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Modelling soil water for direct drilling and conventional cropping strategies in southern N.S.W.

R.A. Fischer and J. Armstrong

CSIRO, Division of Plant Industry. P.O. Box 1600, Canberra City, A.C.T. 2601.

In conjunction with a program of field experiments initiated in 1979 to compare direct drilled crops with ones sown conventionally, we are attempting to model these systems. This communication describes initial efforts to model soil water in the pre-sowing phase. Conventional practice (C) is assumed to consist of an initial cultivation with the first rain (> 15 mm) after January 1, and further cultivations after rain provided at least 20 days has elapsed. There are no pre-sowing cultivations with direct drilling (D). The model presently neglects the effects of green plants or dead residues. The top soil is divided into four 5 cm thick layers; soil evaporation proceeds at a diminishing rate the deeper the layer from which it is occurring. Also, in accord with some observations of reductions in soil evaporation with cultivation (e.g. Greacen and Hignett 1976), soil evaporation from a recently cultivated surface is approximately 50% of that from an uncultivated surface when occurring from the second and deeper layers under otherwise similar conditions. The effect is such that after a rain, a cultivation, and then 100 mm Class A pan evaporation, 10 mm less evaporation from the cultivated compared to uncultivated surface (17 mm vs 27 mm) is predicted. If cultivated layers are fully wet by subsequent rain, conductivity rises to that of the uncultivated layer.

A simulation run used daily pan evaporation and rainfall for Canberra the Jan. 1 to June 30 period in each of 8 years (1971-77; mean total rainfall 353 mm and mean total pan evaporation 710 mm). Assuming zero top soil moisture on Jan. 1 and 10 mm water holding in each 5 cm soil layer, it predicted an average of 5 cultivations commencing on Jan. 24 for C. On average 115 mm (std dev. = 88 m) water was stored in the whole profile, compared to 100 mm (a = 86 mm) or 15 mm less for the uncultivated or D situation.

In order to estimate sowing date probabilities, it was assumed wheat could be sown only if the top layer contained 2-8 mm water (4-10 mm for D) and the second and third layers 7-10 mm and 5-10 mm, respectively. Averaged over the 8 years there were in March. April. May and June, respectively, 2, 3, 6 and 7 sowing days with C, and 1, 3,4 and 6 days with D. This suggests little difference between the two strategies. However the extra moisture stored in the profile with C tended to be concentrated in the fourth layer (15-20 cm). Average water (mm) in this layer was 8.2 (March), 8.1 (April), 9.9 (May) and 10.0 (June) for C. The corresponding figures for D were 4.8, 4.3, 6.2 and 6.0 mm. Refinements to these predictions as field data becomes available are expected.

Greacen, E.L. and Hignett, C.T. (1976). CSIRO Div. Soils Tech. Paper No.27. *

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