*4.2. Nutrient Delivery to Outlets*

*4.2. Nutrient Delivery to Outlets*  Figure 9 shows the loads and yields of NH4+-N delivered to the target outlets in each catchment in PLB. Figure 10 shows the loads and yields of NH4+-N delivered to the target outlets in each catchment in HRB. Fuhe River's downstream catchment had the maximum incremental NH4+-N load delivered to Poyang Lake (Figure 9a). The middle reach of Ganjiang River and the upstream reach of Raohe River had large load deliveries. The upstream Figure 9 shows the loads and yields of NH<sup>4</sup> + -N delivered to the target outlets in each catchment in PLB. Figure 10 shows the loads and yields of NH<sup>4</sup> + -N delivered to the target outlets in each catchment in HRB. Fuhe River's downstream catchment had the maximum incremental NH<sup>4</sup> + -N load delivered to Poyang Lake (Figure 9a). The middle reach of Ganjiang River and the upstream reach of Raohe River had large load deliveries. The upstream reach of Xinjiang River and the downstream reach of Xiushui River had large load deliveries.

**Figure 9.** (**a**) Loads and (**b**) yields of NH4+-N delivered to the target outlets in PLB.

reach of Xinjiang River and the downstream reach of Xiushui River had large load deliv-

Figure 9 shows the loads and yields of NH4+-N delivered to the target outlets in each catchment in PLB. Figure 10 shows the loads and yields of NH4+-N delivered to the target outlets in each catchment in HRB. Fuhe River's downstream catchment had the maximum incremental NH4+-N load delivered to Poyang Lake (Figure 9a). The middle reach of Ganjiang River and the upstream reach of Raohe River had large load deliveries. The upstream reach of Xinjiang River and the downstream reach of Xiushui River had large load deliv-

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

*4.2. Nutrient Delivery to Outlets* 

eries.

**Figure 9.** (**a**) Loads and (**b**) yields of NH4+ **Figure 9.** -N delivered to the target outlets in PLB. (**a**) Loads and (**b**) yields of NH<sup>4</sup> + -N delivered to the target outlets in PLB.

**Figure 10.** (**a**) Loads and (**b**) yields of NH4+-N delivered to the target outlets in HRB. **Figure 10.** (**a**) Loads and (**b**) yields of NH<sup>4</sup> + -N delivered to the target outlets in HRB.

The yield was calculated as the division of the incremental load of a specific reach delivered to the river watershed outlet by the incremental reach area. Yield reflects the intensity of NH4+-N transferred to the river watershed outlets. Such analysis helps to identify major contributing areas to the water quality of outlets. As shown in Figure 9b, the middle reach of Ganjiang River had the highest NH4+-N yields. The downstream portions of Ganjiang River, Fuhe River, Xinjiang River, and Raohe River had high incremental NH4+-N loads, but the downstream portion of Ganjiang River had a larger NH4+-N yield than the others. These regions belong to the central urban areas of major cities in Jiangxi Province, which is perhaps the reason why these catchments have high yields. The yield was calculated as the division of the incremental load of a specific reach delivered to the river watershed outlet by the incremental reach area. Yield reflects the intensity of NH<sup>4</sup> + -N transferred to the river watershed outlets. Such analysis helps to identify major contributing areas to the water quality of outlets. As shown in Figure 9b, the middle reach of Ganjiang River had the highest NH<sup>4</sup> + -N yields. The downstream portions of Ganjiang River, Fuhe River, Xinjiang River, and Raohe River had high incremental NH<sup>4</sup> + -N loads, but the downstream portion of Ganjiang River had a larger NH<sup>4</sup> + -N yield than the others. These regions belong to the central urban areas of major cities in Jiangxi Province, which is perhaps the reason why these catchments have high yields.

In HRB, the downstream reach of Beisanhe River had the maximum incremental NH4+-N load delivery (Figure 10a). The downstream reach of Tuhai-Majia River had a In HRB, the downstream reach of Beisanhe River had the maximum incremental NH<sup>4</sup> + -N load delivery (Figure 10a). The downstream reach of Tuhai-Majia River had a

large incremental NH4+-N load delivery. The downstream reaches of Daqing River and

N yield than their downstream reaches. The middle reaches of Zhangweinan Canal and

As shown in Figure 10b, the downstream reach of Yongding River had the highest

By combining incremental load, load delivery, and yield results, it could be found that the densely populated area in both PLB and HRB contributed the most to the NH4+- N load. However, the difference between the two basins was that the point sources of PLB played the dominant role in the transport of load delivery, whereas the residential land was the dominant sources of load delivery in HRB, as shown in Figure 11. The contributions of farmland in both basins cannot be omitted, either. In PLB, NH4+-N load delivery originating from point sources and farmland accounted for 41.83% and 38.84%, respectively. In HRB, NH4+-N load transport originating from residential land and farmland ac-

Tuhai-Majia River also had high NH4+-N yields.

counted for 40.16% and 36.75%, respectively.

