Geography Reference
In-Depth Information
Figure 11.51. A Mediterranean
vineyard landscape with
(a) discontinuities (artificial channels
and field mosaic of different land
use), (b) low flows in an ephemeral
stream and (c) non-aquatic
vegetation in a dry stream.
flooding events, newer and more significant causes of
water pollution, and greater pressure on the scarce water
resources.
In these regions most of the streams are ephemeral as
they are dry throughout most of the year and generate flow
only during brief periods after rainfall. The main reason for
this is that rainfall events exhibit high intensities and short
durations inducing Hortonian overland flows. Ephemeral
streams provide a number of interesting problems for
hydrologists anxious to answer questions relating to pre-
diction in ungauged catchments and determination of
catchment threshold behaviour (Zehe and Sivapalan,
2009 ), and thus give significant insight into catchment
functioning and hydrological process understanding (Siva-
palan et al., 2003b ). Owing to the lack of hydrological
information on such catchments, estimating and predicting
runoff is a significant challenge.
In agricultural landscapes, questions arise regarding
the occurrence and magnitude of ephemeral streams,
and the discontinuities caused by human activities.
Such landscapes and associated ephemeral streams are
shown in Figure 11.51 . Temporal discontinuities may
be induced by agricultural practices such as tillage,
while spatial discontinuities are created by land use
and roads patterns or infrastructure such as reservoirs
and channels. These discontinuities may largely impact
runoff coefficients (Moussa et al., 2002 ) and the con-
nectivity of water pathways (Dickinson and Whiteley,
1970 ).
The headwater catchment scale is a key scale for investi-
gating these questions because hydrological processes and
landscape management are jointly identifiable (Colin et al.,
2012 ). At
10 000 m 2 ), agricultural
organisation, landscape patterns and hydrological processes
are incompletely represented, while at the catchment scale
(
the plot scale (100
-
100 km 2 ) hydrological processes are smoothed and a
detailed landscape description is difficult. Hence, the spatial
variability of the geographic properties is easier to qualify
exhaustively on small catchments (1 km 2 ) and therefore
easier to relate to hydrological processes (Wood et al.,
1988 ). However, at this scale, all catchments could be
considered as ungauged except for few experimental ones.
The use of hydrological signatures (such as runoff
occurrence frequency, runoff coefficient magnitude, water
balance) to compare and classify catchments (Wagener
et al., 2007 ; Sivapalan, 2005 ) appears as a first step in
choosing appropriate models for poorly understood
hydrological systems (McDonnell and Woods, 2004 ;
Wagener et al., 2005 , 2007 ). The catchment behaviour
signature, defined as a set of indicators related to the main
hydrological processes, is a good way to appreciate simi-
larity and/or dissimilarity between catchment hydrological
functioning. These are the basis for hydrological behav-
iour diagnosis and allow the first propositions for correct-
ive actions. Several approaches have been developed
for estimating hydrological indicators and thus modelling
catchment behaviour. One of
>
them is based on the
'
gauging the ungauged catchments
'
concept
(Barthold
Search WWH ::




Custom Search