Computer simulation, combined with long-term historical weather information, can explain changes in soil water content during the non-growing season, or fallow and how it impacts on drainage in the growing season.
 
A Grains Reserach and Development Corporation (GRDC)-supported study by Dr Perry Dolling of the Department of Agriculture and Food WA and CSIRO researchers showed that water removed by a three to five year perennial pasture phase could reduce yield in the following annual crop.
 
The Agricultural Production Systems Simulator (APSIM) model demonstrated that soil water accumulation during the fallow significantly increased drainage under wheat in the following growing season.
 
A management strategy for growers would be to remove the perennial in spring before the year of the crop, allowing the soil profile to fill with rainfall before sowing.
 
Warm to hot temperatures and variable rainfall typically precludes growing annual crops or pastures during summer in southern WA, leading to an enforced fallow period.
 
Yet fallow rainfall during summer and autumn in southern WA can be significant and increase crop productivity the following growing season from May to September.
 
Fallow water losses are mostly due to evaporation, but also due to transpiration from weeds or water excess. Water excess is the result of runoff and drainage below the root zone.
 
Drainage is a consequence of several factors: heavy rainfall, low atmospheric demand during winter, low water holding capacity of the soil and short-lived, shallow rooted annual species.
 
Soil water deficits at the end of the growing season prevent drainage until rainfall increases the soil water content above field capacity.
 
Using the analogy that soil at the end of summer is an empty 'bucket,' it can take a long time to fill due to the high soil water deficit.
 
This is positive because it can help control salinity and negative because in a dry winter the crop will struggle to grow if the bucket is empty.
 
A dry soil profile is difficult to manage due to the conflicting management requirements of salinity and crop. For this reason, a phase of perennial pasture to control salinity followed by a phase of crop should be considered.
 
Efficiency of soil water storage from rainfall during the fallow is generally low, especially in coarse textured surface soils.
 
An average of 25 per cent of summer water inputs are retained in the soil at the end of the fallow.
 
As growing rainfall decreases, the trade-off between drainage and crop yields increases in importance. This is particularly so for fine textured soils.