Environmental Engineering Reference
In-Depth Information
Department of Agriculture (USDA) to predict sheet and rill erosion. The USLE is given
by:
where
A
= soil loss (kilogrammes per square metre per annum),
R
= rainfall erosivity,
K
= soil erodibility,
L
= slope length,
S
= slope gradient,
C
= cropping and
P
=
conservation practices. Numerical values for each factor can be calculated from standard
tables. USLE predicts only the amount of soil moved, and not the amount of soil moved
out of a field or drainage basin. The latter is estimated by another model, the
sediment
delivery ratio
(SDR). Predictions from SDRs are usually much lower than predictions
from USLEs, because much of the soil moved is deposited downslope in fields, along
fences and hedgerows, as colluvium and as stream alluvium. Where there is severe gully
erosion, however, the reverse can be the case. Reservoirs built for water supply or flood
control can become infilled quickly by eroded soils, and have been used for estimating
erosion rates.
material is uniform and differences between soils in the catena result from different
processes at the surface and in the subsurface along the slope. In the second type one
parent material is superimposed on another, and the slope thus cuts across them both. The
upper part of the catena is on one parent material, and the lower slope on another. Hence
a parent material or geological factor is added to slope effects. In Britain, as in most
countries, both types of catena are well represented.
The soil profile of each member of the catena is related to every other member of that
catena. Hence individual soils are like individual links in a chain. The effect of slope in
giving rise to soils with very different drainage conditions and water content has been
noted already (pp. 401, 403). The sequence of soils formed along a transect from a hill
crest to an adjacent valley bottom is a
hydrological sequence
. An example on an acid
parent material is shown in Figure 19.16. Freely draining soils in the upper part of the
slope have bright, well oxidized colours. Progressively less well drained and increasingly
gleyed soils occupy the lower positions of the catena. A greyish subsoil with ochreous
mottling denotes imperfect drainage. A continuously gleyed horizon with much ochreous
mottling shows poor drainage, whilst a dark peaty surface overlying a blue-grey horizon
reflects very poor drainage. Such sequences are common on uniform parent materials,
especially on glacial tills. In Scotland the Soil Survey maps hydrologic sequences as
soil
associations
when formed on uniform parent materials in the landscape.
The movement of water is the principal reason for the differences in soils downslope
in humid temperate regions. Subsurface lateral flow is more important than overland
flow. Chemical cations and anions will be carried in the water and will thus tend to
accumulate relatively in downslope locations. The increased content of cations
downslope causes a parallel increase in the pH of the mineral horizons below the surface
peat. Often pH can reach 7ยท0 in the Humic Gleysol and the Eutric (i.e. baserich) Histosol.
There is also the increased weathering by hydrolysis because of the increased wetness. If
there are many ferromagnesian minerals in the parent material, weathering of these can
cause exchangeable magnesium to exceed exchangeable calcium in the wetter soils of the
catena. Thus a soil catena often shows a chemical sequence, with more mobile and
