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
• 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.
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...).
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)
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.
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.
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.
2. Functionality Flow chart
of the
WOFOST model
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.
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.
Crop growth (2) Detailed flow chart of the crop growth simulate d by WOFOST
Interception of sunlight
• Solar radiation at top of canopy • Solar radiation within canopy • Intercepted radiation
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)
Growth respiration
Depend on :
• Conversion coefficient per plant organ • Partitioning of assimilates over organs (uses 30 - 40% of all assimilates)
Partitioning of assimilates and development stages
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.
Soil water balance (2) Schematic representa-tion of the different components of the WOFOST soil water balance
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.
3. Summary of data requirements
1. Soil properties: horizon thickness, upper and lower
limits of volumetric water content, volumetric water at saturation, hydraulic conductivity at saturation.
2. Daily weather data: radiation, precipitation, max/min
temperatures, wind speed, and relative air humidity.
3. Crop parameters: temperature sums, photoperiod
response, yield components, …
4. Initial conditions: water content, total nitrogen,
phosphorus and potassium.
Inputs/outputs of the WOFOST model Crops Soils Meteo network Meteo data Meteo data Reference parametersReference parameters Agro-meteorological model (WOFOST) Water balance parameters Water balance parameters Yield (kg/ha) Yield (kg/ha) Inputs Output s
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
Spatial version of WOFOST model : the EU-Crop Growth Monitoring System (CGMS)
Crops Soils Meteo network Remote sensing 0 2000 4000 6000 8000 10000 12000 1975197619771978197919801981198219831984198519861987198819891990199119921993199419951996199719981999 Harvest year Yie ld (kg /h a) Vegetation index Vegetation index Meteo data Meteo data Reference data Reference data Current agricultural data Current agricultural data Agro-meteorological model WOFOST Area Area Water balance parameters Water balance parameters Yield function Production Production District yield Agricultural statistics
4. Comparison with AGROMETSHELL (FAO) Relation between Evapotranspiration and
Station water balance NDVI or other grid ETA grid Agricultural statistics District ETA District Yield 1987 2002 1985
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
More information
• http://agrifish.jrc.it/marsstat/Crop%5FYield%5
FForecasting/METAMP/
