S. Asseng et al., Potential deep drainage under wheat crops in a Mediterranean climate. II. Management opportunities to control drainage, AUST J AGR, 52(1), 2001, pp. 57-66
High rates of deep drainage in Western Australia are contributing to ground
water recharge and secondary salinity. Strategies are being sought to incre
ase water use in cropping systems and to reduce deep drainage. Quantifying
potential drainage through measurements is hampered by the high degree of c
omplexity of these systems as a result of diverse soil types, a range of cr
ops, and in particular the inherent seasonal variability. Simulation models
can provide the appropriate means to extrapolate across time and space. Th
e Agricultural Production Systems Simulator (APSIM) was used to explore the
effect of alternative agronomic practices on wheat production and deep dra
inage for representative soils and rainfall regions of the central wheatbel
t of Western Australia. Soil water profiles were reset each year to the low
er limit of plant-available water, assuming maximum water use in the previo
us crop. The long-term simulation studies showed that management practices
with N fertiliser directed at yield increase were most effective in achievi
ng these aims in the medium to high rainfall regions. The corresponding eff
ect for drainage reduction was marginal. The small effect on drainage contr
ol associated with production increase can be traced to the effect of rainf
all distribution with major occurrences of both rainfall and drainage durin
g winter (June-August) coinciding with the lowest potential atmospheric dem
and for evapotranspiration, in combination with low water-holding capacity
soils. Nitrogen-induced increases in crop transpiration were in conjunction
with reduced soil evaporation, which increased water use efficiency and oc
curred mostly after the main drainage period, but had little effect on deep
drainage within the season. Similar outcomes of enhanced productivity with
minor impact on deep drainage were noted with crops sown at different time
s and with a hypothetical wheat crop having a deeper rooting system.
Simulations without resetting soil water each year enabled the quantificati
on of potential carryover effects on long-term average deep drainage. The c
arry-over of soil water left behind at crop harvest reduced the water stora
ge capacity of the soil in a subsequent year and could increase long-term d
eep drainage substantially, depending on soil type. Improved management inc
reased late water use in the high rainfall region, in particular on better
water-holding soils, and could largely reduce this carry-over effect.
The current wheat-based cropping systems, even with alternative management
practices, continue to be a major threat to sustainability on the low water
-holding soils in the wheatbelt of Western Australia, as a main cause of se
condary salinity.