Geoscience Reference
In-Depth Information
7.1.3.
Global or distributed approaches
Converting climatological data into hydrological data is a difficult process due to
the spatial variability of the climatological phenomena that the hydrologist would
like to use, such as rainfall and temperature. Not only is it made difficult because of
the aforementioned spatial variability, but also because of the different
environments that the data are recorded in (such as the nature of soil and sub-soil,
relief etc), and because of the hydrologists lack of understanding of the laws of
physics that govern the relationships between rainfall, evaporation, and water flows,
etc.
If we take the example of a relatively small drainage basin (with an area of
several hectares to a few hundred square kilometers) the following assumptions can
be made: precipitation and evaporation are uniform for the entire drainage basin.
Under these conditions, the drainage basin is considered a system whose input is
seen as being unique chronicles of data that relate to different climatic factors [LAV
97].
Figure 7.2.
A diagram representing a global model
However, it is not possible to use the global approach for large drainage basins
where rainfall is no longer considered as being uniform for the entirety of the basin.
The drainage basin is divided into sub-basins that are small enough so that,
individually, they can be represented by a global model. Water levels associated
with each of these sub-basins are then combined together and are transformed at the
areas where the different sub-basins cross paths, as can be seen in the diagram
below. For such a semi-distributed model to be successful, the hydrologist must
divide the main drainage basin into smaller, homogenous sub-basins.
It is possible to use distributed models in which a specially adapted grid can be
used to solve issues associated with the spatial variability of the different parameters
that are used in the model. For distributed models, however, one of the main
difficulties is the automatic construction of the drainage model. This main problem
is caused due to the way in which each grid is placed one on top of the other. The
drainage model is the key force behind the agglomeration of primary water levels
that form part of the discharge that occurs at a drainage basin's outlet. Such a
