3.4 Eutrophication of reservoir water quality
Through using the TLI method to derive the TLI ( Σ ) value of each reservoir, we found that the eutrophication level of Huanglishu Reservoir was the most serious during the three years, followed by Shahe Reservoir, and optimal for Chengxi Reservoir. The highest value of TLI( Σ ) in the Huanglishu Reservoir was 52.38 and the lowest value was 40.40, the water body was medium nutrition most of the time, but there existed three periods of time when the TLI( Σ ) was greater than 50, which was in the mild eutrophication status. This result indicates that the water quality of the Huanglishu Reservoir has been mildly polluted. The eutrophication evaluation results of the water body in Shahe Reservoir are similar to the Huanglishu Reservoir, with the fluctuation range of TLI( Σ ) from 40.96 to 51.29, and the water quality evaluation level is mild eutrophication only in October 2020, and the TLI( Σ ) tends to be close to 50 in most of the other periods, which is on the edge of the medium nutrient water quality. The TLI ( Σ ) value of Chengxi Reservoir was always below 50, which did not reach the criterion of mild eutrophication, and the qualitative evaluation of the water body was good, the results indicated that the water body of the reservoir was lightly polluted. As a whole, the water quality class of Huanglishu Reservoir and Shahe Reservoir reached Class III, whereas Chengxi Reservoir was always maintained at Class II (Fig. 3 and Table 1).