Geology Reference
In-Depth Information
surface residue, surface roughness, ridge height,
and the depth distribution of buried residue and
soil biomass. Some of these, like standing stubble
and ridge height, are variables that did not exist
as USLE or RUSLE1 subfactors, but even the more
familiar variables have received new and more
detailed treatment in RUSLE2. In addition to sur-
face and standing residue, RUSLE2 tracks dead
biomass in 24 2.5-cm-thick soil layers in the soil
profile. By default, standing residue decays at a
rate that is a fraction of that of the surface resi-
due, buried residue, or dead roots, which all decay
at a rate controlled by climatic and residue vari-
ables using the same relationships as in RUSLE1.
Mechanical tillage operations are described much
more fully in RUSLE2 database files than in
RUSLE1, in terms of the impact they have on
flattening standing residues, disturbing the soil,
or affecting the growth of vegetation. Soil distur-
bance is described in terms of the fraction of the
soil disturbed, the intensity and depth of soil dis-
turbance, the creation of ridges and random
roughness, and the effect on burying, redistribut-
ing, or re-surfacing residues.
In a vegetation description, users define the
base crop yield, the time course of canopy and
root mass development (a 'growth chart'), and the
characteristics of the residue produced when the
crop dies. RUSLE2 uses this information once a
'begin growth' operation in a management
description calls for that vegetation. The growth
of the vegetation in RUSLE2 is independent of
the location's climate data, so it must be properly
described by the user for the situation being ana-
lysed. Several 'wizards' are available in the
RUSLE2 interface to help users to develop vegeta-
tion descriptions, to define canopy/biomass rela-
tionships, canopy shape and intercepted raindrop
fall height, and yield/flow retardance relation-
ships. A new portion of the program specifically
designed to help the database developer and pro-
gram user properly to account for residue and root
production in perennial vegetation systems is
being developed, and is discussed subsequently.
One key feature added to the vegetation/opera-
tion/management descriptions in RUSLE2 is the
ability of the user to vary crop yield. Vegetation is
described for a specific assumed base yield, but
when the vegetation is actually used within a
management regime, the user can specify a higher
or lower yield value. The vegetation description
includes how the biomass varies with yield,
allowing adjustment of all of the vegetation
parameters by the program.
(v) Changes to the support practice factor
Whereas the RUSLE1 user selects a cover manage-
ment condition that, together with the soil hydro-
logical group, defines a 'runoff index' analogous to
the runoff curve number (
CN
), RUSLE2 calculates
a
CN
internally as a function of soil hydraulic
class, soil biomass, soil consolidation, soil rough-
ness, and soil residue cover, thus reflecting the
combined effects of soil, management and cli-
mate. RUSLE2 calculates runoff for the
P
10y,24h
rainfall event every day. It also calculates sheet
and rill erosion for this index event, and uses
process-based equations to determine sediment
transport, deposition, and fines enrichment.
'Infiltration' is calculated on slope segments with
a low
CN
as the difference between
P
10y,24h
precipi-
tation depth and the 'initial abstraction', taken as
0.2 times the 'maximum retention' parameter, a
transform of the
CN
(USDA-ARS, 2008a). The
RUSLE2 equations for sediment transport capac-
ity and deposition, and robust simplifications of
the equations used in CREAMS, give RUSLE2 the
ability to reflect the effects of spatial variation of
soil erodibility, slope steepness, and cover man-
agement along a slope on detachment, transport
and deposition. This approach results in estimates
of the long-term average sediment production,
erosion rate, transport capacity, deposition, and
sediment characteristics along the slope, as well
as the sediment amount and characteristics of
sediment leaving the slope (Foster
et al
., 2000). In
fact, RUSLE2 goes further than other 'process-
based' models, in that it approximates backwater
effects when it determines the effectiveness of
dense narrow vegetative buffers on sediment trap-
ping (USDA-ARS, 2008a). RUSLE2 also includes
the ability to approximate the effect of simple
impoundments and channels on sediment deliv-
ery and fines enrichment.
