Modelling Catchment and Fluvial Processes and their Interactions - Environmental Modelling: Finding Simplicity in Complexity - page 175

11

Modelling Catchment and
Fluvial Processes and their
Interactions

Mark Mulligan 1 and John Wainwright 2
1 Department of Geography, King's College London, UK
2 Department of Geography, Durham University, UK

evapotranspiration). This local balance controls soil
moisture and the partitioning of the resulting available
water between more rapid surface pathways for flow
such as runoff and slower subsurface pathways such
as throughflow (interflow in the US) or groundwater
recharge. In addition, one has to understand the terrain-
(geomorphology) and geology-controlled topological
network for connectivity of these response units such
that the propagation of water between them can be
understood. This propagation may involve amplification
or diminution of flows depending upon the characteris-
tics of the connected HRUs. Finally, the interaction with
rapid flows provided by permanent channels is added as
a further layer of complexity that has a two-way 'com-
munication' with hillslopes. The interaction of spatially
varying climatic and human impacts must also be taken
into account. Thus, the spatial complexity of catchment
hydrology can be significant, not to mention the fact
that many of these factors change also in time, at scales
from minutes (for the meteorological) to hundreds of
thousands of years (for the topographic and topological).
The types of questions asked of hydrologists at the
catchment scale are diverse. Leaving those indirectly
related to hydrology aside (such as the impact of land-use
change on soil and nutrient erosion, the transport
of point source and non-point source contaminants,
the extent and distribution of salinization, variations
and change in aquatic or wetland environments and

11.1 Introduction: connectivity in
hydrology

In Chapter 10, Baird outlines the complexities of water
flow in soils and hillslopes and the importance of their spa-
tial variability. This chapter takes a landscape view of water
movement and examines the complexity of aggregation
of hillslope processes into nested catchments connected
by stretches of flowing water and the interaction between
hillslopes and channels. We will examine the complex-
ity of catchments (also termed drainage basins and river
basins) in process, time and space and highlight some of
the simple rules that determine their behaviour. Hydro-
logical and computational definitions of catchments will
be used as the basis for discussing the range of models
applied for hydrological modelling at this scale. We then
present some of the state-of-the-art and future directions
in catchment modelling before identifying some of the
gaps in our understanding of catchments and the some
of the simplicities that have emerged from the modelling
of complex catchments.
Studying the hydrology of catchments involves
an understanding of the water balance of individual
hydrological response units (HRUs, Fl ugel, 1995) or
hydrologically similar surfaces (HYSS, Kirkby et al .,
2002). The water balance is controlled by the inter-
action of climate, vegetation, terrain and soils, which
determine the local water balance (precipitation minus