3.4.1 Annual changes
The TLI( Σ ) of the three reservoirs showed large differences between years (Fig. S4). The annual mean TLI( Σ ) values of Huanglishu Reservoir varied in a wide range from 2019 to 2021 (43.92 ~ 46.68), followed by Shahe Reservoir (41.39 ~ 43.10), while there was almost no change in Chengxi Reservoir (44.84 ~ 45.39). Furthermore, the annual average TLI( Σ ) values in Shahe Reservoir are generally lower than those in Huanglishu Reservoir and Chengxi Reservoir, with relatively good water quality. Interestingly, the annual average TLI( Σ ) values of all three reservoirs exhibited maximum values in 2020 and minimum values in 2019, with the TLI( Σ ) of Huanglishu Reservoir (50.21 ~ 43.1) exceeding 50 in 2020, and the nutrient status of the water body was mildly eutrophic in that year, with a water quality level of Class III (Table 1), while none of the other reservoirs reach this standard.