determining where and how much water vapor enters the atmosphere, which in turn influences the global energy balance. The massiveness of the global oceans also introduces a thermal inertia into the climate system: ocean processes are slow in comparison to atmospheric processes, so changes in ocean structure can introduce slow changes of climate. This fact has become more evident to us in recent years as we have come to understand the El Nino/Southern Oscillation (Climate Variability) which we now know causes perturbations of weather in certain regions for several successive months.

2. Ice Masses

Ice masses offer storage of H₂O (Figure 10 ) on time scales ranging from a few months for continental seasonal snow pack to thousands of years for the Antarctic ice mass. They all contribute to the albedo (reflectivity) of solar radiation. In the 1970s when the prospect of global cooling was being studied, calculations were made of how much warming would be introduced by spreading carbon black on polar ice masses.

Antarctica (Figure 11)

The Antarctic ice sheet is a massive expanse of permanent ice whose high reflectance causes this region of the planet to lose more radiant energy through infrared radiation than it gains through illumination by the sun. This excess loss is made up by energy continually being supplied to the Antarctic region from lower latitudes by the atmosphere and ocean. Ice forms at high elevation on the Antarctic continent and slides downslope to the sea to create large floating icebergs that drift away from the polar ice mass. The ocean surface area covered by these icebergs reduces the amount of open ocean and reduces total evaporation and loss of heat energy from polar regions. This important fact only recently is being included in global climate models.
The Antarctic continent, because of its circularly symmetrical shape, high elevation, and extreme cold temperatures, leads to the formation of stationary and persistent stratospheric clouds that supply the conditions for ozone depletion. Like the global ocean, the Antarctic ice mass introduces thermal inertia into the climate system.

Greenland (Figure 10)

The Greenland ice sheet (Figure 12) is far less massive than the Antarctic ice field and is not nearly as cold. The consequences of this are discussed under the unit on Sea-Level Rise. Having parts of its ice mass near the melting point, the Greenland ice field is more vulnerable to decrease in size than is the Antarctic ice sheet, and so its time scale for change is shorter than that of the Antarctic ice sheet.

Mountain glaciers

Mountain glaciers contribute only a small amount of H₂O to the global total, but their locations closer to population centers make them targets for urban and agricultural water supplies. They can experience noticeable change over periods of decades and, indeed, serve as independent indicators of global warming as can be seen in the accompanying plot of changes in glacier termini (Figure 13).

Arctic ice

Arctic ice is not anchored to land as is Antarctic ice. As floating ice, it governs the evaporation and heat loss from ocean water and having thickness far less than Antarctic or Greenland ice, it covers a relatively large surface area despite its relatively small volume. Being of lower elevation and being surrounded by asymmetric distributions of land and ocean, the North Pole does not provide an ideal location for persistent polar stratospheric clouds, although some ozone loss has been detected in this region.

3. Continental Seasonal Snow

Continental seasonal snow also provides recharge for reservoirs used for urban and agricultural purposes. These areas frequently also serve as important recreational sites.

4. Surface Fresh Water

The need of fresh water for humans and terrestrial animals, and the high cost of desalinization have put an increasing demand on global supplies of fresh water. In addition to its uses for direct consumption by humans and animals, and for cleaning, agriculture, and power production, fresh water bodies provide a source of food and ecosystem services previously described. River valleys running through dry regions form riparian zones that have much higher biological diversity than the arid surroundings. Rivers offer an inexpensive form of transportation: a single barge tow can carry the equivalent amount of grain or coal as 900 large trucks . The role of rivers and lakes for recreation and aesthetics should not be overlooked. Their contribution to a high "quality of life" is difficult to quantify but is increasingly recognized in societies having substantial leisure time.
Marshes and wetlands are bodies of fresh surface water that have particularly rich ecological diversity. These bodies support not only their own diversity but provide an ecological service in their ability to break down anthropogenic chemicals such as agricultural fertilizers and pesticides. As we saw in the unit on the Carbon Cycle, these areas also are substantial reservoirs of carbon.

5. Subsurface Water

Soil moisture, permafrost, groundwater, and water in deep aquifers compose different forms of subsurface water that impact or are impacted by global change.
Soil moisture provides the basic nutrient for plant growth and a reservoir for precipitation storage. By supplying moisture to the soil surface and to plants at a regulated rate depending on soil characteristics, soil moisture also influences surface evaporation and evapotranspiration (water loss by plants) and hence the global energy budget. By slowly delivering water to the surface and to plants over a period of weeks to months, soil moisture introduces a seasonal timescale into the climate system. A climate model with an inappropriate soil moisture submodel may allow its simulated soil to dry out too fast or too slow and cause, respectively, excessive or insufficient heating at the soil surface.
Permafrost plays an important role in global change because of the plant material it has locked up in ice. Melting permafrost exposes new plant material to temperatures that accelerate the decay process. This melting, from whatever cause, will lead to new releases of CO₂ and methane, both greenhouse gases, that lead to further global warming (a positive feedback to the climate system).
Ground water includes near surface aquifers that frequently are tapped by wells for human consumption, agricultural irrigation, and industrial uses. Their proximity to the surface and occasional direct connection to surface water, leave ground water supplies vulnerable to contamination from surface pollutants.
Deep aquifers represent water bodies that have been created thousands or more years ago by slow geo-climatic processes, and their water is sometimes referred to as "fossil water". The large Ogallala aquifer in the Central Plains of the US is an example of a deep aquifer.
Volcanic eruptions cause episodic additions of rather small amounts of subsurface water to the atmosphere, but large eruptions put combinations of sulfates and water vapor into the stratosphere where they contribute to a transient (1-2 year) cooling effect on the global climate.

6. Biospheric Water

Biospheric water contributes little to total global water reservoirs but serves important functions of transporting nutrients in plants, keeping plants (and hence the earth’s surface) cool and green (thereby influencing reflection of solar radiation), and serving as a source of nourishment for predators. We will see in the unit on agricultural impacts that increased atmospheric CO₂ increases the water use efficiency of plants, which means they transpire less to the atmosphere. Less transpiring water means less evaporation and therefore warmer vegetation surfaces. Murray (1997) calls this the second greenhouse effect (the first being the direct warming due to greenhouse gases) and he estimates it could lead to an additional 2^oC to 5^oC warming of vegetation surfaces. Plants increase surface evaporation and sublimation (vaporization of snow) by suspending moisture in the form of dew, rainwater, or snow above the ground, thereby exposing much larger moisture surfaces to air movement and phase change.

7. Atmospheric Water

Atmospheric water consists of water vapor and liquid and ice components of clouds. In vapor form, water contributes to the greenhouse effect. The global distribution of water vapor is shown in Figure 14 in terms of g/kg of air, and in a second graph in terms of relative humidity (Figure 15).

8. Clouds

Clouds contribute both to global warming by absorbing infrared radiation from the surface and to global cooling by reflecting solar radiation. The role of water vapor and clouds in moving latent heat already has been discussed. They also contribute to the poleward transport of sensible heat. Atmospheric water is an important mechanism for cleansing the atmosphere of natural and anthropogenic contaminants. One such anthropogenic contaminant, sulfate aerosol, interacts with clouds to cause increased reflection of solar radiation and hence lead to global cooling (Figure 16). This will be discussed further in the unit on the Global Energy Balance .

1-11: Global Hydrological Cycle