Models in Irrigation andWater Management - Practices of Irrigation and On-farm Water Management - page 406

product of the grain numbers per plant times the average kernel weight at physi-
ological maturity. The grain weight is calculated as a function of cultivar-specific
optimum growth rate multiplied by the duration of grain filling, which is reduced
below optimum value when there is an insufficient supply of assimilate from either
the biomass produced or stored biomass in the stem.
10.3.5.2 APSIM
APSIM stands for “Agricultural Production Systems sIMulator.” It is a modeling
framework that provides capabilities to simulate cropping systems over variable
time periods using available meteorological data (McCown et al., 1996; APSIM
Initiative, 2010).
The framework provides a “plug-in-pull-out” facility, allowing users to select
modules for modeling crops and their environments, under a constraint conditions.
Once the required modules have been selected, the behavior of the simulation
is controlled through user-defined management criteria. The user can define and
run simple simulations using options on the menu-bar. For more complex simula-
tions, APSFront includes the capability to use template files to generate the APSIM
parameter files needed for a simulation run.
10.4 Crop Production Function/Yield Model
10.4.1 Definition of Production Function
The relationship between crop growth or crop yield and water use is termed as crop
production function. On the other hand, a mathematical model is an equation or
set of equations which represents the behavior of a system. The behavior of crop
production function can be explained with an example, shown in Fig. 10.4 . The
figure shows the yield curve of wheat that might result from an experiment in which
irrigation water is supplied at different rates. Response of diminishing return type,
as in Fig. 10.4 , are quite common in biology and elsewhere, such as response of
crops to fertilizer and photosynthetic response to light.
10.4.2 Importance of Production Function
The basic information needed to solve problems of optimum water management on
farms consists in a precise knowledge of the water consumption of each crop and
its response to irrigation. In other words, we must know the production functions
in relation to water. Profits and risks inherent in irrigation management deci-
sions depend directly on the underlying crop-water production functions. Irrigation
scheme managers are often confronted with the issue of intensive (full irrigation to
meet evapotranspiration demand) versus extensive (to cover more area with deficit