Runoff and soil loss in a semi-arid shrub invaded Poplar Box (Eucalyptus populnea) woodland.

Abstract

The ridges of the Cobar pediplain generally have only a sparce herbage cover. Much of the incident rainfall is reputed to run off, perpetuating the low pasture productivity and causing erosion. Woody shrubs have often invaded the woodlands of the pediplain. On some gentler sloping (1%) ridges dense patches of shrubs surround large eucalypts and together are known as 'thickets'. Between these thickets relatively sparsely shrubbed areas ('interthickets') occur. Runoff from small plots situated in thicket and interthicket areas of shrub invaded poplar box woodland was recorded during 1975-1981. Half the plots had been partially cleared by pushing over all shrubs with a bulldozer ('shrub-pushed', in contrast to 'unpushed'). Soil losses from the plots were measured over a four year period. Rainfall during the study was 26% greater than the long term average for the area, and provided many runoff events, particularly from intense summer storms. Runoff averaged 26% of rainfall for unpushed interthicket areas while thicket runoff was negligible. Runoff from the shrub-pushed plots was c. 25% less than from their unpushed counterparts, but this difference was not significant at P < 0.05. During the December-March period monthly runoff from unpushed interthicket plots averaged 41% of monthly rainfall in excess of 12 mm, while for the other eight months runoff averaged 24% of monthly rainfall in excess of 9 mm. Soil losses were greater during the stormier part of the year, with long term soil loss rates equivalent to 25 mm per 1000 years from enclosed interthicket plots and 55 mm per 1000 years from the gra7ed interthicket plots. Shrub-pushing appeared to increase soil losses in the short term but after two years soil loss rates decreased to be less than those from unpushed plots. Estimates of likely interthicket runoff based on long term rainfall records had a median value of 77 mm per year, or only 64% of that observed during the study period. Over the 88 years the 5 year moving average of estimated runoff fell to as low as 40 mm per year. The results are discussed in the context of waterspreading.