Geology Reference
In-Depth Information
19
Gully Erosion: Procedures
to Adopt When Modelling Soil Erosion
in Landscapes Affected by Gullying
J.W.A. POESEN
1
, D.B. TORRI
2
AND T. VANWALLEGHEM
3
1
Department of Earth and Environmental Sciences, Katholieke Universiteit Leuven,
GEO-Institute, Celestijnenlaan, Heverlee, Belgium
2
IRPI CNR, Perugia, Italy
3
Department of Agronomy, Institute for Sustainable Agriculture - CSIC,
Finca Alameda del Obispo, Córdoba, Spain
19.1
Why Model Gully Erosion?
Field-based evidence suggests that soil losses from
sheet and rill erosion as measured on runoff plots
or as predicted using most water erosion models
are not realistic indicators of total catchment ero-
sion rates or sediment yield, nor do they indicate
satisfactorily the sources and redistribution of
eroded soil within a catchment (de Vente & Poesen,
2005). Yet soil losses caused by gully erosion have
rarely been accounted for in soil loss assessment
programmes (Poesen
et al
., 2003).
The development of gully channels on-site
causes a significant decrease in soil quality
through very high soil losses (i.e. 10-100 t ha
−1
y
−1
;
Poesen
et al
., 2002) and through the enhanced
drainage and desiccation of the inter-gully areas
which in dry environments may lead to limited
soil water availability and significant crop yield
reductions. In addition, the erosion channels
lower the trafficability of the land, inducing an
extra economic cost for farmers and residents.
Gully erosion often represents a major source
of sediment. Area-specific sediment yield in
Mediterranean environments differs on average
one order of magnitude when comparing non-
gullied with intensively gullied catchments
(Poesen
et al
., 2002). Typically, gully channels
occupy less than 5% of a catchment area, but
their contribution to total catchment sediment
yield is well above this percentage: i.e. from 10%
Most research dealing with soil erosion by water
has focused on sheet (inter-rill) and rill erosion
processes operating at the runoff plot scale. This
can be concluded from two facts: (1) the many run-
off plot studies reporting soil loss rates caused by
sheet and rill erosion for various climatic and land
use conditions; and (2) the use of both empirical
and process-based field-scale and catchment-scale
erosion models addressing mainly sheet and rill
erosion for predicting soil erosion rates under envi-
ronmental change or for establishing soil erosion
risk maps at various scales (see other chapters in
this topic). In Europe, for instance, more than 2200
plot-years of data on annual soil loss by sheet and
rill erosion have been published over the last dec-
ades (Cerdan
et al
., 2006), whereas during the same
period less then 50 gully-years of data on annual
soil loss by gully erosion have been reported
(Poesen
et al
., 2006). However, in many landscapes
and for a range of climatic and land-use conditions,
one can observe the presence and dynamics of vari-
ous types of gullies: ephemeral gullies, permanent
or classical gullies, and bank or edge-of-field gullies.
