In this figure, the Erosion worksheet has been selected. The file is empty and will be filled with values when SCUAF is run. A worksheet can be selected by clicking on its tab.
The units of measurement are as follows:
The worksheets present in the Output file of SCUAF version 5 correspond to specific output menu items in SCUAF version 4, these are indicated by the numbers below:
While SCUAF is not a total plant growth simulation model, one of its major functions is to estimate the effects of changes in soil properties, relative to their initial condition, on the growth of trees and crops (initial condition refers to the state of the soil at the start of modelling).
There are three components to these effects: nutrients, soil carbon, and soil depth.
Nutrients
Nutrients are modelled in the standard manner for soil science, agricultural and forestry research, by comparing plant nutrient requirements with nutrient availability.
In the case of trees, the requirements from the soil may be reduced by deep uptake, the uptake of nutrients by roots extending into weathering rock, or below the soil depth considered in the model (typically the rooting depth of crops).
Requirements and Availability
are estimated separately for nitrogen and phosphorus. If, for either nutrient,
Availability is less than Requirements, plant growth is reduced proportionally,
by using the law of the minimum. For example, (as kg nutrient/ha):
If plant Requirements are N=100 P=10 and Availability is N=65 P=20, then nitrogen
is the limiting nutrient, and plant growth would be reduced to 65/100 or 65%
of its non-nutrient-constrained value. Nitrogen uptake would be 65 kg/ha, but
phosphorus uptake would be reduced to 65/100 ×10 kg/ha. Where both nutrients
are deficient, plant growth is based on the nutrient which causes the greater
reduction.
In agroforestry systems, plant-nutrient requirements are estimated separately for trees and crops. Correspondingly, nutrient availability is calculated for soil under trees and soil under crops. The model allows a proportion of tree roots to grow into the soil under crops (the proportion being specified by the user), and tree/crop competition for nutrients is calculated on the basis of relative root densities. Thus, trees may obtain some of their nutrients from soil under crops, in effect ‘robbing’ the crop of part of the nutrient supply which it would have had if the trees were not present.
Soil carbon
The level of soil carbon,
and hence organic matter, affects plant growth in two ways: through the release
of nutrients by mineralisation of humus, and through its influence on soil physical
properties. The former effect operates as a component in nutrient availability.
The organic matter status is the major variable affecting soil physical properties,
and these in turn influence plant growth in a number of ways, notably through
facilitating the development of root systems, and influencing water-holding
capacity. These multiple effects are modelled through two variables, the carbon
feedback factors for trees and for crops. It is assumed that an increase in
soil carbon will improve plant growth, and a decrease will reduce it; the rates
of increase or decrease are set by the user.
Soil depth
Erosion removes topsoil, causing a progressive reduction in soil depth. This affects plant growth by reducing the rooting zone and lowering the water-holding capacity. The latter is probably the major effect. The effect of soil depth is modelled by soil depth feedback factors, for trees and for crops, set by the user.