Climate Change and the European Water Dimension - Agri ...

Chapter III. The Hydrologic Cycle
III.1. INTRODUCTION
Water covers 70% of the Earth’s surface, and is present in the atmosphere and the
crust; it also comprises a large part of all living matter. Water is the only natural
resource that exists naturally in three forms: liquid, solid (snow, ice) and gas (clouds).
Unlike most mineral resources, it is renewable it exists in an endless cycle, moving
between its gaseous, liquid and solid forms. This “hydrologic cycle” comprises
nature’s method of replenishing, redistributing and purifying the world's natural water
resources.
By hydrologic cycle we mean the circulation (and transformation) of water between
the earth and the atmosphere through different pathways (Figure III.1). Being a
cycle, it has no specific beginning or ending. Rather, liquid water from the Earth’s
surface, particularly the oceans, is either evaporated into a gaseous form and enters
Figure III.1. The hydrologic cycle, with annual volumes of flow given in its units
relative to the annual precipitation on the land surface. Because of its negligible
magnitude compared to other components, aerosol has been removed from the cycle
(Maidment, 1993; with permission).
the atmosphere as water vapor (clouds) or is dispersed in the atmosphere through
aerosols. The atmospheric moisture is eventually returned to the Earth’s surface in
the form of rain, snow or hail. Approximately 100,000 km 3 (about 20% of the total
global annual precipitation) falls onto the land surface of the continents. Except for
that portion evaporated or transpired directly back into the atmosphere, and the
portion moved directly into the atmosphere through aerosols, most of the water
reaching the land surface in the form of rain, snow or hail will eventually find its way
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