Fallow soil evaporation in a grey Vertisol under contrasting wheat stubble management practices in cotton cropping systems

In comparison with incorporating wheat stubble, soil water storage in Vertisols is believed to be enhanced by standing wheat stubble. It is widely believed that a significant proportion of the enhanced water storage is attributable to reductions in soil evaporation. The objective of this study was to quantify the differences in fallow soil evaporation in a Vertisol under two cotton (Gossypium hirsutum L.)-wheat (Triticum aestivum L.) rotation systems where wheat stubble was either retained as an in situ mulch (standing stubble) or incorporated. Soil cores were extracted from the surface 70 mm of beds after the wheat phase of wheat stubble incorporated (post-incorporation) and standing wheat stubble plots in an ongoing cropping systems experiment near Narrabri, NSW during the 2008–09 and 2009–10 summer fallow periods. The cores were saturated, drained and subjected to drying cycles under two evaporation rates (4 and 6 mm d⁻¹) during which evaporation was assessed by weighing the cores. Although cumulative evaporation was generally greater with wheat stubble incorporation than with standing stubble, the differences were small. These results suggest that the more effective water storage observed under the latter practice when rainfall was the major source of water may be not due to large reductions in evaporation but to enhanced infiltration.