A methodological extension of the classic normalised cumulative loads analysis: combination of normalised cumulative loads and hydrograph separation for nutrient exports during precipitation events.

The analysis of nutrient dynamics during precipitation events using normalised cumulative loads (NCL) is a classic and frequently used method of water quality analysis. Normalised pollutographs combine the cumulative nutrient load and cumulative runoff of an event in one plot to better interpret the nutrient export dynamics of a catchment. Essentially, the strength of a first-flush event or an ongoing dilution can be quantified using NCL. However, different runoff generation processes may be superimposed, hampering process analysis. Thus, our approach combines the classical pollutograph with hydrograph separation using stable water isotopes. The pollutograph is set up for each of the separated runoff components (pre-event water and event water) and the export dynamics of the runoff components of the same event are compared with the dynamics of the total runoff via the calculated area under the curve. K-Means cluster analyses were used both to categorise the functional behaviour of the runoff components and to identify any changes in export dynamics between the runoff components.

This method was applied in the catchment area of the ‘Nesselbach’, which is located near Hofgeismar (district of Kassel) in the northern foothills of the West Hessian depression. The catchment area is primarily characterised by agricultural land use, which is accompanied by a small forest and a small settlement. All necessary climate parameters were recorded by a weather station located in the catchment. The sampling period extended from 1 February 2021 to 1 August 2022. Measurements of nitrate concentrations and runoff were recorded using high-resolution optical in situ probes (resolution: 5 min), supported by automatic samplers to collect phosphorus, major ions and isotope samples during 15 precipitation events (resolution: 15 min). All parameters were additionally calibrated by regular manual sampling with laboratory values.

Using the extended method, our results show clear differences in the export dynamics of phosphorus and nitrate, as well as unexpected results in the direct comparison of export dynamics between total runoff and event water. Phosphorus shows similar dynamics in the event water as in the total runoff (of the same event), with a tendency to more extreme values. For nitrate, on the other hand, the dynamics of nitrate export sometimes changed drastically; export dynamics tending towards dilution in the total runoff show wash-off tendencies in the event water and vice versa. The same approach was also used to investigate the export dynamics of the ions Na, Ca, SO₄, K, Mg and Cl, which served to increase the sample size of the analyses carried out and thus to confirm the robustness of the method.