S. Asseng et al., USE OF THE APSIM WHEAT MODEL TO PREDICT YIELD, DRAINAGE, AND NO3(-) LEACHING FOR A DEEP SAND, Australian Journal of Agricultural Research, 49(3), 1998, pp. 363-377
High rates of drainage and leaching of nitrates in deep sands in Weste
rn Australia are contributing to groundwater recharge and soil acidifi
cation in this region. Strategies are being sought to increase water a
nd nitrogen (N) use in the legume-based cropping systems. Choice of ap
propriate management strategies is complicated by the diversity of soi
l types, the range of crops, and the inherent season to season variabi
lity. Simulation models provide the means to extrapolate beyond the bo
unds 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
in soil, nitrate (NO3-) leaching, wheat growth, N uptake, and grain yi
elds obtained from the Agricultural Production Systems Simulator (APSI
M) model when this was initialised with appropriate information on soi
l properties and wheat varieties commonly grown on deep sands in the 5
00 mm rainfall zone west of Moora in Western Australia. The model was
found to give good predictions of soil water content, evapotranspirati
on, deep drainage, and overall NO3- leaching. Temporal changes in inor
ganic N in soil were simulated, although the small concentrations in s
oil inorganic N precluded close matching of paired observed and predic
ted values. Crop growth and N uptake were closely predicted up to anth
esis, but a poor fit between observed and predicted crop growth and N
uptake was noted post anthesis. Reasons for the discrepancies between
modelled and observed values are outlined. The model was run with hist
orical weather data (81 years) and different initial soil water and in
organic soil N profiles to assess the probability of drainage and NO3-
leaching, and the grain yield potentials for wheat grown on deep sand
s in the region west of Moora. Simulation showed that the soil water a
nd the soil inorganic N content at the beginning of each season had no
effect on grain yield, implying that pre-seed soil NO3- was largely l
ost 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 kg N/ha could be leached under wheat following a lupin crop, where
initial soil water contents and soil NO3- contents used in the model
were those measured in a deep sand after late March rainfall. Simulate
d application of N fertiliser at sowing increased both grain yield and
NO3- leaching. Splitting the N application between the time of sowing
and 40 days after sowing decreased NO3- leaching, increased N uptake
by wheat, and increased grain yield, findings which are consistent wit
h agronomic practice. The high drainage and leaching potential of thes
e soils were identified as the main reasons why predicted yields did n
ot approach the French and Schultz potential yield estimates based on
20 kg grain yield per mm of rainfall. When the available cater was red
uced by simulated drainage, simulated grain yields for the fertilised
treatments approached the potential yield line.