THE GLOBAL CYCLE OF WATER

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National Oceanic and Atmospheric Administration

2001

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INTRODUCTION

The global hydrological cycle together with its driving force, solar radiation, forms the basic

resource for primary biological production. It provides the water that is required for the assimilation

of carbon and plays an important role in the supply of nutrients and their transport.

Moreover, the hydrological cycle is responsible for the moderate and favourable temperature

conditions prevailing on Earth through its linkage with the global atmospheric cycle.

The hydrosphere is the interconnection between the biosphere, the atmosphere and the lithosphere,

notably integrating the fluxes of water, energy and geochemical compounds.

Water is able to execute these tasks because of a number of exceptional properties:

1) high and universal dissolving power, essential for distributing geochemical material and to

transport nutrients and to remove waste substances from living organisms

2) high surface tension, causing high capillary forces; together with osmotic forces, this enables

water and solute transport within organisms and maintaining a high cellular tension.

3) large heat capacity and heat of vaporisation, inherent to its role as energy transporter

4) maximum density above freezing point, at 40C; this anomaly causes freezing to proceed from

the surface downward, slowing down both the heat release and the advancement of the freezing

process, thus protecting living organisms

5) high freezing and boiling point relative to its molecular weight, in comparison with similarly

structured compounds, such as H2S and H2Se; compared to these compounds, these temperatures

would be between -50 and -1000C.

All these properties are related to the high cohesion and pseudo-crystalline structure of water.

This structure is caused by the eccentricity of the positive hydrogen nuclei with respect to the

electrons and the oxygen nucleus, which gives the H2O molecule an electrical polarity or dipole

character.

In this chapter we will discuss the composition of the hydrosphere and the basic concepts of the

hydrological cycle and its interaction with atmospheric circulation. Subsequently, we will consider

the individual elements of the hydrological cycle, and their mutual interaction. Finally, attention is

paid to the impact of climatic change and man's interference with the hydrological cycle.