*3.4. Nutrient Estimates*

3.4.1. Nutrient Load

Figure 7a,b show the incremental loads of NH<sup>4</sup> + -N in each catchment in PLB and HRB. The incremental load ranged from 4513 to 1,468,867 kg/year in PLB. The middle reach of Ganjiang River delivered the maximum incremental NH<sup>4</sup> + -N load to Poyang Lake (Figure 7a). Fuhe River's downstream catchment and the upstream reach of Raohe River had large load deliveries. The upstream reach of Xinjiang River and the downstream reach of Ganjiang River had large incremental load.

In HRB, the incremental load ranged from 10 to 1,121,252 kg/year (Figure 7b). The upstream reach of Daqing River had the maximum incremental NH<sup>4</sup> + -N load delivery. The upstream reach of Ziya River and the middle reach of Daqing River made large incremental NH<sup>4</sup> + -N load deliveries. The upstream reaches of Majia River, Beisanhe River, and Zhangweinan Canal; and the downstream portions of Beisanhe River and Tuhai River, had large incremental NH<sup>4</sup> + -N load.

#### 3.4.2. Nutrient Sources

Nutrient source apportionment was analyzed for the major river watersheds in PLB and HRB (Tables 5 and 6, respectively) [57]. On average, NH<sup>4</sup> + -N from point sources (industrial and sewage discharge), farmland, woodland and grassland, and residential land accounted for 43.09%, 32.14%, 10.70%, and 14.07%, respectively, in PLB. Point sources were the main NH<sup>4</sup> + -N sources in the Ganjiang, Fuhe, Xinjiang, and Xiushui River watersheds. In the Raohe River watershed, farmland was the dominant NH<sup>4</sup> + -N source.

**Figure 7.** Incremental loads (kg/a) of NH4+-N in (**a**) PLB and (**b**) HRB. **Figure 7.** Incremental loads (kg/a) of NH<sup>4</sup> + -N in (**a**) PLB and (**b**) HRB.



accounted for 43.09%, 32.14%, 10.70%, and 14.07%, respectively, in PLB. Point sources **Table 6.** Nutrient sources' fluxes and fractions of main rivers in HRB.


Xiushui 628 48.56 397 30.67 146 11.32 122 9.46 1293 100.00 Total 11,359 43.09 8471 32.14 2820 10.70 3709 14.07 26,360 100.00 In HRB, NH<sup>4</sup> + -N from point sources (industrial and sewage discharge), farmland, woodland and grassland, and residential land accounted for, on average, 23.51%, 38.48%,

Raohe 656 15.18 1764 40.85 978 22.66 921 21.32 4318 100.00

eries.

4.14%, and 33.87%, respectively. In Luanhe, Yongding, Ziya, Zhangweinan Canal, and Tuhai-Majia River watersheds, farmland was the primary NH<sup>4</sup> + -N pollution source. In the Beisihe River watershed, residential land was the dominant NH<sup>4</sup> + -N source. Point sources were the main NH<sup>4</sup> + -N sources in the Daqing River watershed.

### **4. Discussion**

#### *4.1. Delivery Fraction*

Figure 8 illustrates the fraction of the load that each reach delivered to the target reach through the drainage lines of rivers. The fraction of NH<sup>4</sup> + -N entering Poyang Lake from rivers ranged from 0.61 to 1.00, with an average of 0.86 (Figure 8a). Meanwhile, the fraction of NH<sup>4</sup> + -N entering Bohai Bay from rivers in HRB ranged from 0 to 0.99, with an average of 0.22 (Figure 8b). As shown in Figure 8, the delivery fraction of a specific reach is related to its distance to the target reach. In both basins, the delivery fraction decreases as the distance increases. The average delivery fraction of HRB is much lower than that of PLB, because the streamflow of PLB is much larger than that of HRB and the terrain of PLB is steeper than that of HRB. *Water* **2022**, *14*, x FOR PEER REVIEW 13 of 19

**Figure 8.** Delivery fractions in (**a**) PLB and (**b**) HRB. **Figure 8.** Delivery fractions in (**a**) PLB and (**b**) HRB.
