There are three elements to modelling the soil in SCUAF: erosion, soil organic matter (represented by carbon), and nutrient cycling. Changes in erosion are modelled for the land use system as a whole, but changes in carbon and nutrients are calculated separately for the soil under trees and soil under crops.
To simplify the input process, the soil section is divided into two worksheets. The Soil worksheet (described on this page) contains the initial soil characteristics as well as erosion parameters; whereas the Cycles worksheet contains parameters related to the processes that affect organic matter and nutrient cycling.
The Soil Depth considered for modelling can be the total Soil Depth, or alternatively, the Depth reached by most Plant Roots. Information is entered per Soil Horizon (maximum 5). SCUAF does not, in most circumstances, deal in Soil Horizons, so the input values are converted into a single soil layer with nutrients measured in kg/ha. Bulk density is used to calculate these values.
Phosphorus differs between Organic (here called an Element),
Available (here called a Nutrient), and Total. Any of the methods for determining
Available Phosphorus may be selected; Outputs showing changes will be in terms
of the same method. Available Phosphorus is expressed as p.p.m. (or mg/kg).
Soil Erosion is estimated on the basis of the Universal Soil Loss Equation (USLE). The basic equation used in SCUAF is:
Soil Erosion (kg/ha/yr) = R * K * S * C(Plant) * P * 1000
where:
R = Rainfall or Climate Factor,
K = Soil Erodibility Factor,
S = Slope Factor,
C = Cover Factor (for Trees and Crops), and
P = Conservation Practices Factor.
The factor of 1000 is to convert from tonnes to kg. The Slope length (L) of the USLE is omitted.
Values of the K, S, C and P Factors are <=1.0. These factors reduce the rate of erosion, e.g. any Factor of 0.5 reduces erosion by half.
The rainfall and slope factors remain constant over time. The soil factor changes with increases or decreases in soil organic matter, and the tree and crop cover factors change with changes in rates of plant growth; the user may also change these factors for different periods. Hence, the calculated rate of erosion increases or decreases with time.
Default values for the Rainfall and Soil Erodibility Factors are taken from the USLE, and for the Cover Factors from the USLE and other research. However, application of USLE values for the Slope Factor results in unrealistically high rates of erosion; default values for the Slope Factor are therefore substantially lower than in the USLE.
If Plant Litter is applied to the Crop, the Crop Cover Factor will be substantially reduced, possibly to 0.1 or less. The program does not do this automatically, and the User should enter a lower figure than the default value where practices of adding litter (crop residues or tree litter) to the Crop are followed, or where minimum tillage is practised. It is also important to note that when the initial NPP is modified, it may also be appropriate to change the cover factor.
If Erosion has been measured, the calculated value of initial Erosion can be adjusted to the observed value by means of an Erosion Adjustment Factor. This Factor will be applied to all subsequent years. This factor can be set directly in the input worksheet, or it can be set interactively by clicking on the "Adjust Erosion" button in the main screen of SCUAF.
Soil Conservation Practices (e.g. terracing) reduce the rate of erosion. The Periods during which Conservation Practices are applied has been specified in the Land-use system worksheet, and the Conservation Factor will be applied only during those Periods.
An adjustment is made for
the specific effect of the tree component in agroforestry systems, called the
tree proportionality factor. Based on the topsoil content of
carbon, nitrogen and phosphorus, together with an enrichment factor for eroded
sediment, the losses of these elements through erosion are calculated. Eroded
sediment usually contains more carbon and nutrients than the topsoil from which
it is derived. This ratio is given by Eroded Sediment Enrichment Factors.