Assessing a 24/7 solution for monitoring water quality loads in small river catchments

Quantifying nutrient and sediment loads in catch- ments is difficult owing to diffuse controls related to storm hydrology. Coarse sampling and interpolation methods are prone to very high uncertainties due to under-representation of high discharge, short duration events. Additionally, im- portant low-flow processes such as diurnal signals linked to point source impacts are missed. Here we demonstrate a so- lution based on a time-integrated approach to sampling with a standard 24 bottle autosampler configured to take a sam- ple every 7 h over a week according to a Plynlimon design. This is evaluated with a number of other sampling strategies using a two-year dataset of sub-hourly discharge and phos- phorus concentration data. The 24/7 solution is shown to be among the least uncertain in estimating load (inter-quartile range: 96 % to 110 % of actual load in year 1 and 97 % to 104 % in year 2) due to the increased frequency raising the probability of sampling storm events and point source sig- nals. The 24/7 solution would appear to be most parsimo- nious in terms of data coverage and certainty, process sig- nal representation, potential laboratory commitment, tech- nology requirements and the ability to be widely deployed in complex catchments.