The capeweed (Arctotheca calendula) content of pastures in the agricultural areas of Western Australia was estimated from coloured aerial photographs taken during flowering. Linear regressions were obtained between a visual score for capeweed content based on colour and the actual capeweed content of calibration sites. Surveys in 1972, 1973 and 1975 showed that 1973 was a year of high capeweed content in all areas compared with 1972 and 1975. The content was higher in lower-rainfall wheatbelt areas, where it averaged about 50% of pasture dry matter in 1973, than in the high-rainfall grazing areas, where the average was 37%. Fluctuations from year to year were followed on fixed sites between 1973 and 1977. The high rainfall sites varied more from year to year in capeweed content than did the low-rainfall sites. A detailed survey of one farm was made between 1972 and 1976 and this confirmed the indications from the other broadscale surveys that 1973 and 1976 were years that favoured capeweed. They were years when germination was followed by a 4-5 week dry period. Soil type and position in the landscape were also shown to influence the capeweed content of pastures.

A computer simulation model* of the water balance for plants growing on coarse soils was developed and tested against field measurements. The inputs for this model are measurable physical parameters. From the close agreement between simulated and observed results, it is suggested that evaporation, soil water storage and deep drainage may be satisfactorily predicted.

No abstract

No abstract