Seasonal dynamics of soil–water pressure in a cracking clay soil

This paper describes the results of an investigation of changes in soil water pressure head (ψ) and its relationship to the macropore network in a cracking clay soil. Four vertical nests, each consisting of three tensiometers positioned at depths of 30 cm, 60 cm and 90 cm below the surface, were monitored continuously over a two-year period to study changes in ψ. On one occasion an anionic tracer (Br-) was applied to investigate the extent of macropore flow. The results revealed considerable temporal variation in ψ with consistent variations between adjacent tensiometer nests. Variations in ψ indicated the seasonal development of a soil macropore system, followed by its subsequent decay and demonstrated the significant effect of rainfall intensity, duration and timing on percolation pathways. Differences in ψ were examined for individual summer rain events which were characterised by differences in precipitation amount and intensity. A total of 79 rain events extending across the period of study were analysed to assess the degree to which time-invariant parameters can be used to describe changes in ψ at a depth of 30 cm below the surface. The results indicated that individual regression models had considerable success in predicting ψ, although the residuals in the regression models were high for the specific case of large summer rain events, and in particular for three events.