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SCUAF
A Model to Estimate Soil Changes Under Agriculture, Agroforestry and Forestry

Physical environment: default values

An important strength of SCUAF is that it provides reasonable default values for a number of different environments.

The Physical Environment variables serve to set the Default Values of a SCUAF model. Because these Default Values are only first approximations, the classes of Environment variables (e.g. climatic classes) are only broadly defined.

The user selects values for the following environmental variables:

Values for these characteristics of the physical environment are set in the Defaults dialog box, accessible by clicking on the "Set Defaults" button. If the user clicks OK after setting Defaults in the dialog box, the appropriate default values for a number of parameters affecting soils and nutrient-cycle processes are written to the active input file. Site information is automatically written to the Info worksheet.

Only the Soil and Cycles worksheets are affected by the defaults. The user still needs to enter the appropriate plant growth rates (NPP) for the particular site. The user can change these parameter values manually at any time by modifying entries in the input worksheets.

All variables in SCUAF are given default values. These are set by the environmental conditions: climate, soil and slope. Examples are:

Some variables are affected by more than one aspect of the environment, e.g. leaching is affected by climate and by soil texture. Others are the same for all environmental conditions, e.g. by default, roots are always initially set at 40% of above-ground biomass.

The dependence of default values on environmental variables is shown in Table 1.

Environment Variables affected
Climate Initial soil conditions, rainfall and cover factors in erosion, rates of plant growth, humus-decomposition constants, atmospheric input of nutrients
Slope Slope factor in erosion
Soil drainage Leaching of nutrients
Soil parent material
Supply of phosphorus by rock weathering
Soil texture Initial soil conditions, soil factor in erosion, leaching of nutrients
Soil reaction Phosphorus fixation onto clay minerals
Initial soil organic matter status Initial soil carbon, organic nitrogen, and organic phosphorus; hence
initial soil fertility

The intended uses of the default values are:

As soon as a user has altered the default value of any variable, this becomes its new value in the current model, and the default value can only be restored by clicking on the “Set Defaults” button and selecting the appropriate environmental characteristics in the "Defaults" form.

Default values are intended to be used only for exploratory or conceptual purposes, and in education. For research, SCUAF must be locally calibrated by the user. The default values should be treated as a fallback—rough approximations for use where more precise data are not available. Users should not expect that the default values will provide an ‘instant’ simulation of their data. The procedure intended is to:


Climate

Climate affects the default values of soil conditions, the climate factor in erosion, plant growth, and many nutrient-cycle processes.

Classes 1-6 are tropical or subtropical. Humid, subhumid and semi-arid are respectively the rain forest, savanna and sahel zones. Highland refers to the tropics, but may also be selected for lower altitudes at higher latitudes. Temperate refers to both warm and cool temperate climates.

    1. Lowland humid
    2. Lowland subhumid
    3. Lowland semi-arid
    4. Highland humid
    5. Highland subhumid
    6. Highland semi-arid
    7. Temperate

Note: values for temperate climates are highly provisional and require more testing against actual data.


Slope

Slope affects the default values of the Slope Factor in Erosion. If the Slope class is Flat, there is no erosion in the model.

Slope classes are those of the FAO/UNESCO Soil Map of the World.

    1. Flat
    2. Gentle
    3. Moderate
    4. Steep


Soil Drainage

Soil Drainage affects the default values of leaching. The choices are:

1 Free
2 Imperfect
3 Poor


Soil Parent Material

Soil Parent Material affects the default value for the supply or Phosphorus by rock weathering.

Felsic refers to light-coloured, silica-rich rocks, e.g. granite, sandstone. Mafic refers to dark-coloured rocks, low in silica, e.g. basalt.

1 Felsic
2 Intermediate
3 Mafic


Soil Texture

Soil Texture affects the default values of Soil properties, Soil Erodibility Factor in Erosion, and Leaching.

1 Medium textured
2 Sandy
3 Clayey


Soil Reaction

Soil Reaction affects the default value of phosphorus fixation.

The classes are bounded by pH values of 5.0, 6.0 and 7.0.

1 Strongly acid
2 Acid
3 Neutral
4 Alkaline


Soil-Fertility Status

Soil-Fertility Status affects the default values of initial Soil Carbon, Nitrogen and Phosphorus (organic, available and total). The choices are:

1 Low
2 Intermediate
3 High

Low is a degraded soil. Intermediate is a ‘normal’ agricultural soil. High is a soil newly cleared of vegetation, or one which has been well fertilized. Under the default values of SCUAF, selecting Low or Intermediate Soil Fertility Status will usually lead to nutrient deficiencies in Year 1.


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