No abstract

Residues of hexazinone in stream water were monitored over a nine-week period after helicopter application of the herbicide at a rate of 2 kg ha-1 to a 46.4 ha experimental catchment. The aerial application was made in December 1981 following conversion of a steeply sloping catchment from native forest to Pinus radiata two years previously. Automatic samplers were used to sample stream water at intervals of 0.25-2.0 h throughout the nine weeks, with the more intensive sampling occurring during and immediately following spraying and during the only substantial storm event. A total of 69 representative samples were analysed, and 4 p.g L-1 hexazinone only was detected in six of these samples, which was well below the maximum recommended concentration for potable water of 600 p.g L-1 • Such low residues were attributed to several factors including the way the spraying operation was conducted (with respect to soil moisture, meteorological conditions and droplet size) and the presence of a 30 m wide vegetation reserve on each side of the stream.

Hewlett's (1977) work using his storm flow separation technique has established that peak rainfall intensities have little predictive value in the determination of storm flow depth or storm peak flow from small upland streams emanating from forested catchments. A logical but disconcerting inference is that storm rainfall intensities have little influence on either the volume of storm flow or peak flow. To obtain additional information a variant of the technique was applied to excellent records from two separate, steep, upland catchments in southern Australia. The catchments were regarded as having quite distinct rainfall response characteristics by hydrologists involved in data collection. Antecedent flow, storm rainfall, and measures of maximum rainfall intensity were supplemented by weighted integrals of rainfall intensity with time to test whether they gave additional insight into the influence of rainfall intensity. As expected, regression analysis showed that knowledge of maximum intensities gave little increase in predictive ability over storm rainfall. However, the time integral of rainfall intensity to the power of 1.5 was better correlated with peak flow than any other measure on both catchments. The results suggest that rainfall intensities are more important in the generation of peak flows than indicated by regression analysis using only peak rainfall intensities. Comparison of independent measurements of the variables using separated rain gauges showed that independent measures of the weighted variables were better correlated than measures of storm maximum intensities, indicating that they are also a more precise measure.