In this study, SHAP was employed to estimate the relative importance of the input features in terms of the prediction results of the XGB with SMOTE. Although the performance of the XGB and RF did not differ significantly after applying SMOTE, here, the XGB was selected because its performance was more stable under a wide range of IR values compared to the performance of RF. After applying SHAP to the prediction results of XGB for each study site, the relative importance (i.e., the mean |SHAP| value) of the water temperature and nutrient-related water quality variables, including T-N and T-P, was higher than that of other environmental variables (Table 5). Note that the importance of water temperature (Cha et al. 2017) and nutrients (Richardson et al. 2019) for cyanobacterial bloom has been reported consistently in previous studies.

Table 5

The five most important input features in the prediction of HABs using XGB with SMOTE

LakesInput features (mean |SHAP value|)
Angye NO3-N (0.0801) Wtemp (0.0703) Prec (0.0629) SS (0.0572) T-N (0.0567)
Daecheong SS (0.1064) PO4-P (0.0604) TOC (0.0594) Irr (0.0535) T-P (0.0338)
Gwanggyo TOC (0.1108) NO3-N (0.1066) T-N (0.0655) T-P (0.0523) SS (0.0514)
Jinyang NO3-N (0.1396) TOC (0.0911) Wtemp (0.0757) T-N (0.0333) SS (0.0324)
Paldang TOC (0.1211) Wspeed (0.0802) Wtemp (0.0724) T-N (0.0600) Irr (0.0560)
Sayeon Wspeed (0.0678) Prec (0.0668) Irr (0.0500) Wtemp (0.0498) T-N (0.0459)
Unmun PO4-P (0.1924) T-P (0.1195) TOC (0.0811) Wtemp (0.0414) T-N (0.0298)
Yeongcheon TOC (0.1228) Wtemp (0.0686) SS (0.0529) T-P (0.0464) T-N (0.0418)
LakesInput features (mean |SHAP value|)
Angye NO3-N (0.0801) Wtemp (0.0703) Prec (0.0629) SS (0.0572) T-N (0.0567)
Daecheong SS (0.1064) PO4-P (0.0604) TOC (0.0594) Irr (0.0535) T-P (0.0338)
Gwanggyo TOC (0.1108) NO3-N (0.1066) T-N (0.0655) T-P (0.0523) SS (0.0514)
Jinyang NO3-N (0.1396) TOC (0.0911) Wtemp (0.0757) T-N (0.0333) SS (0.0324)
Paldang TOC (0.1211) Wspeed (0.0802) Wtemp (0.0724) T-N (0.0600) Irr (0.0560)
Sayeon Wspeed (0.0678) Prec (0.0668) Irr (0.0500) Wtemp (0.0498) T-N (0.0459)
Unmun PO4-P (0.1924) T-P (0.1195) TOC (0.0811) Wtemp (0.0414) T-N (0.0298)
Yeongcheon TOC (0.1228) Wtemp (0.0686) SS (0.0529) T-P (0.0464) T-N (0.0418)

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