Geoscience Reference
In-Depth Information
hydroclimatology emphasizes the time-average movement of energy and water.
Such movement occurs in two directions, both out of and into the atmosphere.
Consequently, the present text is motivated not only by the need to understand the
global and regional scale atmospheric features that affect the weather in a specific
catchment, but also to understand how the surface-atmosphere exchanges that
operate inside a catchment contribute along with those from nearby catchments to
determine the subsequent state of the atmosphere downwind.
Broadly speaking, hydrometeorology differs from hydroclimatology in much
the same way that meteorology differs from climatology. Hydrometeorologists
therefore tend to be more interested in activity at shorter time scales than
hydroclimatologists. They are particularly concerned with the physics, mathematics,
and statistics of the processes and phenomena involved in exchanges between
the atmosphere and ground that typically occur over hours or days. Sometimes
these short-term features are described statistically. Hydrometeorologists may, for
example, analyze precipitation data to compute the historical statistics of intense
storms and flood hazards. However, hydrometeorologists are also interested
in seeking basic physical understanding of surface exchanges of water and energy.
This commonly involves the study of processes that act in the vegetation covering
the ground, or the soil and rock beneath the ground, or in lower levels of the
atmosphere where most atmospheric water vapor is found. The present text
includes some description of the statistical approaches used in hydrometeorology
but gives greater prominence to providing an understanding of fundamental
hydrometeorological processes.
Water in the Earth system
Although there have been several studies which have attempted to quantify where
water is to be found across the globe, the magnitude of the Earth's water reservoirs
and how much water flows between these reservoirs still remains poorly defined.
Table 1.1 gives estimates of the size of the eight main reservoirs together with the
approximate proportion of the entire world's water stored in each reservoir and
an estimate of the turnover time for the water. The magnitude of the groundwater
reservoir and the associated residence time is complicated by the fact that a large
proportion of the water in this reservoir is 'fossil water' stored in deep aquifers
which were created over thousands of years by slow geo-climatic processes. The
amount of such fossil water stored is very difficult to estimate globally. Defining
a residence time for oceans is also complicated. This is because oceans usually
have a fairly shallow layer of surface water on the order of 100m deep that
interacts comparatively readily with the atmospheric and terrestrial reservoirs,
but this layer overlies a much deeper, slower moving, and more isolated reservoir
of saline water.
Clearly oceans are by far the largest reservoir of water on Earth, which means
that a vast proportion of water on the Earth is salt water. The majority of Earth's
