1. CGMS/WOFOST model principles
Bernard TYCHON
University of Liège
Department of Environmental Sciences and Management
B-6700 Arlon Belgium
Presentation based on Decrem, Gommes, Supit and van Diepen’s documents.
Training on the Mars Crop Yield Forecasting System – IPSC – JRC
Ispra, Italy, 29-30/11/2005

2. WOFOST = model developed by the Centre for World Food Studies in Wageningen, the Netherlands.
WOFOST is a dynamic, mechanistic model that simulates crop growth on the basis of the underlying processes, such as photosynthesis, respiration etc.
CGMS = spatial version of WOFOST

3. The WOFOST approach
WOFOST simulates the crop life cycle from sowing to maturity.
Meteorological data (rain, temperature, wind speed, global radiation, air humidity) are needed as input.
Model parameters include soil moisture content at field capacity and wilting point, and other parameters on saturated water flow. Also information on site-specific crop management is requested.
The crop growth model includes parameters for European crops (Wheat, Grain Maize, Barley, Rice, Sugar Beet, Potato, Field Bean, Soy Bean, Oilseed Rape, Sunflower, etc.) and for tropical crops (Sorghum, Millet, Cassava, Groundnut, Sweet potato...).

4. 1. Production levels
WOFOST is able to predict yields in several production levels:
Production level 1: Potential (radiation and T° limited)
Production level 2: Water limited
Production level 3: Nutrient limited (not used in CGMS)

5. Production level 1: Potential (radiation and temperature limited)
Growth occurs in conditions with abundant plant nutrients and water all the time.
The growth rate of vegetation is determined by weather conditions  very intensively managed irrigated crops.
The only inputs to the model are temperature and radiation.

6. Production level 2: Water limited
Growth is limited by water shortage at least part of the time  intensively managed dryland crops.
The model must determine water stress and its effect on the photosynthetic and growth processes.
Beside temperature and radiation, another input to the model is precipitation.

7. Production level 3: Nutrient limited
Growth is limited, at least part of the time, by shortage of nitrogen (N), phosphorus (P) or potassium (K), and water or weather at other times.  usual dryland crops even if ‘well-fertilized’.
The model must determine soil nutrients dynamics, plant uptake, nutrient use in the plant, and effects of nutrient stress on photosynthesis, partition and growth.

8. 2. Functionality
Flow chart of the WOFOST model

9. Weather The meteorological data used by WOFOST are:
maximum temperature,
minimum temperature,
global radiation,
wind speed,
vapour pressure,
rainfall.
The Penman method is used to calculate the evapotranspiration.
The global radiation is estimated using the Ångström formula when no actual data are available. The Ångström formula uses the sunshine duration as input.

10. Crop growth (1)
Crop growth depends on the daily net assimilation, which depends on the intercepted light.
Reduction of the transpiration due to water stress results in a reduced production of assimilates.
The assimilates are partitioned over the various plant organs.

11. Crop growth (2)
Detailed flow chart of the crop growth simulated by WOFOST

12. Interception of sunlight
Solar radiation at top of canopy
Solar radiation within canopy
Intercepted radiation
CO2 assimilation

14. Maintenance respiration
Propotional with
Biomass of living plant organs
Maintenance coefficient per plant organ
Temperature (Q10 factor : doubling with 10°C)
(uses 15 – 30 % of all assimilates)

15. Growth respiration
Depend on :
Conversion coefficient per plant organ
Partitioning of assimilates over organs
(uses % of all assimilates)

16. Partitioning of assimilates and development stages

17. Soil water balance (1)
A crop growth model also has to keep track of the soil moisture content to determine when and to what degree a crop is exposed to water stress.
WOFOST uses a water balance, which compares incoming water in the root zone with outgoing water and quantifies the difference between the two as a change in the soil moisture content.

18. Soil water balance (2)
Schematic representa-tion of the different components of the WOFOST soil water balance
Bucket model, one layer

19. Nutrient use (not used in CGMS)
Influence of nutrients (nitrogen, phosphate and potassium) on the yield is calculated on a yearly basis.
First the potential supplies of nitrogen, phosphorus and potassium are calculated.
In a second step, the actual uptake of each nutrient is calculated as a function of the potential supply of that nutrient, in order to obtain a yield estimate.

20. 3. Summary of data requirements
Soil properties: horizon thickness, upper and lower limits of volumetric water content, volumetric water at saturation, hydraulic conductivity at saturation.
Daily weather data: radiation, precipitation, max/min temperatures, wind speed, and relative air humidity.
Crop parameters: temperature sums, photoperiod response, yield components, …
Initial conditions: water content, total nitrogen, phosphorus and potassium.
Management conditions: sowing and harvest dates.

21. Inputs/outputs of the WOFOST model
Meteo data
Meteo network
Water balance parameters
Agro- meteorological model (WOFOST)
Yield (kg/ha)
Crops
Reference parameters
Soils
Inputs
Outputs

22. Some results

23. Applications of WOFOST/CGMS
ANALYSIS
inter-annual yield variability
yield variability over different soil types or over a range of agro-hydrological conditions
sowing strategies
effects of climate change
PREDICTION
regional yield forecasts
regional assessments of crop yield potential in the form of maximum yield levels
estimation of maximum benefits from irrigation or from fertilizer use
detection of adverse growing conditions (drought, …) by simulation

24. Spatial version of WOFOST model : the EU-Crop Growth Monitoring System (CGMS)
Remote sensing
Vegetation index
Water balance parameters
Meteo data
Production
Meteo network
Agro- meteorological model WOFOST
Yield function
Crops
Reference data
Soils
Area
Current agricultural data
Agricultural statistics
District yield

25. 4. Comparison with AGROMETSHELL (FAO)
Relation between Evapotranspiration and assimilation [F(mesophyll resistance)]

26. Agricultural statistics
Station water balance
NDVI or other grid
ETA grid
Agricultural statistics
District ETA
District Yield
1987
2002
1985

28. 5. A final warning
A more complex model does not necessary means better results than a simpler model.
As each parameter estimate and process formulation has its own inaccuracy, these errors accumulate in the prediction of final yield.
The model must be validated over the expected range of inputs, just like a statistical model.
Expertise is still required for a good use of WOFOST/CGMS as well as for AGROMETSHELL

29. More information