S. Asseng et al., Potential deep drainage under wheat crops in a Mediterranean climate. I. Temporal and spatial variability, AUST J AGR, 52(1), 2001, pp. 45-56
High rates of deep drainage (water loss below the root-zone) in Western Aus
tralia are contributing to groundwater recharge and secondary salinity. How
ever, quantifying potential drainage through measurements is hampered by th
e high degree of complexity of these systems as a result of diverse soil ty
pes, a range of crops, different rainfall regions, and in particular the in
herent season-to-season variability. Simulation models can provide the appr
opriate means to extrapolate across time and space. The Agricultural Produc
tion Systems Simulator (APSIM) was used to analyse deep drainage under whea
t crops in the Mediterranean climate of the central Western Australian whea
tbelt. In addition to rigorous model testing elsewhere, comparisons between
simulated and observed soil water loss, evapotranspiration, and deep drain
age for different soil types and seasons confirmed the reasonable performan
ce of the APSIM model.
The APSIM model was run with historical weather records (70-90 years) acros
s 2 transects from the coast (high rainfall zone) to the eastern edge of th
e wheatbelt (low rainfall zone). Soils were classified as 5 major types: de
ep sand, deep loamy sand, acid loamy sand, shallow duplex (waterlogging), a
nd clay soil (non-waterlogging). Simulations were carried out on these soil
types with historical weather records, assuming current crop management an
d cultivars. Soil water profiles were reset each year to the lower limit of
plant-available water, assuming maximum water use in the previous crop. Re
sults stressed the high degree of seasonal variability of deep drainage ran
ging from 0 to 386 mm at Moora in the high rainfall region (461 mm/year ave
rage rainfall), from 0 to 296 mm at Wongan Hills in the medium rainfall reg
ion (386 mm/year average rainfall), and from 0 to 234 mm at Merredin in the
low rainfall region (310 mm/year average rainfall). The largest amounts of
drainage occurred in soils with lowest extractable water-holding capacitie
s. Estimates of annual drainage varied with soil type and location. For exa
mple, average (s.d.) annual drainage at Moora, Wongan Hills, and Merredin w
as 134 (73), 90 (61), and 36 (43) mm on a sand, and 57 (64), 26 (43), and 4
(18) mm on a clay soil, respectively. These values are an order of magnitu
de higher than drainage reported elsewhere under native vegetation. When no
t resetting the soil each year, carry-over of water left behind in the soil
reduced the water storage capacity in the subsequent year, increasing long
-term average deep drainage, depending on soil type and rainfall region. Th
e analyses revealed the extent of the excess water problem that currently t
hreatens the sustainability of the wheat-based farming systems in Western A
ustralia.