Crop Yield Forecasting

FIGURE 1.4.
Crop Growth Model simplified scheme
Source: modified from Baruth 2013.
The WOFOST model uses a set of input variables – including 46 crop parameters, the crop calendar,
and regional cultivars some parameters of which have been modified – to produce the following
output variables: 1) crop development stage; 2) crop total biomass and yield under potential and
water-limited conditions; 3) crop leaf area index under potential and water-limited conditions; and 4)
soil moisture, transpiration.
The plant’s physiological age is defined by its development stage, each of which is characterized
by the formation or appearance of different organs. The most important phenological change
occurs from the vegetative to the reproductive stage, which determines a large redirection of
dry matter allocation towards the plant organs. Because many physiological and morphological
processes change once the phenological stage begins, its accurate quantification is essential
for any crop growth simulation model. For many annual crops, the development stage can
be conveniently expressed as a dimensionless variable, having a value of 0 at seedling
emergence, of 1 at flowering and of 2 at maturity. To mark the beginning of the growing
season, either the sowing or emergence date may be chosen; however, if the sowing date
is selected, the WOFOST model determines the emergence date and thus the beginning
of the crop growth simulation. The crop emergence can be defined as a function of the
sum of effective daily temperatures since the sowing date. Emergence takes place when
the sum of effective daily temperatures reaches the emergence threshold temperature. This
threshold temperature is crop-specific and should be provided by the user. The daily effective
temperature depends on the base temperature, below which no phenological processes
Crop Yield Forecasting: Methodological and Institutional Aspects 39