This paper investigates factors controlling soil water content changes during the non-growing summer–autumn season or fallow (December–May) in annual farming systems in southern Western Australia. This was achieved by examining variation in available soil water storage to a depth of 1.0–1.5 m at 3 sites within 13 seasons. Reasons for the variation were examined using the Agricultural Production Systems Simulator (APSIM). This paper also investigated whether water accumulation during the summer–autumn period (fallow) contributed to drainage during the following growing season (May–November). This was achieved by determining the relationship between soil water content at the end of the fallow period (1 May) and the amount of drainage below 2.5 m by using APSIM coupled to historical weather records at 3 locations.

At the end of the fallow, 24 mm (or 25%) of rain falling during the fallow was retained in the soil. Evaporation was the main loss of soil water during fallow periods (mean of 60 mm). Other losses included transpiration from plant cover (mean of 12 mm) and drainage below the root zone and runoff (combined mean of 13 mm). Evaporation and transpiration losses of soil water were concentrated in the surface 0.3 m. The use of APSIM to determine changes in the soil water content during the fallow indicated the importance of plants to soil water losses, the potential for higher evaporation than previously reported, and the possibility of an extended period (4–6 weeks) of drainage in sandy soils after large rainfall events (>50 mm).

Soil water accumulation during the fallow period had a significant effect on simulated drainage under wheat in the following growing season. By the end of fallow there was limited ability of the soil to store water before drainage occurred due to rainfall during the fallow and the small soil water deficit under annual farming systems (1–67 mm). A 1-mm increase in soil wetness at the end of the fallow resulted in a 0.7–1-mm increase in simulated drainage during the growing season.