Geography Reference
In-Depth Information
Figure 10.2. Comparison of the Vermilion catchment in Illinois (3341 km²) dominated by short flow paths due to tile drainage (top) and the
Gurk catchment in Austria (230 km²) dominated by long flow paths in deeply weathered bedrock (bottom). Photos: B. Rhoads, Creative
Commons License.
10.2.1 Processes
Runoff variability across the landscape arises from inter-
actions between the spatio-temporal variability of precipi-
tation and the spatial heterogeneity of soils, topography,
vegetation and the stream network morphology. As dis-
cussed in detail in Chapter 4 , the processes affecting runoff
variability are physical (e.g., open channel flow), chemical
(e.g., soil cracking and associated changes in infiltration)
and biological (e.g., transpiration by plants and soil disrup-
tion by animals such as earth worms).
Runoff is generated by rainfall or melting snow. Runoff
generation varies temporally and spatially due to the inter-
play of wetting and drying phases. During the wetting
phase, runoff at the hillslopes may be generated by various
mechanisms ( Figure 10.3 ): infiltration excess runoff (or
Hortonian overland flow) occurs when the rate of rainfall
on a surface exceeds the rate at which water can infiltrate
the ground. In Figure 10.3a the surface ponding is due to
soil compaction; saturation excess runoff occurs in areas
with shallow water tables when the soil is saturated and
the depression storage filled, and rain continues to fall.
In Figure 10.3b the water table is close to the surface
due to the vicinity of a stream; and subsurface storm
flow occurs when water infiltrated on an up-slope portion
of the catchment exfiltrates closer to the channel as is
the case in Figure 10.3c . There is a diversity of more
complex phenomena such as runon infiltration where
surface runoff generated on an up-slope portion of the
catchment infiltrates closer to the channel ( Figure 10.3d ).
All of these processes at the hillslope scale depend on the
soil physical characteristics (e.g., hydraulic conductivity),
the characteristics of macropores, the layering of the
subsurface (e.g., soil depth) and the evolution of the soil
characteristics along the hillslope (catena). Equally impor-
tant, the soil moisture state controls all of these phenomena
in a number of ways, e.g., through the occurrence of
shallow water tables and the activation of fast flow paths.
During the drying phase, the amount of runoff may be
reduced in a number of possible ways. Part of the water
may evaporate from the soil surface or transpire from the
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