Surface runoff experiments on the eastern Darling Downs

Abstract

Rainfall and runoff were measured from 1976 to 1980 on four 1 ha catchments at Greenmount, about 25 km south of Toowoomba, to study the effects of crop and residue management practices. For the first 2 years, three cropping strategies were carried out - summer cropping, winter cropping, and double cropping. In the following 2 years, four summer fallow conditions and a summer crop were used in the experiment. On average, about 4% of rainfall became runoff, and this was highly variable from storm to storm and among the surface treatments. U.S. Department of Agriculture curve numbers were calculated for each occurrence of runoff and grouped according to stubble management practice. For low, medium and high antecedent moisture, curve numbers averaged 70, 82 and 95 where stubble was burnt, compared with 55, 65 and 75 where stubble was incorporated or mulched. Time of concentration of the catchments was found to be in the order of 10-15 min from studies of unit hydrographs and rational method runoff coefficients. Runoff coefficients varied with amount of surface cover and rainfall intensity, and were in the same range of values found elsewhere in Australia. A multiple linear regression was derived to relate runoff coefficient to surface cover and rainfall intensity. The highest observed runoff rates from the experiment were used to check estimates of design flood flows from the procedures in the Queensland Soil Conservation Handbook and in 'Australian Rainfall and Runoff. The estimates from these design manuals compared very well with the recorded data.