Geoscience Reference
In-Depth Information
erosion and landslide hazards tend to be related to the pore water pressure distribution
and the local flow velocities.
Appropriate parameter values
But even for the same formulation, it may also be necessary to adopt different parameter
values depending on the flow regime. The formulation of river flow usually requires dif-
ferent values of the roughness parameter in the Gauckler-Manning or Chezy equations,
depending on whether it islowflowwithin the regular channel or high flow with flooding
outside the banks. Similarly, in the description of hillslope outflow by some of the sub-
surface parameterizations of Chapter 10, the appropriate values of the effective hydraulic
conductivity
k
0
and of the thickness of the flow region
η
0
, used to represent stormflow
conditions with active macropores, will be considerably larger than those appropriate
for conditions of baseflow, after the water tables have subsided and many of the macro-
pores in the upper soil layers have emptied and are no longer active. Actually, because
of the high flow velocities, subsurface stormflows may not be of the Darcy type, and it
may be necessary to use Forchheimer's Equation (8.34) with an additional transmission
parameter beside the hydraulic conductivity.
Ultimately, the performance, in a general sense, of any kind of parameterization
and of the resulting model, has to be judged on the basisofits ability to simulate or
replicate observations of the variables of interest. As mentioned in Chapter 1, parsimony
and robustness are important additional considerations. Different aspects of the modeling
issue have been treated by Klemes (1986), Morton (1993), and Woolhiser (1996), among
others.
11.3.2 How to put it all together? Distributed versus lumped approach
As already explained in Chapter 1, scale is the appropriate criterion to classify the
different methodologies. Accordingly, one can distinguish two general classes of models
that have been used in the past to simulate streamflow generation. In the
distributed
models, also called
runoff routing
models, the computational scales are much smaller
than the flow domain characterizing the catchment, whereas in the
lumped
models the
computational scale is essentially of the same order as that of the catchment.
The main feature of the distributed approach is that the basin outflow is obtained by
tracking the water through its different transport phases in the basin interior. In brief,
these phases are surface and subsurface transport into the stream channel network, in
response to precipitation after it reaches the ground surface, and the subsequent open
channel flow to the basin outlet; between precipitation episodes the basin outflow is
dominated by baseflow and evaporation processes. The different mechanisms in each of
these transport phases may be described by combining some of the formulations of the
relevant processes, as presented in Chapters 2-10. These formulations invariably involve
a number of assumptions neglecting certain aspects of the flow, which are considered to
be less important; this means that they can be only simplified representations of reality.
The distributed approach has been receiving increasing acceptance in recent years with
the advent of digital computation and with the growing availability of higher-resolution
