Geoscience Reference
In-Depth Information
Figure 9.1
Three-dimensional view of an ensemble of three REWs, including a portion of atmosphere (after
Reggiani and Rientjes, 2005, with kind permission of the American Geophysical Union).
catchment shaped, linked by channel reach control volumes and reflecting the topographic structure of
the catchment (Reggiani and Rientjes, 2005).
This scale independence is a major advantage of this framework but we also expect that the nature
of hydrological fluxes should be scale dependent and exhibit some nonlinearities (Beven, 2006b). The
REW balance equations include these scale dependencies and nonlinearities but only implicitly. They
do so through the flux terms in the balance equations at the boundaries of the chosen landscape units. In
applying a model based on the REW concepts, these flux terms (of mass, energy and momentum) must
be specified in some way. We expect hydrological fluxes to be nonlinearly related to storage (perhaps
with some hysteresis between wetting and drying) and to be dependent on the scale of the element. This
is what is called the
closure problem
. Addressing the closure problem is exactly what is required to
develop the new generation of hydrological models; indeed, Beven (2006b) has called the solution to
the closure problem the “Holy Grail” of scientific hydrology. It is worth saying that none of the current
implementations of the REW concepts have provided entirely satisfactory solutions to the closure problem
(see Section 9.4); hence further research is required. The framework provides, however, a general structure
for thinking about how the complex nature of hydrological responses within a REW landscape unit might
best be represented by the functional relationships specified for the fluxes at different scales.
