Geoscience Reference
In-Depth Information
atmosphere. Thus processes in the water cycle play a central role in governing weather
and climate.
1.4
METHODOLOGIES AND PROCEDURES
This topic aims primarily to describe the occurrence and the transport of water in its
continuous circulation over the landsurfaces of the Earth. Before starting this task, it is
worthwhile to review briefly the different strategies that are available and that can be
used for this purpose.
1.4.1
Statistical analysis and data transformation
As observed in Section 1.2, one of the main practical objectives of hydrologic analysis
is the determination of the quantity of water, in storage or in transit, to be found at
a given time and place, free of any direct human control. When a reliable record of
observed hydrologic data is available, a great deal can be learned simply by a statistical
analysis of this record. Although such an approach is proper for stationary systems in
the prediction of long-term behavior for general planning purposes, it cannot be used
for short-term and emergency forecasting, for example, during floods, or for day-to-day
resource management decisions. Furthermore, reliable records are available for only a
few locations over a limited period of time, and practically never where needed. Therefore
in hydrology the problem is often such, that a method must be devised to transform some
available data, which are of no direct interest, to the required hydrologic information.
For instance, the problem may consist of determining the rate of flow in a river at a given
location either from a known flow rate at some other point upstream or downstream,
or from a known rainfall distribution over the upstream river basin. In other cases, the
problem may consist of deducing the basin evapotranspiration from soil and vegetation
on the basis of available meteorological data.
1.4.2
The “physical” versus the “systems” approach
The hydrologic literature is replete with attempts at classifying the methodologies and
paradigms that have been used to transform hydrologic input into hydrologic output
information. Until a few years ago it had become customary to consider two contrasting
approaches, namely the “physical” approach and the “systems” approach. In the physical
approach the input-output relationship is sought by the solution of the known conser-
vation equations of fluid mechanics and thermodynamics with appropriate boundary
conditions to describe the flow and transport of water throughout the hydrologic cycle.
This approach has obvious limitations; the physiographic and geomorphic characteristics
of most hydrologic systems are so complicated and variable, and the degree of uncertainty
in the boundary conditions so large, that solutions are feasible only for certain highly
simplified situations. In other words, the properties of natural catchments can never be
measured accurately enough, and solutions, based on internal descriptions starting from