Luan River had large incremental NH4+-N load deliveries.

large incremental NH<sup>4</sup> + -N load delivery. The downstream reaches of Daqing River and Luan River had large incremental NH<sup>4</sup> + -N load deliveries.

As shown in Figure 10b, the downstream reach of Yongding River had the highest NH<sup>4</sup> + -N yield. The middle reaches of Beisanhe River and Daqing River had higher NH<sup>4</sup> + -N yield than their downstream reaches. The middle reaches of Zhangweinan Canal and Tuhai-Majia River also had high NH<sup>4</sup> + -N yields.

By combining incremental load, load delivery, and yield results, it could be found that the densely populated area in both PLB and HRB contributed the most to the NH<sup>4</sup> + -N load. However, the difference between the two basins was that the point sources of PLB played the dominant role in the transport of load delivery, whereas the residential land was the dominant sources of load delivery in HRB, as shown in Figure 11. The contributions of farmland in both basins cannot be omitted, either. In PLB, NH<sup>4</sup> + -N load delivery originating from point sources and farmland accounted for 41.83% and 38.84%, respectively. In HRB, NH<sup>4</sup> + -N load transport originating from residential land and farmland accounted for 40.16% and 36.75%, respectively. *Water* **2022**, *14*, x FOR PEER REVIEW 15 of 19

**Figure 11.** Comparison of land use, incremental load, and load delivery among (**a**) PLB and (**b**) HRB. **Figure 11.** Comparison of land use, incremental load, and load delivery among (**a**) PLB and (**b**) HRB.

The phenomenon above might be explained by the following reasons. Firstly, the proportion of residential land in HRB was much bigger than that in PLB, and thus the residential land contributed more to the load delivered to the outlets in HRB. Secondly, the streamflow of PLB is much larger than that of HRB, which leads to the decrease of NH4+-N removal in streamflow and makes the point sources the dominant sources in PLB. Lastly, the urbanization and point sources management of PLB might be inferior to those The phenomenon above might be explained by the following reasons. Firstly, the proportion of residential land in HRB was much bigger than that in PLB, and thus the residential land contributed more to the load delivered to the outlets in HRB. Secondly, the streamflow of PLB is much larger than that of HRB, which leads to the decrease of NH<sup>4</sup> + -N removal in streamflow and makes the point sources the dominant sources in PLB. Lastly, the urbanization and point sources management of PLB might be inferior to those of HRB.

#### of HRB. *4.3. Strategy for Nutrient Management*

*4.3. Strategy for Nutrient Management*  Nutrient delivery abatement is vital for the water quality of the receiving waterbody. Nutrient delivery abatement is vital for the water quality of the receiving waterbody. The results of SPARROW models in this study are meaningful for evaluating NH<sup>4</sup> + -N load transport, critical regions of high load delivery, and dominant nutrient sources.

The results of SPARROW models in this study are meaningful for evaluating NH4+-N load transport, critical regions of high load delivery, and dominant nutrient sources. Based on the studies above, policies of enhancement were proposed. Since PLB has more water resources and steeper terrain, some upstream and middle reaches still transport large amounts of NH4+-N load to the outlets, similar to the reaches around the lake. Therefore, it is crucial that more attention should be paid to the reaches around the lake and the upstream and middle reaches, which deliver a large amount of NH4+-N load to outlets, especially the centers of big cities. At the same time, the point sources and farmland were recognized as the dominant sources contributing to the load delivered to the outlets in PLB. Consequently, it is important to enhance the management of the point sources in these reaches, such as municipal and industrial wastewater. In addition, since PLB already contains high proportions of woodland and grassland, it is important to es-Based on the studies above, policies of enhancement were proposed. Since PLB has more water resources and steeper terrain, some upstream and middle reaches still transport large amounts of NH<sup>4</sup> + -N load to the outlets, similar to the reaches around the lake. Therefore, it is crucial that more attention should be paid to the reaches around the lake and the upstream and middle reaches, which deliver a large amount of NH<sup>4</sup> + -N load to outlets, especially the centers of big cities. At the same time, the point sources and farmland were recognized as the dominant sources contributing to the load delivered to the outlets in PLB. Consequently, it is important to enhance the management of the point sources in these reaches, such as municipal and industrial wastewater. In addition, since PLB already contains high proportions of woodland and grassland, it is important to establish buffer zones along rivers by planting vegetation or building wetlands, in order to increase the absorption of NH<sup>4</sup> + -N.

