SIMULATION OF SOIL-WATER STORAGE AND SOWING DAY PROBABILITIES WITH FALLOW AND NO-FALLOW IN SOUTHERN NEW-SOUTH-WALES .2. STOCHASTICITY AND MANAGEMENT TACTICS

This paper examines annual variation in the effects of fallow on available soil water at sowing and on sowing day probabilities, derived from simulation using the soil water/weed growth model described in Part I (Fischer et al., 1990). The simulation assumed a typical red-brown earth soil profile and was continuous for the period 1943-1983 at Wagga Wagga (mean annual rainfall 566 mm) and at Griffith (420 mm), sites in the wheatbelt of southern New South Wales. The extra water stored on 1 May by long fallow (1 September start) compared to short fallow (November-December start), and short fallow compared to no-fallow, was moderately closely related (r2 = 0·44 to 0·53) to rain during the early part of the respective fallow periods. Effects on sowing time were estimated assuming wheat sowing required 15 days commencing on 2 May. On average, the median sowing date was 10-14 days later for no-fallow compared to fallow, being considerably later in some years due to topsoil dryness and somewhat earlier in fewer years due to better trafficability in the absence of fallow. Considering only the cost of tillage and the benefit of extra stored water and more timely sowing, increased duration of fallow did not show a positive net benefit in all years. Tactics whereby fallowing was only initiated according to criteria which might increase the chance of a positive outcome were investigated. The best tactics tested were based on fallow initiation if available soil water exceeded 40 mm. However, the improvement in cumulative net benefit never exceeded 30-50% of the difference in net benefit given by a fixed fallowing strategy (fallow in all years on a given date) and that which would have been given by perfect foresight (fallow only in those years giving positive net benefit). © 1990.