52
2. Formulating
the
Problem
tity of water as well as to incorporate the physical capacities of the arcs
(rivers, canals) in the network. Various degrees of detail can be incorporated in the physical constraints, but the mass balances may include
terms representing (1) runoff-rainfall in the watersheds, (2) channel flow
at downstream points for an upstream input, (3) groundwater models, (4)
water control works, such as reservoirs or lakes, including evaporation,
(5) factors for municipal and industrial net uses and crop irrigation, and
(6) requirements for such factors as navigation, recreation, and water
quality.
While the detail of the river basin mass balances can be as complex or as
simple as the analyst desires, obviously there exists some optimum degree
of complexity in which a compromise is found between the accuracy of the
model representation and the ease of its solution. The model developed
here is a compromise. It encompasses sufficient subsystems and elements to
be easily extended to represent a real hydrological system, yet its solution
is not so complicated as to make its use impractical.
A hypothetical river basin rather than an actual one was selected for the
modeling in order to make the model as general as possible. No actual basin
contains precisely the elements incorporated in the model, although many
of the elements are found in all basins. By forming a hypothetical system
containing elements from many different river basins, we obtain a more
flexible model than would be possible using a single, real river basin.
Figure 2.1 shows a hypothetical river basin that includes the following
major features: (1) channel streamflow, (2) canal flow, (3) runoff-rainfall
input, (4) reservoirs (existing and potential), (5) stream junctions, (6) a
constant-level lake, (7) irrigation removal, (8) municipal removal and
return, and (9) industrial removal and return. Figure 2.1 does not include
the following features that may be present in some water resources systems:
(1) a bay or estuary, (2) groundwater supply and/or recharge, (3) evaporation and transpiration (except as included in a specific subsystem), (4)
sedimentation, (5) navigational requirements, (6) flood control requirements, (7) water quality requirements, or (8) recreational and wildlife
requirements.
Because all the basin subsystems are connected by the flow of the river
downstream, there are no feedback or recycle loops of information, and it
is easy to connect the subsystem elements to form the total basin model.
As a practical matter, it is important for all the elements to have common
dimensions (units) and time bases in the flow of information connecting
the subsystem models through common variables. For example, the release
from a reservoir, which is the output for a reservoir model, becomes an
input variable to the adjacent downstream channel flow model. Obviously,
2. Formulating
the
Problem
tity of water as well as to incorporate the physical capacities of the arcs
(rivers, canals) in the network. Various degrees of detail can be incorporated in the physical constraints, but the mass balances may include
terms representing (1) runoff-rainfall in the watersheds, (2) channel flow
at downstream points for an upstream input, (3) groundwater models, (4)
water control works, such as reservoirs or lakes, including evaporation,
(5) factors for municipal and industrial net uses and crop irrigation, and
(6) requirements for such factors as navigation, recreation, and water
quality.
While the detail of the river basin mass balances can be as complex or as
simple as the analyst desires, obviously there exists some optimum degree
of complexity in which a compromise is found between the accuracy of the
model representation and the ease of its solution. The model developed
here is a compromise. It encompasses sufficient subsystems and elements to
be easily extended to represent a real hydrological system, yet its solution
is not so complicated as to make its use impractical.
A hypothetical river basin rather than an actual one was selected for the
modeling in order to make the model as general as possible. No actual basin
contains precisely the elements incorporated in the model, although many
of the elements are found in all basins. By forming a hypothetical system
containing elements from many different river basins, we obtain a more
flexible model than would be possible using a single, real river basin.
Figure 2.1 shows a hypothetical river basin that includes the following
major features: (1) channel streamflow, (2) canal flow, (3) runoff-rainfall
input, (4) reservoirs (existing and potential), (5) stream junctions, (6) a
constant-level lake, (7) irrigation removal, (8) municipal removal and
return, and (9) industrial removal and return. Figure 2.1 does not include
the following features that may be present in some water resources systems:
(1) a bay or estuary, (2) groundwater supply and/or recharge, (3) evaporation and transpiration (except as included in a specific subsystem), (4)
sedimentation, (5) navigational requirements, (6) flood control requirements, (7) water quality requirements, or (8) recreational and wildlife
requirements.
Because all the basin subsystems are connected by the flow of the river
downstream, there are no feedback or recycle loops of information, and it
is easy to connect the subsystem elements to form the total basin model.
As a practical matter, it is important for all the elements to have common
dimensions (units) and time bases in the flow of information connecting
the subsystem models through common variables. For example, the release
from a reservoir, which is the output for a reservoir model, becomes an
input variable to the adjacent downstream channel flow model. Obviously,