Land-to-water transfer of nutrients: What knowledge can be gained by combined analysis of river water quality and flow records?

Abstract

Waikato Regional Council currently collects monthly grab samples for monitoring of water quality at 114 sites throughout the region, both from the Waikato River and from smaller stream and rivers. The samples are analysed for a range of water quality parameters, and since 2004, long term water quality trend analysis has been carried out at 5-yearly intervals. Although water flow has not been recorded at many of the sites, flow adjustment has routinely been carried out as part of this process, in an attempt to remove the effects of flow trends from the analysis. However, access to flow records that are matched to water quality sampling sites also allows a wider range of analyses to be performed. The purpose of this report was to investigate potential advantages of establishing flow recording in parallel to the existing monthly water quality sampling programme, in order to allow a greater degree of information to be extracted from the water quality data. The report identified 26 water quality sampling sites across the Waikato Region where continuous flow records were available, either at the sampling site, or nearby. These flow records were used in several ways to better understand the time series of total nitrogen (TN), nitrate-nitrite nitrogen (NNN), ammoniacal nitrogen (NH4), total phosphorus (TP), dissolved reactive phosphorus (DRP) and the non reactive phosphorus fraction (TP-DRP) collected at these sites. Silica concentration (Si) and electrical conductivity (EC) were also considered, as possible indicators of water age. Four sets of analyses were carried out.