Environmental Engineering Reference
In-Depth Information
1
At the local scale, how does a given change in
land use or management affect local-scale runoff
generation?
2
How does a local-scale effect propagate down-
stream, and how do many different local-scale
effects combine to affect the flood hydrograph at
larger catchment scales?
3
How can adverse effects be mitigated using
economically and environmentally acceptable
measures?
their saturated hydraulic conductivity, land
management can significantly affect the local
generation of surface and subsurface runoff. Man-
agement practices that cause soil compaction at
the surface reduce the infiltration capacity of the
soil and can lead to infiltration-excess runoff.
Similarly, practices that leave weakly structured
soilswith little or no vegetative cover can also lead
to infiltration-excess runoff, as a result of the rapid
formation of a surface crust with very low mois-
ture storage capacity and hydraulic conductivity.
Practices that cause compaction at the base of
a plough layer can also lead to saturation-excess
surface runoff, and to subsurface runoff by rapid
lateral throughflow in the upper soil layers. Apart
from the soil degradation factors, several other
factors associated with land use and management
can potentially influence runoff generation. For
example, the maintenance of land drains has
declined since the 1980s when subsidies ceased,
and many of these may have become blocked and
do not function effectively.
The landscape within a catchment is a complex
mosaic of elements, all with different responses
and overlain by a range of land management
practices, so there is the key issue of how the
responses of these elements combine to generate
the overall catchment response. As runoff is rout-
ed from the local scale to the catchment scale, the
shape of the flood hydrograph will reflect increas-
ingly the properties of the channel network, such
as its geometry, the slopes and roughnesses of
individual stretches, and attenuation induced by
floodplain storage effects when out-of-bank flood-
ing occurs. However, the magnitude of the flood
peak will also reflect the volume and timing of
runoff from landscape elements delivered into
the channel network, and the extent to which the
timings of the peaks of tributary hydrographs are
in phase or out of phase with the main channel
hydrograph or with each other. This will all vary
as a function of the magnitude of the flood, as
travel times are a function of water depth, and will
depend on the spatial distribution of rainfall over
the catchment.
When considering impact, therefore, the main
questions are:
Evidence for impacts and mitigation
The following is a brief summary of current knowl-
edge about local-scale impacts at the farm plot/
hillslope and catchment-scale impacts. The scope
of the summary is confined primarily to UK stud-
ies, supplemented by overseas studies in temperate
environments with similar land use/management
practices. The findings from the overseas studies
are generally in close agreement with those from
the UK. Further details can be found in O'Connell
et al. (2005, 2007).
Local-scale impacts
Local surface runoff can increase as a result of
a number of modern farm management practices
such as:
.
increased stocking densities on grassland (UK
studies: Heathwaite et al. 1989, 1990; USA: Rauzi
and Smith 1973);
.
the prevalence of autumn-sown cereals
(Belgium: Bielders et al. 2003; UK: Palmer 2003b;
Denmark: Sibbesen et al. 1994);
.
the increase of maize crops (UK: Clements and
Donaldson 2002; Netherlands: Kwaad
and
Mulligen 1991);
.
the production of fine seedbeds (UK: Edwards
et al. 1994; Speirs and Frost 1985);
.
trafficking on wet soils (UK: Davies et al. 1973;
France: Papy and Douyer 1991; USA: Young and
Voorhees 1982).
There does not appear to be a strong link with
soil type, but sandy, silty and slowly permeable
seasonally wet soils are more susceptible than
others. Reduced infiltration and increased surface
