Run-Off and Soil Movement on Mid-Slopes in North-East Queensland [Australia] Grazed Woodlands.

Abstract

Run-off, bedload and sediment concentration data were collected over a five-year period from unbounded catchments in grazed and exclosed pastures in woodlands. Cover varied from 4% during drought conditions to almost 100% in exclosed areas after above-average rainfall. High bedload soil loss, sediment concentration and run-off percentages were associated with low cover (<30%). Run-off as a percentage of rainfall increased linearly with rainfall intensity; decreased linearly with cover; decreased slightly as soil moisture status declined; and reached a maximum at intermediate rainfall events. Interactions between these factors were observed. Run-off was up to 30% of rainfall in moderate rainfall events (30-40 mm) where maximum rainfall intensity over any 15 minute period (I15) exceeded 70 mm/h. When soil moisture status was high, mean run-off exceeded 30% for 40-80 mm rainfall events. For all rainfall event sizes, run-off exceeded 20% where I15 exceeded 60 mm/h. Cover had very little effect on run-off when rainfall intensity was low (I15<20 mm/h), soil water deficit was low (<10 mm) or when rainfall events were >75 mm or <10 mm. Bedload plus suspended sediment loads ranged from negligible to 1000 kg/ha/a, depending principally on cover. Soil movement from areas with >40-50% cover was very low. Pastures dominated by Bothriochloa pertusa (a stoloniferous, naturalised grass) had lower run-off and lower rates of soil movement than pastures dominated by Heteropogon contortus (a native tussocky perennial grass) when compared at the same level of cover. Differences between grazed and exclosed areas could be attributed solely to differences in cover.

Hydrological heterogeneity in Mediterranean reclaimed slopes: runoff and sediment yield at the patch and slope scales along a gradient of overland flow

“…This phenomenon has been scarcely analysed: most previous studies depict hydrological heterogeneity for stable systems throughout the world where sources and sinks are coupled under a dynamic equilibrium state (Ludwig et al, 1997(Ludwig et al, , 2005, or address the stability of coupled systems under several types of disturbances, mainly fires and overgrazing, which reduce vegetation cover, and thus runoff obstruction, increasing runoff and erosion rates (Wilcox et al, 2003;McIvor et al, 1995;Scanlan et al, 1996). Our research describes the variation of the sink-source pattern in a gradient of ecological recovery (driven by a gradient of overland flow volume) after slope reclamation.…”

Characterisation of Hydrological Response to Rainfall at Multi Spatio-Temporal Scales in Savannas of Semi-Arid Australia

“…The influence of ground cover on runoff and soil movement in small plots highlights the relationship between sparse (<40%) ground cover and runoff generation [19]. However, for very large events with high rainfall intensities (>100 mm/h), cover had little influence on runoff [18,19]. In all these aforementioned studies, only temporal variability of rainfall was considered, not spatial variability throughout the catchment.…”

“…At smaller scales (i.e., plot), studies have established that annual runoff varies considerably with the spatial distribution of ground cover [17]; however, cover may have little effect on overland flow during very large rainfall events (>100 mm with intensities between 45 and 60 mm/h) when the landscape is inundated with surface runoff due to Hortonian overland flow processes [18][19][20]. Roth [21] showed that high ground cover values (>75%) can promote infiltration during high intensity events.…”

Characterisation of Hydrological Response to Rainfall at Multi Spatio-Temporal Scales in Savannas of Semi-Arid Australia

“…The influence of ground cover on runoff and soil movement in small plots highlights the relationship between sparse (<40%) ground cover and runoff generation [19]. However, for very large events with high rainfall intensities (> 100 mm/h), cover had little influence on runoff [18,19]. Several studies in the GBR have shown that converting forest to pasture can increase runoff by ~80% at sub-catchment scales [27] and ~40% for river basin scales [28].…”

“…At the smaller scale (i.e., plot), studies have established that annual runoff varies considerably with the spatial distribution of ground cover [17], however, cover may have little effect on overland flow during very large rainfall events (>100 mm with intensities between 45-60 mm/hr) when the landscape is inundated with surface runoff due to Hortonian overland flow processes [18][19][20]. Roth [21] showed that high ground cover values (>75%) can promote infiltration during high intensity events.…”