The Soil-Plant-Atmosphere model (SPA, Williams et. al 1996) is a process-based model that simulates ecosystem photosynthesis and water balance at fine temporal and spatial scales (30 minute time-step, multiple canopy and soil layers).
The detailed soil and rooting routines are described in a follow up paper (Williams et al 2001) .
The scale of parametrization (leaf-level) and prediction (canopy level) have been designed to allow the model to diagnose eddy flux data, and to provide a tool for scaling up leaf level processes to canopy and landscape scales.
The model drought routines were extensively tested in Ponderosa pine ecosystems of the US, with comparisons against sapflow data (Williams et al 2001) and flux data (Schwarz et al 2004) for different aged stands.
A detailed investigation of drought impacts on Amazonian rain forest was undertaking by linking SPA modelling to a rainfall exclusion experiment and detailed field measurements at the leaf level (Fisher et al 2006) and the stand scale (Fisher et al 2007).
The model is written in FORTRAN 90. The code is divided among several files, each associated with a particular component of the overall model. This means that the core files can be shared among users, who should only have to alter the input/output file and the main program file in order to customise the model for their own uses.
The model is freely available to interested researchers as source code
You can see a schematic of the model and a flow diagram of the model code here.
Also available are spreadsheets (excel):
To download the key model papers, look in my publications, linked to the left
If you would like to see and use the model code, available in FORTRAN 90, please contact me by email (email@example.com).
1. Misson, L., J. A. Panek and A. H. Goldstein (2004) "A comparison of three approaches to modeling leaf gas exchange
annually drought-stressed ponderosa pine forests", Tree Physiology 24, 529-541. This interesting paper compares predictions for a droughted pine system using three different models (SPA, Ball-Berry, Jarvis).
The authors only vary one parameter (the iota efficiency parameter) when tuning SPA. I would expect that site-specific values of tree and soil conductance would improve the fits even further.
2. Lee, Y H & L Mahrt (2004) "Comparison of heat and moisture fluxes from a modified soil-plant-atmosphere model with observations from BOREAS", JGR-A 109 (D8) art. no. D08103.
This study evaluates the prediction of heat and moisture fluxes from a new land surface scheme with eddy correlation data collected at the old aspen site during the Boreal Ecosystem-Atmosphere Study (BOREAS) in 1994. The model used in this study couples a multilayer vegetation model (components from SPA) with a soil model. Inclusion of organic material in the upper soil layer is required to adequately simulate exchange between the soil and subcanopy air. Comparisons between the model and observations are discussed to reveal model misrepresentation of some aspects of the diurnal variation of subcanopy processes.
3. Stauch, VJ & AJ Jarvis (2006) "A semi-parametric gap-filling model for eddy covariance CO2 flux time series data". Global Change Biology, 12: 1707-16. SPA is used to provide synthetic data sets for gap filling study.