Model Choices

Most model options are boolean options and can be enabled in the .ini-file with ‘1’ and disabled with ‘0’. However, some options expect a string value. The possible strings are described together with the model option. Two large groups of model choices can be distinguished: ‘Model options’ and ‘Output options’. The Model options describe how the model will run: which options and/or extensions of WaTEM/SEDEM are used? The output choices describe which output that must be generated. On this page we describe the model choices for a basic run of WaTEM/SEDEM. Model choices that enable optional model extensions (e.g. incorporation of erosion management practices) are described in a seperate section.

Model options

L model

WaTEM/SEDEM allows the user to choose between two models to calculate the L-factor. The L-factor defines the impact of the slope length and is used in the calculation of RUSLE and transport capacity (TC).

The L-model is calculated according to the work of Desmet and Govers (1996):

\[L = \frac{(A+D^2)^{m+1}-A^{m+1}}{D^{m+2}.x^m.22,13^m}\]
with

  • \(A\): upstream area for every raster pixel (\(\text{m}^2\)).

  • \(D\): grid resolution \((m)\).

  • \(m\): length exponent \((-)\).

  • \(x\): factor incorporating the flow direction \((-)\).

\(x\) is calculated as a function of the aspect \(\alpha\) of the pixel:

\[x = |sin(\alpha)| + |cos(\alpha)|\]

The upstream area \(A\) in a pixel is determined by the stream flow algorithm, and considers the parcel trapping efficiencies and the parcel connectivities.

For the computation of \(m\), however, two options exist:

1. Van Oost et al. 2003:

Van Oost et al. (2003) uses an \(m\) depending on the surface of the upstream area \(A\). If the upstream area is smaller than \(A_{ref}\) = 10.000 ha, \(m\) is calculated as:

\[m = 0.3 + (\frac{A}{A_{ref}})^c\]

otherwise \(m\) is set to 0.72. in the model \(c\) is ‘hard coded’ as 0.8, meaning that this value is fixed for this model and cannot be changed by the user.

2. McCool et al. (1989):

McCool et al. (1989) calculates \(m\) as:

\[m = \frac{\beta}{\beta + 1}\]

with \(\beta\):

\[\beta = \frac{\frac{sin(\theta)}{0.0896}}{3.sin^{0.8}(\theta) + 0.56}\]

where \(\theta\) stands for the slope of the pixel in percentages.

The preferred method (i.e. Van Oost et al. (2003) or McCool et al. (1989, 1987)) can be selected by setting the model choice L model to ‘Desmet1996_Vanoost2003’ or ‘Desmet1996_McCool’, respectively, in the ini-file. This should be done in the following manner (mind the quotes):

L model='Desmet1996_Vanoost2003'

This is the default value for this model option. or:

L model='Desmet1996_McCool'

S model

WaTEM/SEDEM allows the user to choose between two models to calculate the S-factor. The S-factor defines the effect of slope steepness and is used in the calculation of RUSLE and transport capacity (TC).

Both models are a function of \(\theta\): the inclination angle or slope (%). The computation of the inclination angle is based on the four cardinal neighbouring pixels (Zevenbergen and Thorne, 1987).

The two S-models are:

1. Nearing (1997):

\[S = -1,5+\frac{17}{1+e^{2,3-6.1.\sin{\theta}}}\]

2. McCool et al. (1987)

McCool et al. (1987) distinguishes between two cases, namely:

\[100.tan(\theta) < 9.0; and: 100.tan(\theta) \geq 9.0\]

In the first case, S is calculated as:

\[S = (10.8.sin(\theta)) + 0.03\]

In the other case, S is calculated as:

\[S = (16.8.sin(\theta)) - 0.5\]

The preferred method (i.e. Nearing (1997) or McCool et al. (1987)) can be selected by setting the model choice S model to ‘Nearing1997’ or ‘McCool1987’, respectively, in the ini-file. This should be done in the following manner (mind the quotes):

S model='Nearing1997'

This is the default method to calculate the S-factor. or:

S model='McCool1987'

TC Model

The Transport Capacity (TC) can be calculated in two ways in WaTEM/SEDEM. The default method is the method proposed by Van Oost et al. (2000):

\[TC = kTC.R.K.(LS - 4.12.S_g^{0.8})\]

with

Most studies using WaTEM/SEDEM use this method by Van Oost et al. (2000). It can be activated in WaTEM/SEDEM by setting TC model to ‘VanOost2000’ in the ini-file (mind the quotes):

TC model='VanOost2000'

However, a second method, proposed by Verstraeten et al. (2007), can be used as well, namely:

\[TC = kTC.R.K.A^{1.4}.S_g^{1.4}\]

with

  • \(A\): the upstream area \((m^2)\) of the pixel

A detailed description and comparison of both TC models can be found in Verstraeten et al. (2007).

The method of Verstraeten et al. (2007) can be activated in WaTEM/SEDEM by setting TC model to ‘Verstraeten2007’ in the ini-file (mind the quotes):

TC model='Verstraeten2007'

Only Routing

By enabling the Only Routing option, only the routing algorithm will be run. This means that the WaTEM/SEDEM calculations are disabled, and no sediment calculations are done. When using this option only a limited model output will be returned by the model.

This option is usefull in large catchments to evaluate the routing without calculating the sediment transport or discharges. It is enabled in the ini-file as follows:

Only Routing = 1

The default is: Only Routing = 0

Calculate tillage erosion

This option enables the tillage erosion model of Van Oost et al. (2000). We refer to the dedicated section for more information about this model. This option can be enabled by writing the following in the ini-file:

Calculate Tillage Erosion = 1

The default is: Calculate Tillage Erosion = 0

Output

The user has the option to generate extra (or change characteristics of the) output by defining following keys in the [Output]-section of the .ini-file.

Saga_Grids

(bool, default false): write output rasters as Saga Grids. If false, Idrisi rasters are written.

write aspect

(bool, default false): write AspectMap.rst

write LS factor

(bool, default false): write LS.rst

write upstream area

(bool, default false): write UPAREA.rst

write slope

(bool, default false): write SLOPE.rst

write routing table

(bool, default false): writes routing.txt and routing_missing.txt

write routing column/row

(bool, default false): writes routing_colrow.txt

write RUSLE

(bool, default false): writes RUSLE.rst

write sediment export

(bool, default false): writes SediExport_kg.rst, SediOut_kg.rst, and SediIn_kg.rst

write water erosion

(bool, default false): writes WATEREROS (kg per gridcel).rst and WATEREROS (mm per gridcel).rst