Figure 7 Proportional contribution of four potential nitrate
pollution sources (a) surface water in November 2021, (b) groundwater in
November 2021, (c) surface water in April 2022, (d) groundwater in April
2022
3.5 Uncertainty Analysis and Sensitivity
Analysis
Sensitivity and uncertainty analysis of Bayesian isotope mixing models
are essential parts of nitrogen source apportionment research.
Uncertainty analysis can evaluate the uncertainty of the nitrate source
allocation results and help identify the main sources of pollution. By
calculating the UI90 of each potential nitrate source,
the uncertainty level of the nitrate source allocation results
calculated by the SIAR model can be evaluated (Ji et al., 2017). The
5%-95% interval can control 90% of the interval and eliminate the
influence of extreme values on the results. Taking the contribution
rates of MS in surface water in April 2022 as an example, although the
maximum contribution rate reached 0.96, the contribution rate in the
90% interval was only 0.527 (UI90=0.586). Sensitivity analysis can
determine which end-members have the greatest range effect on the
nitrate source allocation results, thereby helping to optimize
experimental design and improve the accuracy of model predictions.
The uncertainties in the surface water and groundwater contributions for
the two sampling periods are shown in Figure 8. As can be seen from
Figure 8, for surface water, the percentage contribution of AD is
relatively stable, with UI90 of 0.115 and 0.072 for AD
in November 2021 and April 2022, respectively, ranging from 7.7% to
16.1% with 90% probability, although its maximum percentage can reach
24.8%. Larger uncertainties were associated with SN
(UI90=0.584 and 0.718) and NF
(UI90=0.416 and 0.737), and M&S
(UI90=0.416 and 0.586) showed moderate uncertainties.
For groundwater, the larger uncertainties came from SN
(UI90 = 0.735 and 0.651) and NF (UI90 =
0.703 and 0.54), with M&S showing moderate uncertainties
(UI90 = 0.452 and 0.623) and AD showing similarly
minimal uncertainties (UI90 = 0.157 and 0.161).
Groundwater has a greater uncertainty than surface water, which is
related to the conditions of groundwater occurrence. Pollutants emitted
by human activities pass through the unsaturated zone before entering
the aquifer, during which they undergo various complex biogeochemical
reactions under the influence of microorganisms. There is greater
uncertainty in groundwater in April 2022 compared to November 2021,
which is related to increased human activities in April such as
fertilization for spring crops.