Soil and Hillslope (Eco)Hydrology - Environmental Modelling: Finding Simplicity in Complexity - page 160

et al . (2007) also make specific mention of the linkages
between ecological and hydrological processes.
McDonnell et al . (2007) note a prevailing concern
among hydrologists - 'The more we explore, the more
heterogeneous and complex [ sic ] nature appears to
be' - but differ from Hopp and McDonnell (2009) in
that they suggest that the appeal to uniqueness (or
'complicatedness') is misguided. They argue that we
should develop new hydrological theory and models
based around our observations of pattern. There is also
explicit recognition by the authors that complexity may
have relatively simple explanations (so complexity may
not be that complicated). Yet, despite this recognition,
eco-hydrological feedbacks are not discussed in any
detail. Indeed, while mention is made of ecological
optimality theory - which may be alluding to the contro-
versial idea that ecohydrological systems evolve so that
water use by plants is minimized (see Eagleson, 2002, and
the subsequent evaluation by Kerkhoff et al ., 2004) - the
authors seem to suggest that patterns of drainage within
hillslopes may arise from a tendency to minimize the
work done in moving water from the hillslope to its base.
A 'least-work hillslope' may not evolve in the presence
of plants and other soil organisms; ecohydrological
structures might develop that impede water flow through
and over the hillslope (Couwenberg and Josten, 2005),
and imposing targets or constraints on how a hillslope
may evolve is akin to engineering a hillslope to have
certain properties which is at odds with the idea that
patterns may emerge from interactions between physical
and biological processes across a range of scales.
Hopp et al . (2009) discuss a proposed hardware-
modelling experiment at the Biosphere 2 facility in
Arizona designed to elucidate some key features of
hillslope hydrological behaviour. The rationale for the
experiment recognizes the importance of describing
ecohydrological pattern and of finding explanations
for pattern formation and maintenance. In addition,
the authors are explicit about the sorts of research
problems that need to be addressed, and identify two
broad questions: 'How does water move through the
landscape to streams?' and 'How does vegetation affect
these flowpaths?' They also list some more specific and
more obviously-ecohydrological questions such as:

The Biosphere 2 Hillslope Experiment will undoubt-
edly produce valuable information on hillslopes as
ecohydrological entities, and the thinking behind the
experiment reflects in many ways the content of this
chapter. However, it is odd that the authors also
ask: 'When does the heterogeneity introduced by the
vegetation and weathering processes disable our ability
to predict the water and energy balances?'
By investigating pattern and its causes we should be
better placed to predict hillslope hydrological response.
It is also argued in this chapter that field observation
and computer modelling can complement the hardware-
modelling approach discussed by Hopp et al . (2009).
Notwithstanding Hopp et al . (2009), relatively few
hydrologists seek to describe pattern and very few indeed
attempt to explain it. The converse is true of landscape
ecologists who, over the last 10-20 years, have increas-
ingly looked at the types of vegetation patterns that occur
on hillslopes and the processes that might explain these
patterns. The type of patterning that occurs may depend
on a range of factors including the type of vegetation
present, climate and water availability, and slope shape
and gradient. Models and field-based studies have been
used to improve understanding of vegetation patterning
due to burning ( cf . Peterson, 2002), due to growth cycles
of the dominant plant species comprising the hillslope
vegetation ( cf . Hendry and McGlade, 1995), and due
to ecohydrological interactions between the vegetation
and the soil (e.g. Rietkerk et al ., 2002; van de Kop-
pel and Rietkerk, 2004; Couwenberg and Joosten, 2005;
Mueller et al ., 2007, 2008). Models used to simulate pat-
tern development range from simple cellular automata
(CA) (e.g. Peterson, 2002) to more complete descriptions
based on plant growth and soil processes (e.g. Rietkerk
et al ., 2002). Some of these vegetation patterning mod-
els may be regarded as naıve in that they ignore the
interrelationship between vegetation dynamics and soil
hydro-physical properties. For example, Peterson (2002)
presents a simple fire model in which vegetation patterns
arise solely from interactions between fire and a prob-
abilistic function of time since a fire last occurred in a
model cell, the latter representing the accumulation of fuel
(as biomass) in a cell due to vegetation growth over time.
Although the purpose of the work reported in Peterson's
(2002) paper was not to look at a real ecosystem - his
interest was in the effect of 'ecological memory' (see
Section 10.4 below) on pattern dynamics - a vegetation-
pattern model that considers fire might also benefit from
a consideration of how fire affects soil hydrological prop-
erties such as infiltration. Fire can cause soils to become

How will the structure of the soil change over time
due to geochemical and biological factors? How will
these changes in turn alter the hydraulic properties and
flow pathways through the subsurface (including the
formation of preferential flow networks)?