Process realism: flow paths and storage - Runoff Prediction in Ungauged Basins: Synthesis across Processes, Places and Scales - page 63

Table 4.1. Correlations between mean transit time (MTT) and various catchment characteristics. Most important control
in a region highlighted in bold italics

Catchment characteristics R² Location Study

Topographic aspect 0.28 Arizona Broxton et al.( 2009 )

Flow paths length/gradient 0.30 Europe, USA Tetzlaff et al.( 2009b )

Flow path gradient 0.08 Europe, USA Tetzlaff et al.( 2009b )

Bedrock infiltration 0.85 Japan Katsuyama et al.( 2010 )

Flow paths length 0.98 New Zealand Dunn et al.( 2007 )

Distance from divide 0.50 New Zealand Vaché and McDonnell ( 2006 )

Topographic wetness index 0.92 Oregon Tetzlaff et al.( 2009b )

Flow paths length/gradient 0.91 Oregon McGuire et al.( 2005 )

Upslope area 0.88 Oregon Tetzlaff et al.( 2009b )

Flow paths length 0.72 Oregon McGuire et al.( 2005 )

Flow path gradient 0.65 Oregon McGuire et al.( 2005 )

Distance from stream 0.62 Oregon Tetzlaff et al.( 2009b )

Proportion responsive soil cover 0.94 Scotland Soulsby et al.( 2006 )

Downslope index gradient 0.83 Scotland Tetzlaff et al.( 2009b )

Proportion responsive soil cover 0.80 Scotland Hrachowitz et al.( 2009 )

Proportion responsive soil cover 0.76 Scotland Tetzlaff et al.( 2009a )

Flow paths length/gradient 0.74 Scotland Tetzlaff et al.( 2009b )

Flow path gradient 0.67 Scotland Soulsby and Tetzlaff ( 2008 )

Proportion responsive soil cover 0.60 Scotland Soulsby and Tetzlaff ( 2008 )

Drainage density 0.59 Scotland Hrachowitz et al.( 2009 )

Flow path gradient 0.42 Scotland Tetzlaff et al.( 2009a )

Precipitation 0.25 Scotland Hrachowitz et al.( 2009 )

Proportion of wetlands 0.59 Sweden Lyon et al.( 2010 )

Flow path gradient 0.52 Sweden Lyon et al.( 2010 )

Flow paths length/gradient 0.43 Sweden Lyon et al.( 2010 )

Peclet number 0.40 Sweden Lyon et al.( 2010 )

Flow paths length/gradient 0.32 Sweden Tetzlaff et al.( 2009b )

areas of Arizona, Broxton et al.( 2009 ) pointed out that the
south-facing slopes consistently accommodated faster
responding flow paths due to the more rapid snowmelt as
compared to the north-facing slopes. It is possible that this
effect is also related to higher biomass production on the
south-facing slopes as a result of the co-evolution of soil
structure and vegetation. In the northern hemisphere,
south-facing slopes offer more direct radiation input,
which favours biomass production, root growth and litter-
fall-feeding small organisms, which altogether is condu-
cive for the development of preferential pathways.
In a comparison of catchments from Europe and North
America, Tetzlaff et al.( 2009b ) found that transit times
tended to be lower in the steepest catchments. In flatter
areas topographic control weakened, in particular where
less permeable soils gave rise to overland flow and lower
transit times. Katsuyama et al.( 2010 ), studying the Kiryu
catchment in Japan, which is underlain by saprolite, argued
that bedrock permeability and groundwater dynamics are
first-order controls on the dominant flow pathways. In
northern Sweden, where mire wetlands are an important

part of the landscape, Lyon et al.( 2010 ) found that transit
times decrease with the proportion of wetlands. Table 4.1
summarises a number of comparative studies on transit
times organised by region. The table highlights the role
of topography in Oregon and New Zealand and the role of
responsive soils in Scotland. An example of MTTs plotted
against catchment characteristics is shown in Figure 4.10 .
Few studies have related MTT to climate and/or wet-
ness conditions. This, however, is crucial for realistically
representing processes that are active at multiple time
scales, depending on catchment wetness (e.g., Hrachowitz
et al., 2010 ). For example, Heidbüchel et al.( 2012 ) found
MTTs of 360 days in a semi-arid catchment in Arizona
contrasting the 144 days found in a humid catchment in
Switzerland. The longer transit times in the semi-arid
catchment are due to the very long dry periods in spring
and autumn when runoff is mainly derived from ground-
water storage, while the humid catchment receives rainfall
throughout the year, reflecting catchment water storage
that is more often closer to full capacity, leading to shorter
transit times.