Impact of calculation method, sampling frequency and hysteresis on suspended solids and total phosphorus load estimations in cold climate

Load calculations of nutrients and suspended solids (SS) transported by rivers are usually based on discrete water samples. Water quality changes in cold climate regions often occur very rapidly and therefore discrete samples are unrepresentative of the range of water quality occurring. This leads to errors of varying magnitude in load calculation. High-resolution turbidity data were used to determine the SS and total phosphorus (TP), and paired with discharge to determine loads from two small catchments in southern Finland. The effect of sampling frequency was investigated by artificially sub-sampling the high frequency concentrations. Regardless of the sampling frequency, the TP load was more likely underestimated while using discrete samples. To achieve ±20% accuracy compared with the reference load, daily sampling should be performed. Hysteresis was detected to have an impact on TP load. Hysteresis analysis also revealed the main source of the TP to be in the fields of the catchment. Continuous measuring proved to be a valuable method for defining loads and short-term fluctuations in water quality in small clayey watercourses in a boreal cold climate, where the climate change will increase the frequency of winter floods.