2.6.2 Disposition to Erosion
Based on the approach of Renard et al. (1997) the rain-run-off erosivity factor (R)
used in the Revised Universal Soil Loss Equation (RUSLE) was estimated from
climatic data for the years 2007–2012. The estimation resulted in figures of
R ¼ 351 for Chieng Khoi and R ¼ 413 for Muong Lum, which is located at higher
altitudes. An analysis of rainfall events in Chieng Khoi for the relatively dry year of
2009 and the relatively wet year of 2010 (Fig. 2.16) revealed that in 2009 there were
4 and in 2010 16 events with between 20 and 30 mm of rain a day, 4 and 5 events
with 40–60 mm of rain a day and 2 and 3 events with more than 60 mm. At the end
of May 2010 there was a 9 day period of rain which produced 212 mm in total.
The areas’ soil properties led to moderate estimates of soil erodibility, with K
factors ranging from 0.14 to 0.31 in Chieng Khoi and 0.17–0.34 in Muong Lum
(Cong 2011); however, soils derived from siltstone, but not from limestone and
clayey shale, frequently showed surface sealing, causing high run-off and erosion
rates. This susceptibility to sealing was due to high silt content in combination with
a lack of agents responsible for stabilizing soil aggregates such as soil organic
matter and exchangeable bivalent cations.
The landscape of the study area is characterized by steep slopes, and both sites
have a high proportion of steep land (>30 % inclination), values being 49 % in
Chieng Khoi and 56 % in Muong Lum (see Table 2.16a). The area of cretaceous
clastic sediments at lower altitudes is characterized by rounded hill tops sitting
150–250 m above the surrounding valleys, while in the limestone areas, the upper
parts of the hills and mountains are very steep, show rock outcrops and are covered
by natural forest. This structure is reflected in the high percentage (40 %) of very
steep slopes (>50 %) in the Muong Lom catchment (Table 2.16a).
Previously, the slopes were under traditional rain-fed stationary slash-and-burn
agriculture, which did not use chemical fertilizers. The principal agricultural
products on the slopes at this time were maize (Zea mays L.) and cassava (Manihot
esculenta Crantz). However, triggered by population increase, resettlement and
market development, land use has intensified over the last decade or so, connected
with a reduction in and finally a demise of slash and burn practices and an increase
in the erosive slope length (the L-factor of USLE). Common soil erosion patterns in
the research area were found to include: (1) Sheet erosion and a dense net of small
rills on hilltops and upper slopes, (2) an increasing depth of the rills on the midslopes, and (3) the development of deep gullies (down to 30 cm in depth) on the
lower slopes – due to the increasing amounts and velocities of run-off during rain
events, with intensities exceeding the infiltration capacity.
The resulting increased erosion rates have been caused, first of all, by a lack of
soil cover at the beginning of the wet season. Erosion measurements on Wischmeier
plots under maize (Fig. 2.16) and on a steep slope (30–38 % – for a detailed
description of the experimental site, see Chap. 7), showed that severe erosion events
happened only until the end of June, although intensive rain storms were observed
until the end of the wet season in September. Erosion reduced after June because
94
K. Stahr et al.
Based on the approach of Renard et al. (1997) the rain-run-off erosivity factor (R)
used in the Revised Universal Soil Loss Equation (RUSLE) was estimated from
climatic data for the years 2007–2012. The estimation resulted in figures of
R ¼ 351 for Chieng Khoi and R ¼ 413 for Muong Lum, which is located at higher
altitudes. An analysis of rainfall events in Chieng Khoi for the relatively dry year of
2009 and the relatively wet year of 2010 (Fig. 2.16) revealed that in 2009 there were
4 and in 2010 16 events with between 20 and 30 mm of rain a day, 4 and 5 events
with 40–60 mm of rain a day and 2 and 3 events with more than 60 mm. At the end
of May 2010 there was a 9 day period of rain which produced 212 mm in total.
The areas’ soil properties led to moderate estimates of soil erodibility, with K
factors ranging from 0.14 to 0.31 in Chieng Khoi and 0.17–0.34 in Muong Lum
(Cong 2011); however, soils derived from siltstone, but not from limestone and
clayey shale, frequently showed surface sealing, causing high run-off and erosion
rates. This susceptibility to sealing was due to high silt content in combination with
a lack of agents responsible for stabilizing soil aggregates such as soil organic
matter and exchangeable bivalent cations.
The landscape of the study area is characterized by steep slopes, and both sites
have a high proportion of steep land (>30 % inclination), values being 49 % in
Chieng Khoi and 56 % in Muong Lum (see Table 2.16a). The area of cretaceous
clastic sediments at lower altitudes is characterized by rounded hill tops sitting
150–250 m above the surrounding valleys, while in the limestone areas, the upper
parts of the hills and mountains are very steep, show rock outcrops and are covered
by natural forest. This structure is reflected in the high percentage (40 %) of very
steep slopes (>50 %) in the Muong Lom catchment (Table 2.16a).
Previously, the slopes were under traditional rain-fed stationary slash-and-burn
agriculture, which did not use chemical fertilizers. The principal agricultural
products on the slopes at this time were maize (Zea mays L.) and cassava (Manihot
esculenta Crantz). However, triggered by population increase, resettlement and
market development, land use has intensified over the last decade or so, connected
with a reduction in and finally a demise of slash and burn practices and an increase
in the erosive slope length (the L-factor of USLE). Common soil erosion patterns in
the research area were found to include: (1) Sheet erosion and a dense net of small
rills on hilltops and upper slopes, (2) an increasing depth of the rills on the midslopes, and (3) the development of deep gullies (down to 30 cm in depth) on the
lower slopes – due to the increasing amounts and velocities of run-off during rain
events, with intensities exceeding the infiltration capacity.
The resulting increased erosion rates have been caused, first of all, by a lack of
soil cover at the beginning of the wet season. Erosion measurements on Wischmeier
plots under maize (Fig. 2.16) and on a steep slope (30–38 % – for a detailed
description of the experimental site, see Chap. 7), showed that severe erosion events
happened only until the end of June, although intensive rain storms were observed
until the end of the wet season in September. Erosion reduced after June because
94
K. Stahr et al.