Impacts of sampling size on estimation accuracy of road runoff event mean concentration-taking time-interval sampling method for example
CHEN Ying, WANG Zhao, ZHAO Jian-qiang, YANG Wen-juan
Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of Ministry of Education, School of Environmental Science and Engineering, Chang'an University, Xi'an 710054, China
Abstract：In order to investigate the impacts of sample size on estimation accuracy of road runoff event mean concentration (EMC), road runoff samples over the whole time range of 3 storm events occurred at Taibai Interchange of the south second-ring road in Xi'an, Shaanxi province were collected by the manual time-interval sampling method from August to November 2014, and concentrations of SS, COD, Pb, Zn, dissolved Pb and dissolved Zn of the samples were determined. Then, self-compiled VBA programing based on random sampling method was utilized for the estimation. The results indicated that, in different types of rainfall events, pollutant concentrations in the initial duration of runoff were relatively high, fluctuated sharply and were affected by the rainfall intensity significantly. However, pollutant concentrations in mid and late duration of runoff were decreased and tended to be stable. For improving the estimation accuracy of EMC, runoff samples should be collected in the whole duration of rainfall event, and in the initial duration of runoff, the runoff samples should be collected intensively. The estimation accuracy of EMC was significantly influenced by sampling size when using time-interval sampling method. The estimated EMC based on concentrations of samples and runoff volume of corresponding periods dispersed widely when sample size was small. However, the dispersion of the estimated EMC decreased with the increasing of sample size, the estimated EMC tended to be stable as well. The maximum relative error between the estimated EMC based on different pollutant indexes in different runoff events and the approximate true value of EMC was decreased with the increasing of sample size. However, the degrees of decrease which were related to the fluctuation of water quality and volume of runoff were quite different. If the sample size was less than 10, the maximum relative error between the estimated EMC and the approximate true value of EMC was close to 40%, and the error could be reduced to 30% and 20% when the sample size were increased to 13 and 19 respectively.
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