Introduction - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 8

1.5.2 Synthesis across places
Overcoming the fragmentation across places is particularly
difficult as catchments are indeed tremendously different.
The approach adopted in this topic to synthesise across
places is built on the notion of

path towards synthesis is to resort to the case histories of
thousands of catchments around the world to make a
prediction for a particular catchment. Similarity is the
foundation of this synthesis, and is therefore a key theme
of the topic.
Hydrological similarity will help bring order to the
current cacophony of catchment processes, models and
data setups that bedevil the science today. Similarity is
therefore the natural vehicle to organise the synthesis so
as to assist towards holistic understanding of hydrological
processes everywhere. Hydrological similarity will also
help runoff prediction in ungauged basins, since it can help
exploit the knowledge of hydrological processes at various
levels of detail. To assist with predictions we therefore
need to learn from what everybody has learned around
the world, and pool together the wisdom and experience
from many countries and the diversity of approaches. The
concept of similarity makes different places comparable,
and in this way assists in the generalisation of the under-
standing gained from one catchment to the collective
understanding of how different catchments function under
different conditions.
It is recognised in this topic that a critically important
part of synthesis across places is therefore a comparative
assessment of how well different methods for runoff pre-
dictions work in ungauged basins. A consistent and coher-
ent assessment of the performance of methods is therefore
another of the cornerstones of this topic.

. As a central
theme throughout the topic, this notion of hydrological
similarity is used to compare different catchments and
landscape units, to learn from their similarities and differ-
ences. We look at different places at the same time. Again,
the analogy with the medical doctor is appropriate here.
The medical profession has two options to understand a
patient

' similarity '

'

s medical condition, how the condition can be
inferred from particular symptoms, to predict the future
evolution of that person

s health and to decide on any
treatment. The first option is to look at this particular
patient in much detail, including biopsy or surgery, to
identify exactly the root cause of the symptoms. The
second option is to pool the findings from many patients
and to learn from their case histories. The crucial step is
then to transfer the knowledge obtained from the large
group of people to the particular person being treated. Each
human being is different, but there are many common
characteristics. Doctors pool together the information from
many people and analyse the differences and the similar-
ities. How will cancer evolve for a given state of the body?
Clearly, doctors will resort to the case histories of thou-
sands of patients around the world to make a prediction for
that particular patient. The two options are complementary,
and the medical profession has adopted a combination of
these two approaches ever since the profession organised
itself, if not before.
Hydrology could operate in a similar fashion. We could
pool the information on many catchments together, and
analyse their differences and similarities. How will runoff
evolve for a given state of the catchment? Clearly, a viable

'

1.5.3 Synthesis across scales
Hydrological processes occur at all scales, from micro-
scopic water flow in soil pores to global-scale interactions
of soil moisture and climate. Consequently, hydrological
analysis has been performed at many scales, from the

Figure 1.6. Harmonisation of
process understanding and models
through comparative evaluation
across places (taken from PUB
Science Plan, see SSG, 2003 ). The
figure depicts the synthesis
(harmonisation) that can be achieved
through organising the activities and
outcomes along the axes of climate,
scale and methods, which has been
the motivation behind this topic.