Agriculture Reference
In-Depth Information
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Technical Description
Growth is described at the level of whole plant and organs. Integration is performed
with daily time steps using the Euler's method. Users may select different methods
to calculate water redistribution in the soil profile and reference evapotranspiration.
Water redistribution in the soil profile is handled by a simple cascading approach
or by a finite difference approach to determine soil water fluxes. CropSyst offers
three options to calculate reference ET. In decreasing order of required weather data
inputs, these options are Pemman-Monteith model, the Preiestley-Tayler model, and
a simpler implementation of Preiestley-Tayler model which requires only temper-
ature. Crop ET is determined from a crop coefficient at full canopy and ground
coverage determined by canopy leaf area index. Irrigation can be set for deficit
irrigation.
Crop development is simulated based on thermal time required to reach specific
growth stages. The thermal time (degree days) are accumulated from planting. The
accumulation of thermal time may be accelerated by water stress. Thermal time may
be also modulated by photo-period and vernalization requirement whenever perti-
nent. Given the common pathway for carbon and vapor exchange of leaves, there is
a conservative relationship between crop transpiration and biomass production. The
increase of leaf area during the vegetative period, expressed as leaf area per unit of
soil area (leaf area index, LAI), is calculated as a function of biomass accumula-
tion, specific leaf area, and a partitioning coefficient. Leaf area duration, specified
in terms of thermal time and modulated by water stress, determines canopy senes-
cence. Root growth is synchronized with canopy growth, and root density (by soil
layer, a function of root depth penetration). The prediction of yield is based on the
determination of a harvest index. Although an approach based on the prediction of
yield components could be used. The harvest index is determined using as base the
unstressed harvest index, a required crop input parameter, modified according to
crop stress (water and nitrogen) intensity and sensitivity during flowering and grain
filling.
Models Inputs
Four input data files are required to run CropSyst: Location, Soil, Crop, and
Management files. A “Simulation Control file” combines the input files as desired
to produce specific simulation runs. In addition, the “Control file” determines the
start and ending day for the simulation, and set the values of all parameters required
for initialization.
Model Output
The model outputs are yield (grain yield and biomass), water balance, nitrogen bal-
ance, residue production and decomposition, and soil erosion. There are three types
of output variables:
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