SCUAF is a computer model
which predicts the effects on soils of specific land-use systems under given
environmental conditions. It is designed to include the distinctive features
of agroforestry — that is, land-use systems which include both trees and crops.
SCUAF can also be used to simulate land use under agriculture or forestry, treating
these as limiting cases of agroforestry (with either 0 or 100 percent trees
replacing crops).
The major advantage of SCUAF is its ease of operation. To anyone familiar with
basic soil–plant relationships, including nutrient cycling, the processes involved
are largely self-explanatory from their descriptions in the inputs
section. The document by Young et al (1998) provides
a detailed description of the processes modelled in SCUAF.
SCUAF is a process–response model. In outline, the user specifies:
The land-use system is based on two plant components: trees and crops. The primary basis for description of this system are the proportions of trees and crops in each successive year. Other elements of the land-use system are additions (organic additions, fertilizers), removals (harvest, losses), prunings (of the trees) and transfers (e.g. transfer of tree prunings to soil under crops). Plants are modelled in terms of four components (leaf, fruit, wood and root), and each component is defined in terms of three elements (Carbon, Nitrogen and Phosphorus).
The model provides an annual simulation of:
The soil conditions and processes covered are:
Besides conventional crop harvest, the harvest may include crop residues, and harvests of fruit, fodder, or wood from trees.
SCUAF does not model soil water or its availability to plants. This was tried in previous versions, but it was found to be essential to simulate soil water changes on a short-term (e.g. 10-day) basis, something that is not possible within the annual framework of SCUAF.
SCUAF is not a plant growth simulation model. The user enters the initial rates of plant growth (trees, crops) as biomass increases per year. The model then estimates the effects of changes in soil properties upon subsequent rates of plant growth.
All of the values employed in the model, parameters and variables, are accessible to the user. There is a set of default values that varies according to the physical environment (climate, soil, slope, etc.). This has two purposes: to provide reasonable estimates for variables that have not been measured, and to save time when rapid runs of the model are needed (e.g. in education). However, the user can readily modify all values to correspond with field measurements or their own preferred estimates.
SCUAF is primarily intended for simulation over periods in the order of 10–20 years, i.e. for the assessment of land-use sustainability within the medium term. It can also be applied to long-term simulation provided the user is aware of the pitfalls inherent in long-term extrapolation.
Units
The unit of time is the year. The unit of area is the hectare.
The unit for input of the land-use system is fraction of area, e.g. 0.4 if trees cover 40% of the area considered. Although it is convenient in discussion to refer to areas as percentages they are entered as fractions.
Most of the other SCUAF inputs, as well as internal calculations, are in terms of kilograms per hectare (kg/ha) for stock variables, or kilograms per hectare per year (kg/ha/yr) for flow variables (rates).
The kg may refer to plant
dry matter (DM), plant or soil carbon (C), nitrogen (N) or phosphorus (P),
e.g. kg DM/ha/year.
Exceptions are made for inputs which in soil science are conventionally expressed
in other units, e.g. soil-horizon percentages. Soil depth is expressed in centimetres
(cm).
Acknowledgements
Versions 1 and 2 of SCUAF were developed with the support of the International Centre for Research in Agroforestry (ICRAF), Nairobi, Kenya. Peter Muraya’s expert knowledge and dedication in programming and reprogramming enhanced the model’s reliability and flexibility. Version 4 was developed in conjunction with the Centre for Resource and Environmental Studies of the Australian National University, within a project entitled: ‘Improving smallholder farming systems in Imperata areas of Southeast Asia: a bioeconomic modelling approach’. The project was funded by the Australian Centre for International Agricultural Research (ACIAR) and the Center for International Forestry Research (CIFOR).
Version 4 of SCUAF was published as an ACIAR Report by Anthony Young, Kenneth Menz, Peter Muraya and Chrysogon Smith. These help files contain excerpts taken directly from that publication.
Version 5 of SCUAF represents a complete rewrite of the programming code. All the equations used in the original model were retained. However, the model was re-designed using an object-oriented approach to represent the complex interactions between soils, trees and crops in a more efficient and less error-prone way. A few minor errors found in version 4 were corrected. Development of this version was funded by ACIAR within a project entitled:"Economic potential of land-use change and forestry for carbon sequestration and poverty reduction".