A synthesis framework for runoff prediction in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 18

Figure 2.7. Runoff generation and surface water groundwater interactions under typical arid and humid conditions. After Erhard-Cassegrain and

Margat ( 1979 ) in Falkenmark and Chapman ( 1989 ). Photos: (left) O. Dahan, (right) P. Haas.

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as it will help find patterns along spatial gradients of
climate and landscape characteristics. And the comparative
hydrology approach will bring order into the diversity
across scales as the land surface is organised into catch-
ments of all sizes, nested within each other, and different
properties may then emerge at different scales that can be
studied, interpreted and explained by the approach. Bro-
nowski ( 1956 , p. 23) brilliantly described this natural,
otherwise normal, scientific process in the following
words:

All science is the search for unity in hidden likenesses The

progress of science is the discovery at each step of a new order
which gives unity to what had long seemed unlike

…

For order
does not display itself of itself; if it can be said to be there at
all, it is not there for the mere looking

…

Figure 2.8. Prediction of runoff signatures in ungauged basins
through climate, catchment and runoff similarity (the numbers in the
box refer to the various chapters of this topic).

order must be discovered
and, in a deep sense, it must be created. What we see, as we
see it, is mere disorder.

…

infiltration (and which is highly episodic) down to a deep
aquifer. Part of the stream reaches will be losing reaches,
with flow in the river infiltrating through the river bed to
recharge the underlying aquifer. In contrast, in a humid
catchment precipitation will be higher, and infiltration will
be less episodic. Part of the stream reaches will be gaining
reaches, where the groundwater recharges the streamflow.
Hydrological similarity in terms of similarity of pro-
cesses is difficult to identify in a real-world setting as only
partial knowledge of the hydrological processes is avail-
able. Based on the rationale that runoff is the result of the
interplay of climate and catchment processes, one can
therefore split up the more generic similarity into runoff
similarity, climate similarity and catchment similarity
( Figure 2.8 ). The comparative hydrology approach then

2.2.2 Hydrological similarity
The success of the comparative hydrology approach hinges
on the concepts of similarity and dissimilarity. If one
compares many catchments, some of them will appear
more similar with respect to a particular characteristic
and this similarity will guide the interpretation of the
different emergent patterns. Catchments can be considered
hydrologically similar, in a general way, if they filter
climate variability in similar fashion, as expressed by their
(scaled) hydrological signatures. This similarity may be
brought about by similar trajectories of co-evolution of
climate, vegetation, soils and landscape. The concept of
similarity and dissimilarity of processes is illustrated in
Figure 2.7 . In arid catchments, there is relatively little
precipitation, much of which evaporates, and there is little