2.3.1 Transport capacity and sediment routing
Transport capacity (TC) is the maximum potential sediment that can exit
down slope of a grid cell per unit length (kg m-1).
For each cell in arable and non-arable land use types, different TC
values were considered and the original TC which considers the transport
capacity as a function of potential rill and interrill erosion was used
in this study (Van Oost et al., 2000; A. J. Van Rompaey et al., 2002; A.
J. Van Rompaey et al., 2001).
TC = KTC * R * K * (LS(i,j) - 4.1SIR) (5)
Where:
TC = Transport capacity (kg m-1 y-1)
KTC = Transport capacity coefficient (m)
SIR = inter-rill slope gradient (mm-1)
KTC describes the proportionality between the potential for rill erosion
and TC. It can be interpreted as the theoretical upslope distance that
is needed to produce enough sediment to reach the TC at the grid cell,
assuming a uniform slope and runoff discharge.
The inter rill slope gradient is calculated based on Govers and Poesen
(1988) as follows.
SIR = 6.86S0.8 (6)
Where: S is slope gradient (m m-1)
WATEM/SEDEM employs a routing algorithm to transfer the eroded sediment
from the source to the river network using a multiple flow algorithm
(Desmet & Govers, 1996; Haregeweyn et al., 2013; A. J. Van Rompaey et
al., 2001). The original TC equation (Equation 5) allows the model to
represent gully erosion through flow concentrations and preferential
channel pathways connected with rivers (A. J. Van Rompaey et al., 2001;
Verstraeten et al., 2007). The routing algorithms in treated catchment
areas can alter the TC and sediment deposition patterns, while causing
little change in predicted total erosion and sediment yield (Takken et
al., 1999; Takken et al., 2005; Vigiak et al., 2006). Thus, following
the flow path, the sediment is transferred downslope if the local
transport capacity is higher than the incoming sediment volume. If the
transport capacity is lower than the incoming sediment volume, sediment
deposition occurs. The output of the model consists of a map indicating
the amount of soil erosion or deposition at each pixel depending on
transport capacity.
To investigate sediment connectivity, a sediment delivery ratio (SDR)
approach that takes into account the spatial distribution of gross
erosion and deposition processes was used, as suggested by Atkinson
(1995); Ferro, Di Stefano, Giordano, and Rizzo (1998). Hence, attempts
to model connectivity have been made by studying the SDR in order to
accommodate gross erosion estimate of soil loss to values observed at a
catchment outlets (Ferro & Porto, 2000; A. J. Van Rompaey et al., 2001)
as follow:
SDR = SY/E (7)
Where,
SDR = the sediment delivery ratio
SY = sediment yield
E = gross erosion per unit area