Use of the APSIM wheat model to predict yield, drainage, and NO3- leaching for a deep sand
Authors: Asseng S. , Anderson G. C. , Dunin F. X. , Fillery I. R. P. , Dolling P. J. , Keating B. A. 
Published: August 2004

High rates of drainage and leaching of nitrates in deep sands in Western
Australia are contributing to groundwater recharge and soil acidification in
this region. Strategies are being soughtto increase water and nitrogen (N) use
in the legume-based cropping systems. Choice of appropriate management
strategies is complicated by the diversity of soil types, the range of crops,
and the inherent season to season variability. Simulation models provide the
means to extrapolate beyond the bounds of experimental data if accurate
predictions of key processes can be demonstrated. This paper evaluates the
accuracy of predictions of soil water content, evapotranspiration, drainage,
inorganic N content insoil, nitrate
(NO-3) leaching, wheat growth, N
uptake, and grain yields obtained from the Agricultural Production Systems
Simulator (APSIM) model when this was initialised with appropriate
information on soil properties and wheat varieties commonly grown on deep sands
in the 500 mm rainfall zone west of Moora in Western Australia. The model was
found to give good predictions of soil water content,evapotranspiration, deep
drainage, and overall NO-3
leaching. Temporal changes in inorganic N insoil were simulated, although the
small concentrations in soil inorganic N precluded close matching of paired
observed and predicted values. Crop growth and N uptake were closely predicted
up to anthesis, but a poor fit between observed and predicted crop growth and N
uptake was noted postanthesis. Reasons for the discrepancies between modelled
and observed values are outlined.
The model was run with historical weather data (81 years) and different
initial soil water and inorganic soil N profiles to assess the probability of
drainage and NO-3 leaching, and
the grain yield potentials for wheat grown on deep sands in the region west of
Moora. Simulation showed that thesoil water and the soil inorganic N content
at the beginning of each season had no effect on grain yield, implying that
pre-seed soil NO-3 was largely
lost from the soil by leaching. There was a 50% probability that 141 mm
of winter rainfall could drain below 1·5 m and a 50% probability
that 53 kgN/ha could be leached under wheat following a lupin crop, where
initial soil water contents andsoil
NO-3 contents used in the model
were those measured in a deep sand after late March rainfall. Simulated
application of N fertiliser at sowing increased both grain yield and
NO-3 leaching. Splitting the N
application between the time of sowing and 40 days after sowing decreased
NO-3 leaching,increased N uptake
by wheat, and increased grain yield, findings which are consistent with
agronomic practice.
The high drainage and leaching potential of these soils were identified as the
main reasons why predicted yields did not approach the French and Schultz
potential yield estimates based on 20 kg grain yield per mm of rainfall. When
the available water was reduced by simulated drainage, simulated grain yields
for the fertilised treatments approached the potential yield line.