Comparison of Record-Extension Techniques for Water Quality Variables

The extension of records at monthly, weekly or daily time steps at a short-record gauge from another continuously measured gauge is termed “record extension”. Ordinary least squares regression (OLS) of the flows, or any hydrological or water quality variable, is a traditional and still common record-extension technique. However, its purpose is to generate optimal estimates of each daily (or monthly) record, rather than the population characteristics, for which the OLS tends to underestimate the variance. The line of organic correlation (LOC) was developed to correct this bias. On the other hand, the Kendall-Theil robust line (KTRL) method has been proposed as an analogue of OLS, its advantage being its robustness in the presence of extreme values. In this study, four record-extension techniques are described, and their properties are explored. These techniques are OLS, LOC, KTRL and a new technique (KTRL2), which includes the advantage of LOC in reducing the bias in estimating the variance and the advantage of KTRL in being robust in the presence of extreme values. A Monte-Carlo study is conducted to examine these four techniques for bias, standard error of moment estimates and full range of percentiles. An empirical examination is made of the preservation of historic water quality concentration characteristics using records from the Nile Delta water quality monitoring network in Egypt. The Monte-Carlo study showed that the OLS and KTRL techniques are shown to have serious deficiencies as record-extension techniques, while the LOC and KTRL2 techniques show results that are nearly similar. Using real water quality records, the KTRL2 is shown to lead to better results than the other techniques.