tablish buffer zones along rivers by planting vegetation or building wetlands, in order to increase the absorption of NH4+-N. Owing to relatively fewer water sources and more plain terrain in HRB, the middle Owing to relatively fewer water sources and more plain terrain in HRB, the middle and downstream reaches contributed more to the NH<sup>4</sup> + -N load delivered to the outlets. Hence, the management among these reaches should be enhanced. In HRB, the residential land

and downstream reaches contributed more to the NH4+-N load delivered to the outlets. Hence, the management among these reaches should be enhanced. In HRB, the residential

quently, controlling the domestic pollution and reducing the usage of chemical fertilizers are feasible measures to be undertaken urgently. In addition, increasing the woodland and grassland coverage to enhance the retention of nutrients in land areas may be a sound

The SPARROW model is a spatial explicitly method to address nutrient load and streamflow transport in watersheds. This study developed SPARROW models in two multi-rivers basins in China, which cover large areas in the north and south of China, respectively. These two basins have quite different conditions in many aspects, including weather, water resources, and land use. The SPARROW models were used to evaluate NH4+-N load streamflow transport, critical regions of high load delivery, and dominant

measure to reduce the NH4+-N load delivered to the outlets.

**5. Conclusions** 

and farmland both have critical positions in the delivery of NH<sup>4</sup> + -N loads to the outlets, due to the large amount of residential land and the high population density. Consequently, controlling the domestic pollution and reducing the usage of chemical fertilizers are feasible measures to be undertaken urgently. In addition, increasing the woodland and grassland coverage to enhance the retention of nutrients in land areas may be a sound measure to reduce the NH<sup>4</sup> + -N load delivered to the outlets.

#### **5. Conclusions**

The SPARROW model is a spatial explicitly method to address nutrient load and streamflow transport in watersheds. This study developed SPARROW models in two multi-rivers basins in China, which cover large areas in the north and south of China, respectively. These two basins have quite different conditions in many aspects, including weather, water resources, and land use. The SPARROW models were used to evaluate NH<sup>4</sup> + -N load streamflow transport, critical regions of high load delivery, and dominant nutrient sources in these two basins, which further provided basin-specific advice to the authorities. Based on the results of this study, the point sources and the farmland are the dominant source of NH<sup>4</sup> + -N entering Poyang Lake, while the residential land and farmland are the major sources of NH<sup>4</sup> + -N entering Bohai Bay. The following measures should be taken: In PLB, especially among the centers of big cities, it is important to enhance the management of the point sources, such as municipal and industrial wastewater. In addition, it is also advised to establish buffer zones along rivers by planting vegetation or building wetlands, in order to increase the absorption of NH<sup>4</sup> + -N. In HRB, controlling the domestic pollution and reducing the usage of chemical fertilizers are feasible measures that should be urgently considered. Moreover, increasing the woodland and grassland coverage to enhance the retention of nutrients in land areas may be a sound measure for the reduction of the NH<sup>4</sup> + -N load delivered to the outlets.

The SPARROW models built for PLB and HRB can serve as references for future uses for different basins with various conditions, extending this model's scope and adaptability. Remarkably, at twice the size of PLB, HRB has more plain terrain, less water resources and more long-distance canals, which lead to more difficult in its modelling. Hence, the SPARROW model developed in HRB is a worthy possibility for similar research in future.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/w14020209/s1, Table S1: Summary of water resources of the main rivers in PLB, Table S2: Summary of water resources of the main rivers in HRB, Table S3: Parameters of GWLF models in PLB.

**Author Contributions:** Conceptualization, Y.W.; Data curation, H.C.; Formal analysis, J.Y.; Project administration, L.G.; Software, J.Y. and G.L.; Supervision, Y.W.; Visualization, H.C.; Writing—original draft, J.Y.; Writing—review & editing, P.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07301-001).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Publicly available datasets of water resources were analyzed in this study. This data can be found here: Haihe River Water Resources Bulletin 2017, http://www.hwcc. gov.cn/hwcc/static/szygb/gongbao2017/index.html (accessed on 19 November 2021); Jiangxi Water Resources Bulletin 2017, http://slt.jiangxi.gov.cn/resource/uploadfile/file/20180917/2018091711225 7428.pdf (accessed on 19 November 2021).

**Acknowledgments:** We wish to thank the Jiangxi Academy of Environmental Sciences for providing streamflow and water quality data of PLB.

**Conflicts of Interest:** The authors declare no conflict of interest.
