Prediction of low flows in ungauged basins - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 181

Figure 8.14. Susceptibility of rocks
to springs estimated by
hydrogeological factors in a
mountain region in Emilia Romagna,
Italy. The susceptibility can be used
for estimating low flows in ungauged
basins. After Cervi et al. ( 2007 ).

provide the hydroclimatic context. Field surveys help to
identify the physical catchment structure that controls the
spatio-temporal patterns of low flows. Synoptic runoff
measurements (i.e., spot measurements at several locations
along the stream network in a short time interval) can be
very valuable for determining the main sources of flow
during low flow periods. Among other things, these
methods can be used for testing distributed hydrological
models (e.g., Engeland et al., 2002 ). In dry climates, vege-
tation on the valley bottom can be a valuable proxy for low
flow characteristics. Lowering the water table and/or redu-
cing overbank flooding may result in changes in the dens-
ity, productivity and species composition of wetland and
riparian vegetation (Smakhtin, 2001 ). Johnson ( 1998 )
showed, for instance, that changes of up to 25% can result
from adding or removing forestry. An overview of the
effect of changes in the vegetation on low flows in humid
regions is given in Price ( 2011 ).
Another type of proxy information for predicting low
flows is the spatial distribution of springs, which is closely
linked to the catchment

developed a method of identifying zones of susceptibility
to springs based on a number of geological factors, includ-
ing hydrogeological facies, surface deposits and distance
from faults. They identified different classes for each of
these factors by analysing existing geological maps, aerial
photographs, field surveys and additional permeability
measurements. They then related these factors to the loca-
tion of springs by a Bayesian method, which allowed them
to map the susceptibility zones ( Figure 8.14 ). The zones of
highest susceptibility are located mostly where the perme-
able flysch overlies clay formations. The position of the
springs is determined by the tectonics that has caused the
overlay of hydrogeological structures with different per-
meabilities. The superficial deposits, however, are less
important, but springs occur where lithotypes of contrast-
ing characteristics are in contact.

8.5 Comparative assessment
The aim of the comparative assessment of low flow pre-
dictions in ungauged basins is to learn from the similarities
and differences between catchments in different places,
and to interpret the differences in performance in terms
of the underlying climate

s geology. There is often a direct
correlation between the spatial arrangement of geological
units in a basin and low flow runoff (Rogers and Armbrus-
ter, 1990 ; Gustard et al., 1987 ; Musiake et al., 1984 ). Cervi
et al.( 2007 ) and Cervi ( 2009 ) proposed using the spatial
distribution of perennial springs to assist in the estimation
of

'

landscape controls. Understand-
ing these controls sheds light on the nature of catchments
as complex systems and provides guidance on the methods

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low flows in ungauged mountain basins. They