A synthesis framework for runoff prediction in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 17

the idea here is to compare many catchments with contrast-
ing characteristics in order to understand the process con-
trols of the catchments viewed as complex systems.
Falkenmark and Chapman ( 1989 , p. 12) summarised their
approach as follows:

The Darwinian approach values holistic understanding of the
behaviour of the given landscape. It embraces the history of a
given place, including those features that are relics of historical
events, as central to understanding both its present and its future.
The Darwinian approach gains predictive power by connecting
a given site to several sites located along critical gradients.
Laws in the Darwinian approach will seek to explain patterns
of variability and commonality across several sites.

The term was coined to describe the

study of the character of hydrological processes as influenced by
climate and the nature of the earth

' comparative hydrology '

s surface and subsurface.
Emphasis is placed on understanding the interactions between
hydrology and the ecosystem, and determining to what extent
hydrological predictions may be transferred from one area to
another.

'

The Darwinian approach therefore contrasts sharply with
the Newtonian approach, which remains the dominant
paradigm in physics, and even in hydrology, and builds
on the application of universal laws (Harte, 2002 ). The
Darwinian approach, on the other hand, is the dominant
paradigm in ecology and emphasises patterns and the his-
tory of the place. Much of the insight and power of the
Darwinian approach comes from comparing similar and
dissimilar places and then attempting to generalise, just as
Darwin learned by comparing species from around the
world. The Newtonian approach generalises by discover-
ing universal laws governing particular processes through
experimentation and mathematical derivations, whereas
the Darwinian approach generalises through discovering
patterns through comparative analyses across places and
seeking explanations of how they came about.
How then does the comparative hydrology approach
help in the synthesis across processes, places and scales,
the focus of this topic? We consider each catchment as a
result of nature

They note, however (p. 9):

It should be remembered that the topic represents no more
than a first effort to draw attention to the field of comparative
hydrology, and we sincerely hope that by doing so, further research
in the field will be stimulated. In our understanding, comparative
hydrology should develop into a basically analytical science.
The heavy descriptive content in the late sections of this
textbook should therefore be accepted as an infant disease, as few
analytical studies stressing similarities and differences between
hydrological zones are yet available.

The present topic builds on the comparative hydrology
approach of Falkenmark and Chapman, and attempts to
do so in a quantitative way to generalise beyond individual
catchments.
One of the strengths of comparative hydrology is that it
allows the examination of processes in a more holistic way
than does normal modelling. In a model, only those pro-
cesses and scales actually represented in the model can be
analysed, while in the comparative hydrology approach we
can see the summary effect and interplay of all relevant
processes if the data from the catchments of contrasting
characteristics are compared. Also, the comparative
hydrology approach provides an opportunity to exploit
multiple development histories. Different catchments have
evolved in different ways as a result of different climates
and geologies and that historical legacy is apparent at one
time in many places. This concept can be illustrated by the
example of the medical doctor in Figure 1.5 . Instead of
dissecting each patient to look inside the body for the cause
of a reported ailment, the doctor may choose to look
around the world to see the case histories of a larger
population of people with similar ailments before prescrib-
ing a treatment.
The comparative approach used for generalisation can be
deemed a Darwinian approach. Charles Darwin conducted a
comparative analysis of wildlife and fossils that he had
collected during his world trip, and came up with the
principle of natural selection by being able to generalise
the patterns he saw in the record he assembled. As Sivapalan
et al.( 2011a , p. 5) put it in a hydrological context:

s myriad experiments. Each catchment
represents a sample, a distinct outcome, one of an infin-
ite variety, but resulting from a combination of the same
co-evolutionary earth system processes, and underpinned
by common, yet unknown, organising principles:
water flow processes, land-forming processes and life-
sustaining processes. But these same organising prin-
ciples may manifest themselves in different ways in
different climates and geologies, so they may look ran-
domly different. The comparative hydrology approach
may be a useful framework to study these apparently
random (or unique) catchments as a way to develop
common understanding.
Just as a jigsaw puzzle looks random at first, once it
starts to fall into place it begins to reveal interesting pat-
terns and indeed connections. In other words, the goal of
comparative hydrology is to ultimately bring order into
what otherwise looks disordered, to find new connections
where none existed, and it will therefore be the hallmark of
the synthesis we propose. The comparative hydrology
approach will bring order into a diversity of hydrological
processes, just the way Darwin found order amongst other-
wise different species, or the way the periodic table of
chemical elements brought order to a seemingly unrelated
collection of elements. The comparative hydrology
approach will bring order into the diversity across places,

'