Uncertainties in estimated phosphorus loads as a function of different sampling frequencies and common calculation methods

Abstract

Water quality monitoring programs are often based upon low-frequency regular sampling regimes from which loads are estimated. In this study, stream flow (Q) and phosphorus concentrations (C) were measured at 2-hourly intervals over a 10-week period between October and December 2006 in a tributary of Loch Leven, Scotland. The dataset was deconstructed to emulate different weekly, daily and composite sampling strategies, the aim being to highlight the large amount of uncertainty and imprecision in estimating total (TP) and soluble reactive (SRP) phosphorus loads on the basis of commonly applied sampling strategies and calculation methods. When based on the full dataset, phosphorus (P) loads estimated from the 2-hourly data were 459 kg TP, 351 kg particulate P (PP) and 78 kg SRP. In contrast, P loads estimated from different weekly, daily and composite sampling regimes and determined by applying seven different calculation methods ranged from 22 to 5028 kg TP, 13 to 4588 kg PP and 7 to 286 kg SRP. The results of this study highlight the large amount of uncertainty and imprecision associated with estimating P loads and contributes to the body of evidence that high-frequency monitoring is necessary if P loads to standing water bodies are to be quantified accurately and the effects of nutrient management programs interpreted correctly.