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Chapter 5 ....continued
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5.6 |
Soil property data inputs |
Soil tables are built using program Topog_soil. Two different soil tables can be generated using Topog_soil, the options being: Options 1 is to be used in association with the Richard's equation-based soil moisture accounting schemes (ie. the NR and RK schemes). Option 2 is only to be used in association with the Simplified Bucket Model (SBM) soil water accounting scheme.
To run Topog_soil, type: $ _soilthen select one of the two soil table options listed. The input requirements for these two options are described in sections 5.6.3 and 5.6.4, respectively.
Under both options, the user is asked if data is to be entered for the macropore flow and sediment transport options.
5.6.1 The macropore flow option
This option allows the user to represent the effects of preferential flow on soil water dynamics. Macropore flow is only activated under saturated soil conditions.
If the user answers "y" to the following question in Topog_soil:
Enter < y > for macropore data, else < n >then data values for three parameters must be provided. These are:
- soil macropore volume (cm3/cm3); this must not be larger than the available soil moisture content you specify (qs - qr)
- soil macropore conductivity (m/day); this applies to the horizontal and lateral transfer of soil water
- soil macropore length scale (m); this is the depth that the macropore extends to from the surface
5.6.2 The sediment transport option
This option is only used if sediment transport is to be computed. If the user answers "y" to the following question in Topog_soil:
Enter < y > for sediment transport data, else < n >then data values for eight parameters must be provided. A description of each of these is provided in section 5.11.2.
5.6.3 Richard's equation soil table
This scheme is based on the Broadbridge and White (1988) soil property model. This in turn, is based on the Fokker-Planck equation and describes the soil water retention curves using three independent parameters. These curves have continuous slope under all circumstances and thus promote model stability.
Inputs Units Typical values Description Ks m/d 0.001 - 50.0 saturated hydraulic conductivity qs m3/m3 0.30 - 0.80 volumetric soil moisture content at saturation qn m3/m3 0.01 - 0.2 volumetric soil moisture content at air-dry C * 1.001 - 2.0 the soil structure parameter High values of C produce soil characteristic profiles indicative of well structured macropore soils, low values of C would be used for poorly drained soils with well developed capillary fringes lc m 0.01-0.2 capillary length scale, determined from sorptivity and C D % 0.1 the most can change per iteration (% of qs - n) y min m -50 - -150 the driest psi value on the table n 150 - 250 number of points in soil table Though all of the input parameters to the Broadbridge and White soil property model are measurable, we recognise that few model users will have access to such data or methods to obtain these. We therefore suggest the following 'default' values for the following generic soil textures:
Texture Ks (m/day) qs qr lc (m) C Sand > 1.0 0.30-0.40 0.05-0.10 0.02-0.05 1.01-1.02 Loamy Sand > 1.0 0.35-0.45 0.05-0.10 0.02-0.05 1.02-1.05 Sandy Loam 0.5-5.0 0.40-0.50 0.05-0.15 0.05-0.10 1.02-1.05 Silty Loam 0.50-2.0 0.45-0.50 0.10-0.20 0.25-0.50 1.05-1.20 Loam 0.50-1.0 0.40-0.50 0.10-0.20 0.10-0.20 1.40-1.50 Sandy Clay Loam 0.25-0.75 0.35-0.45 0.10-0.20 0.10-0.20 1.40-1.50 Silty Clay Loam 0.10-0.50 0.40-0.50 0.15-0.25 0.10-0.20 1.20-1.30 Clay Loam 0.10-0.25 0.45-0.55 0.20-0.30 0.25-0.50 1.20-1.40 Sandy Clay 0.10-0.25 0.40-0.50 0.15-0.25 0.05-0.10 1.10-1.20 Silty Clay 0.05-0.20 0.45-0.55 0.25-0.35 0.20-0.50 1.05-1.20 Clay 0.01-0.20 0.45-0.55 0.25-0.35 0.20-0.50 1.30-1.50 Heavy Clay < 0.01 0.40-0.60 0.05-0.20 0.05-2.00 1.50-2.00
Recommended "default" values of Broadbridge-White soil parameters for generic soil textures.
5.6.4 The Simplified Bucket Model soil table
This soil table can only be used when applying the SBM soil moisture accounting scheme. Unlike option 1, all of the key data is output to a single line.
The last two columns in the output file record the soil hydraulic conductivity versus depth. These last two columns of data are not used by Topog_dynamic; they are provided to let the user visualise the influence of the m parameter on Ks decline through the soil profile. This can be viewed using Topog_chart.
Inputs Units Typical values Description Sp qs - qr soil specific storage m 0.2 - 20 soil conductivity decay parameter a unsaturated/saturated intercompartment throttling term yo m pressure potential at oven dryness qr m3/m3 0.01 - 0.1 volumetric soil moisture content at oven dryness qs m3/m3 0.30 - 0.80 volumetric soil moisture content at saturation Ks m/day 0.001 - 50.0 saturated hydraulic conductivity
5.7 |
Soil profile data inputs |
Each catchment element must be associated with a .nodes file. A separate .nodes file must be built for each soil profile represented in the catchment.
5.7.1 Using the Richards equation soil moisture accounting scheme
The .nodes file has three columns. The first column specifies the depth below the surface (in m) of each node used in the soil water module. The second column is an integer value, specifying which soil table is allocated to each node. A value of 1 means that the first soil table listed in the .par file is to be used. A value of 2 means that the second listed file should be used, and so on. The third column contains the initial soil water solute concentration (in kg/m2) for each node, and is only used if the solute transport module is invoked.
An example .nodes file for a 3 m deep soil with three layers is shown below. Here, soil table 2 is allocated to all nodes between 0 and 0.5 m, soil table 1 is allocated to all nodes between 0.5 and 2.0 m, and soil table 3 is allocated to the remaining nodes.
# Depth (m) Soil# Solute (g/L) 0.0 2 0.007325 # Soil 2 Kyabram clay 0.01 2 0.007325 0.05 2 0.00325 0.1 2 0.0025 # 0.2 2 0.00325 # removed for testing 0.3 2 0.007325 0.5 1 0.007325 # Max root depth 0.75 1 0.000625 # Soil 1 Murray Silt 1.0 1 0.001325 1.25 1 0.001325 1.50 1 0.001325 1.75 1 0.001325 2.0 3 0.001325 2.5 3 0.001325 3.0 3 0.001325 Header lines with comments can be inserted into the .nodes file, provided they commence with a '#' as the first character on the line. Comments at the end of the data on a line can also be inserted, commencing with a '#'. Lines of data can be commented out by inserting '#' as the first character on that line, as shown in the example above.
To ensure model stability, and to model soil/litter evaporation credibly, it is necessary to space nodes carefully. Internodal spacing should be small at the top of the profile, but can increase down through the profile, to a maximum of about 1 m. The lower the saturated hydraulic conductivity and the higher the rainfall, the closer the nodes should be.
Each soil type nominated in the profile must be allocated to at least three nodes.
If using the plant growth module, each vegetation type represented in the catchment requires a list of initial root carbon values. These values must be provided for each of the nodes listed in the .nodes file. More information on the root carbon values is provided in section 5.8.
5.7.2 Using the SBM soil moisture accounting scheme
When using the SBM soil moisture accounting scheme, only two nodes need be specified, as shown in the example below:
Only a single soil type may be nominated in each .nodes file, though neighbouring elements may be allocated different .nodes files, and hence different soil tables. If multiple soil profile types are nominated, a polygon file defining the distribution of these within the catchment must also be specified (see section 5.12).
| Take me out of frames | Chapter 5 continuted ...... |