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Cover effect on runoff in grazing country

In the grazing country, ground cover varies considerably over time and in space as a result of climatic variations, soils, and the grazing intensity. At present, soil erosion prediction for the  grazing land is based on rainfall as the only factor to represent the effect of climate and catchment hydrology, although it is widely known that runoff decreases as the level of ground cover increases, other factors being the same (Fig.1 below). As runoff amount and the peak runoff are sensitive to the level of ground cover in grazing land, methods are needed to estimate runoff amount and the peak runoff rate that vary in space for application in the Great Barrier Reef catchments.

For this DES supported project, methods have been developed to predict spatiotemporal variations in runoff and the peak runoff rate from runoff totals and the peak rainfall intensity that are available at the sub-catchment scale (50-100 km2).  Two alternative methods were developed to disaggregate the runoff total associated with the outlet of each sub-catchment into runoff amount for individual 30-m grid cells.

The first method assumes an exponential decrease in the volumetric runoff coefficient as the ground cover increases, and the second assumes a linear decrease in the SCS Curve Number. In either case, the decrease in runoff is truncated at a threshold ground cover, beyond which the effect of cover on runoff is negligible. For prediction of the peak runoff rate, the scaling technique that requires rainfall and runoff amounts in addition to the peak rainfall intensity as input was developed.

Fig. 2 (above) shows an example of predicted runoff as a function of rainfall and ground cover that varies explicitly in space.  The methodology has been extensively tested using field data from a number of sites in Queensland.  Work is under way to implement this new methodology to predict runoff and peak runoff rate as a function of ground cover for grazing land in the Great Barrier Reef catchments.

Publications:

Yu, B. 2020. Prediction of cover-dependent runoff amount and peak runoff rate for grazing land in Queensland, Technical Report https://science.des.qld.gov.au/__data/assets/pdf_file/0018/202653/qwmn-prediction-cover-dependent-runoff-peak-runoff-rate-grazing-land-qld.pdf

Tiwari, J., B. Yu, B. Fentie, and R. Ellis. 2020. Probability distribution of groundcover for runoff prediction in rangeland in the Burnett- Mary Region, Queensland. The Rangeland Journal, https://doi.org/10.1071/RJ19082

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