Effect of cultivation and raindrop impact on the surface hydraulic properties of an Alfisol under wheat

Abstract

In tilled soils, hydraulic properties required as input to process-based water balance models may undergo temporal changes during the cropping cycle, particularly as a result of raindrop impact. This study was conducted to investigate the effect of raindrop induced aggregate breakdown on changes to soil hydraulic properties in an Alfisol sown to wheat under minimum tillage and conventional cultivation at Kapunda, South Australia. Unsaturated hydraulic properties of the sandy loam A horizon were measured at two stages in the cropping cycle during Year 7 of the trial using tension infiltrometers. The surface was also subjected to simulated rainfall and the hydraulic properties re-measured after the soil water content had returned to the initial pre-rain value. Results showed that a pre-seeding, the total porosity was greater under conventional cultivation (CC) (0.56 m 3 m −3), that under direct drilling (DD) (0.48 m 3 m −3). Much of the additional porosity under CC was restricted to large voids that were air filled at a matric head of just −20 mm. In consequence, at this supply potential, the hydraulic conductivity was significantly greater under DD (35.6 mm h −1) compared with CC (15.3 mm h −1). The sorptivity also followed a similar trend (11.2 mm per 30 min under CC compared with 18.4 mm per 30 min under DD). Later in the cropping cycle, the hydraulic properties displayed no significant differences between treatments. This was attributed to consolidation of the conventionally cultivated seedbed under natural rainfall, with total porosity decreasing from 0.56 at pre-seeding to 0.48 at post-harvest. Simulated raindrop impact had no effect on hydraulic conductivity or sorptivity measured at a supply potential of −40 mm, but at a supply potential of −20 mm the measured hydraulic conductivity was reduced to 4.5 mm h −1 at pre-seeding and 8.6 mm h −1 at post-harvest. These values were similar to the pre-rainfall simulation measurements at −40 mm and suggest that the effect of aggregate disruption from raindrop impact was predominantly restricted to pores of equivalent radius >0.75 mm. Time to incipient ponding was predicted adequately using soil properties obtained at a supply potential of −40 mm prior to rainfall, primarily because infiltration of intense rainfall into dry soil was dominated by sorptivity. It is concluded that tension infiltrometers provide a rapid and practical method for surveying temporal changes to surface hydraulic properties and that at this site there is no sustained influence of tillage on the measured hydraulic properties.