Koci, Jack, Sidle, Roy C., Kinsey-Henderson, Anne E., Bartley, Rebecca, Wilkinson, Scott N., Hawdon, Aaron A., Jarihani, Ben, Roth, Christian H., and Hogarth, Luke (2020) Effect of reduced grazing pressure on sediment and nutrient yields in savanna rangeland streams draining to the Great Barrier Reef. Journal of Hydrology, 582. 124520.

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Abstract

Excess sediment and nutrient yields from degraded rangelands have detrimental off-site ecological and on-site economic impacts, yet the effect of reduced grazing pressure on water quality is not fully understood. This study compares 15-year records of runoff, sediment and nutrient yields amongst three ephemeral headwater catchments (10.5–13.5 km2) with similar wet/dry tropical climate and landform characteristics, but contrasting cattle grazing pressure. The catchments are located within the Burdekin River basin which is the largest source of sediment and particulate nutrients to the Great Barrier Reef, Australia. Estimated mean annual total suspended sediment (TSS), total nitrogen (TN) and total phosphorus (TP) loads are: 0.6 t ha−1, 1.3 and 0.3 kg ha−1, respectively, in a catchment from which cattle were excluded over the study period (‘Main Creek’); 1.5 t ha−1, 1.4 and 0.4 kg ha−1, respectively, in a catchment where grazing pressure was reduced from approximately 25 to 13 cattle per km2 at the commencement of monitoring (‘Weany Creek’); and 1.4 t ha−1, 3.0 and 0.5 kg ha−1, respectively, in a catchment which maintained higher levels of grazing pressure (‘Wheel Creek’, approximately 25 head per km2). Higher annual sediment loads in the grazed catchments compared to the ungrazed catchment are likely due to degradation of soil water storage capacity, soil surface protection and gully erosion rates, but could also be influenced by differences in catchment geomorphic attributes (e.g., hydrological and sediment connectivity). Events with runoff >20 mm (16–25% of all events) generated 79–85% of the total runoff volume and 71–78% of the total TSS, TN, and TP loads. At the event timescale, there are not clear differences in runoff, sediment and nutrient loads among the catchments, attributed to variability in catchment conditions (e.g., antecedent soil moisture, rainfall intensity, vegetation) that occur within- and between events. It is concluded that recovery of degraded savanna rangelands following reduction in livestock grazing pressure takes decades and is strongly influenced by climate. Measuring water quality responses to land management change in variable climates requires nested spatial monitoring over long time scales that also includes factors that can influence the response (e.g., climate, soil properties, vegetation and land use).

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