Forecasting methods

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Various authors have proposed to subdivide crop yield in three components: mean yield, multi-annual trend and residual variation (e.g. Vossen, 1989; Dagnelie et al. , 1983; Dennet et al. , 1980; Odumodu and Griffits, 1980). It is assumed that the interacting effects of climate, soil, management, technology, etc. determine the mean yield. Observed national, regional and sub-regional yields show a trend in time. The trend is mainly due to long-term economic and technological dynamics such as increased fertiliser application, improved crop management methods, new high yielding varieties, etc. The third component, the residual variation, is considered to be the variation among years (Dennet et al. , 1980). It is exactly this part which should be explained by weather, crop and remote sensing indicators.

According to Dennet et al. (1980) and Odumodu and Griffits (1980), the technological time trend should be removed from the crop yield time series, assuming that the residual variation is independent of that trend. This approach can be summarised as (Vossen, 1989):

Palm and Dagnelie (1993) fitted various time trend functions to national yield series (ton.ha -1 ) of several crops for 9 EU member states. Regressions were executed for the period prior to 1983 and a forecast for 1983 was made. This procedure was repeated for successive years up till 1988. The prediction results were compared with national yield values. Of the tested functions a quadratic function of time performed best. However, differences with a simple linear trend function were small. In a next step, these authors removed the trend from the yield series using the quadratic function. The residuals for the period prior to 1983 were regressed against various meteorological parameters and a prediction for 1983 was made. Again, this procedure was repeated for successive years up till 1988. This was done for 19 Departments in France . Comparing the predicted and official yield series demonstrated that the applied meteorological variables did not improve the prediction accuracy.

Swanson and Nyankori (1979) for corn and soybean production in the USA , Sakamoto (1978) for wheat production in South Australia , Agrawal and Jain (1982) for rice yields in the Raipur District in India , considered the technological time-trend dependent on the residual variation. According to Winter and Musick (1993), Hough (1990) and Smith (1975), weather affects farm management practices such as planted area, timing of field operations, application of inputs, etc. Hence, the time trend should be analysed simultaneously with the explaining variables. This approach can be summarised as (Vossen, 1989):

Swanson and Nyankori (1979) showed that the time trend was underestimated when weather data were not analysed simultaneously with the time trend. Similar results were found for millet in Botswana (Vossen, 1989).

The previous equation does not account for the interaction between crop growth and weather variability. Also root characteristics and soil physical properties are not accounted for. Therefore Vossen (1990b, 1992) proposed to use crop growth simulation results to describe year-to-year yield variation. In a crop growth simulation model weather and soil characteristics are summarised and crop characteristics, including yield form the output, i.e. simulation results quantitatively represent the influence of weather variables on crop growth. The yield can be written as:

Prediction model

Official statistics of regional mean yields are predicted by the CGMS using one of the following simulated predictors (see Crop Simulation):

  • Potential above ground biomass (ton.ha-1 dry weight)
  • Water limited above ground biomass (ton.ha-1 dry weight)
  • Potential storage organs biomass (ton.ha-1 dry weight)
  • Water limited storage organs biomass (ton.ha-1 dry weight)

Originally, it was intended to predict yields by solely using the water limited weight of storage organs in the prediction model. Later on, the other three were added. Water limited yield, for instance, is inappropriate for a region with a lot of irrigation. Furthermore drought stress can be strongly reduced in case of groundwater influence. This factor is not included in the CGMS. The simulated biomass indicators were added because these are more robust, and less sensitive to modelling errors in the distribution of assimilates. Moreover they also allow yield prediction during the growing season, when grain filling has not yet started or grains are still very small (de Koning et al. , 1993). In the current system all of the crop parameters can be used as predictors including Leaf Area Index, Soil Moisture and Development Stage. With little effort any indicator can be added, such as the ratio between the estimated actual crop transpiration and potential crop transpiration (Ta/Tp, see Crop Simulation) or remote sensing indicators (see Remote Sensing).

The statistical sub system of the CGMS uses a combination of a linear time trend and crop growth simulation results as proposed by Vossen (1990b, 1992). This prediction model can be described as:

Trend analysis

Other prediction models

scenario analysis in SPSS