Watershed scale complexity and multiple modes of adjustment: One difficulty in
describing water and matter related processes at the catchment scale arises from the
complex and non-linear nature of watersheds (Table 3.1). Ecosystems at the
watershed scale often contain many interfering variables – or to use a statistical
term – too many degrees of freedom. This means that given similar initial
conditions and process dynamics, there are multiple states to which an ecosystem
may adjust (Phillips 2003). From the field researchers point of view, the same
system output information can be evoked by different process combinations and
intensities, and as a result, physically based and spatially distributed models that
aim to describe detailed water and matter flows at the watershed scale often fail
(Beven 2001) when tested for site to site transferability or tested against the
knowledge of the field researcher (Seibert and McDonnell 2002; Vache ´ and
McDonnell 2006). The ability of watersheds to adjust into multiple possible system
states given the same processes often ends up with virtually contradictory results
when comparing sites, scales and different studies, without knowledge of how hill
slope scale processes are integrated to form the watershed scale response. This has
the potential to impair sustainable resource management strategies and consequently calls for further research to be undertaken to understand the scaling and
connectivity of processes – from field plots to slopes and watersheds. Furthermore,
non-linear mechanisms need a focus, such as the threshold behavior of landslide
initiation (Ziegler et al. 2009), or time lags in the response of watershed scale
sediment delivery to hill slope scale soil erosion or soil conservation measures
(Chappell 1983; Bruijnzeel 2004).
Validity limits of process domains: Another issue complicating the discussion of
water management strategies is that of concepts, those that picture complex process
combinations. Among these, the sponge concept (Calder 2002) states that tree roots,
forest litter and undisturbed soil under the forest are responsible for high water
infiltration rates that, in effect, can cause a more sustained response in stream
discharge (Table 3.2). In contrast, increased soil disturbance and soil compaction
(leading to an absence of the sponge effect), clearly increases the speed of water
flow into streams. While this concept has a physical basis, the range and scale of
conditions to which it applies need to be taken into account. During extreme flood
events, with soils already wetted, other physiographic parameters govern the timing
and amount of water discharge from a watershed. Among these are the water
storage capacities of soils (Bruijnzeel 2004), the stream channel geometry (Ziegler
et al. 2009) and with increasing scale, also the routing of flood waves through the
river network, including reservoir management (see Sect. 3.3).
Suitability of proxies for process domains: A second example of the difficulty to
be found in describing process domains is based on a process combination similar to
top soil erosion from permanent arable land, a scenario which can also be found
within mountainous deciduous forests. These forests are defoliated at the onset of
the monsoon season, which often starts with highly erosive rainfall events. At this
time, soils below the defoliated canopies have no or reduced ground cover, made
worse when soil surface litter layers and the understorey have also been removed by
human activities. This situation can also lead to severe erosion under forest cover
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H.L. Fro ¨hlich et al.
describing water and matter related processes at the catchment scale arises from the
complex and non-linear nature of watersheds (Table 3.1). Ecosystems at the
watershed scale often contain many interfering variables – or to use a statistical
term – too many degrees of freedom. This means that given similar initial
conditions and process dynamics, there are multiple states to which an ecosystem
may adjust (Phillips 2003). From the field researchers point of view, the same
system output information can be evoked by different process combinations and
intensities, and as a result, physically based and spatially distributed models that
aim to describe detailed water and matter flows at the watershed scale often fail
(Beven 2001) when tested for site to site transferability or tested against the
knowledge of the field researcher (Seibert and McDonnell 2002; Vache ´ and
McDonnell 2006). The ability of watersheds to adjust into multiple possible system
states given the same processes often ends up with virtually contradictory results
when comparing sites, scales and different studies, without knowledge of how hill
slope scale processes are integrated to form the watershed scale response. This has
the potential to impair sustainable resource management strategies and consequently calls for further research to be undertaken to understand the scaling and
connectivity of processes – from field plots to slopes and watersheds. Furthermore,
non-linear mechanisms need a focus, such as the threshold behavior of landslide
initiation (Ziegler et al. 2009), or time lags in the response of watershed scale
sediment delivery to hill slope scale soil erosion or soil conservation measures
(Chappell 1983; Bruijnzeel 2004).
Validity limits of process domains: Another issue complicating the discussion of
water management strategies is that of concepts, those that picture complex process
combinations. Among these, the sponge concept (Calder 2002) states that tree roots,
forest litter and undisturbed soil under the forest are responsible for high water
infiltration rates that, in effect, can cause a more sustained response in stream
discharge (Table 3.2). In contrast, increased soil disturbance and soil compaction
(leading to an absence of the sponge effect), clearly increases the speed of water
flow into streams. While this concept has a physical basis, the range and scale of
conditions to which it applies need to be taken into account. During extreme flood
events, with soils already wetted, other physiographic parameters govern the timing
and amount of water discharge from a watershed. Among these are the water
storage capacities of soils (Bruijnzeel 2004), the stream channel geometry (Ziegler
et al. 2009) and with increasing scale, also the routing of flood waves through the
river network, including reservoir management (see Sect. 3.3).
Suitability of proxies for process domains: A second example of the difficulty to
be found in describing process domains is based on a process combination similar to
top soil erosion from permanent arable land, a scenario which can also be found
within mountainous deciduous forests. These forests are defoliated at the onset of
the monsoon season, which often starts with highly erosive rainfall events. At this
time, soils below the defoliated canopies have no or reduced ground cover, made
worse when soil surface litter layers and the understorey have also been removed by
human activities. This situation can also lead to severe erosion under forest cover
114
H.L. Fro ¨hlich et al.