*2.1. Study Area*

PLB is located in southern China, between 113◦ E–118◦ E and 24◦ N–30◦ N, as shown in Figure 1a. The drainage area of PLB is 162,271 km<sup>2</sup> , of which 156,743 km<sup>2</sup> is located in Jiangxi Province (the total land area of Jiangxi Province is 166,946 km<sup>2</sup> ), accounting for 97% of the drainage area of PLB and 94% of the total land area of Jiangxi Province [46]. PLB belongs to a subtropical warm and humid monsoon climate zone. The average annual temperature is about 16.3–19.5 ◦C, generally increasing from north to south. The vegetation type is subtropical, evergreen, broad-leaved forest. PLB is composed of five main tributary rivers, namely, Ganjiang River, Fuhe River, Xinjiang River, Raohe River, and Xiushui River, as well as several smaller rivers. These rivers finally flow into Poyang Lake, and the outflow of Poyang Lake inject into Yangtze River through the Hukou station. The span of PLB covers a length of 620 km from north to south and a width of 490 km from east to west. The basin is surrounded by mountains in the east, west, and south; has hills and valley plains crisscrossing the central part; and there is the Poyang Lake plain in the north. The terrain of PLB is high in the south and low in the north, which is conducive to water convergence. water convergence.

and valley plains crisscrossing the central part; and there is the Poyang Lake plain in the north. The terrain of PLB is high in the south and low in the north, which is conducive to

**Figure 1.** Location and Digital Elevation Model (DEM) of (**a**) PLB and (**b**) HRB. **Figure 1.** Location and Digital Elevation Model (DEM) of (**a**) PLB and (**b**) HRB.

The annual average total amount of water resources in PLB was 163.06 billion m3, while the annual average total amount of water uses is 17.67 billion m3 and the annual average quantity of wastewater is 4.39 billion m3. The total water resources of Ganjiang River, Fuhe River, Xinjiang River, Raohe River, and Xiushui River are 89.13, 20.19, 21.11, 15.77, and 16.86 billion m3, respectively. The average annual runoff in PLB into Poyang Lake is 142.73 billion m3. The annual average runoff totals of Ganjiang River, Fuhe River, Xinjiang River, Raohe River, and Xiushui River into Poyang Lake are 81.26, 16.20, 18.53, 14.04, and12.70 billion m3, respectively. The annual average total amount of water resources in PLB was 163.06 billion m<sup>3</sup> , while the annual average total amount of water uses is 17.67 billion m<sup>3</sup> and the annual average quantity of wastewater is 4.39 billion m<sup>3</sup> . The total water resources of Ganjiang River, Fuhe River, Xinjiang River, Raohe River, and Xiushui River are 89.13, 20.19, 21.11, 15.77, and 16.86 billion m<sup>3</sup> , respectively. The average annual runoff in PLB into Poyang Lake is 142.73 billion m<sup>3</sup> . The annual average runoff totals of Ganjiang River, Fuhe River, Xinjiang River, Raohe River, and Xiushui River into Poyang Lake are 81.26, 16.20, 18.53, 14.04, and 12.70 billion m<sup>3</sup> , respectively.

HRB is located in northern China, between 112° E–120° E and 35° N–43° N, as shown in Figure 1b. The scope of HRB covers eight provinces (or cities). The total area of Beijing and Tianjin, 91% of the area of Hebei province, 38% of the area of Shanxi Province, 20% of the area of Shandong Province, 9.2% of the area of Henan province, 13,600 km2 of Inner Mongolia Autonomous Region, and 1700 km2 of Liaoning Province belong to Haihe River Basin—318,200 km2 in total. The climate of HRB is semi-humid and semi-arid. It is located in the temperate East Asian monsoon climate area. The annual average temperature is 1.5–14.0 °C [47]. The vegetation types in HRB are various due to the monsoon climate. HRB is constituted by seven major river watersheds, Luan River, Beisanhe River, Yongding River, Daqing River, Ziya River, Zhangweinan Canal, and Tuhai-Majia River. These rivers mostly flow from west to east, and finally discharge into Bohai Bay. HRB is bordered by the Shanxi Plateau and Yellow River Basin in the west, the Mongolia Plateau and the inland river basin in the north, the Yellow River in the south, and the Bohai Sea in the HRB is located in northern China, between 112◦ E–120◦ E and 35◦ N–43◦ N, as shown in Figure 1b. The scope of HRB covers eight provinces (or cities). The total area of Beijing and Tianjin, 91% of the area of Hebei province, 38% of the area of Shanxi Province, 20% of the area of Shandong Province, 9.2% of the area of Henan province, 13,600 km<sup>2</sup> of Inner Mongolia Autonomous Region, and 1700 km<sup>2</sup> of Liaoning Province belong to Haihe River Basin—318,200 km<sup>2</sup> in total. The climate of HRB is semi-humid and semi-arid. It is located in the temperate East Asian monsoon climate area. The annual average temperature is 1.5–14.0 ◦C [47]. The vegetation types in HRB are various due to the monsoon climate. HRB is constituted by seven major river watersheds, Luan River, Beisanhe River, Yongding River, Daqing River, Ziya River, Zhangweinan Canal, and Tuhai-Majia River. These rivers mostly flow from west to east, and finally discharge into Bohai Bay. HRB is bordered by the Shanxi Plateau and Yellow River Basin in the west, the Mongolia Plateau and the inland river basin in the north, the Yellow River in the south, and the Bohai Sea in the east. The total terrain of HRB is high in the northwest and low in the southeast, which is composed of three landforms: plateau, mountain, and plain. Plateaus and mountains are located in the north and west of HRB, covering an area of 189,400 km<sup>2</sup> , accounting for 60% of the basin area. The east and southeast of HRB are covered by a plain, which covers

128,400 km<sup>2</sup> , accounting for 40% of the basin's area. The annual average total amount of water resources in HRB was 30.30 billion m<sup>3</sup> , while the annual average total amount of water uses is 37.00 billion m<sup>3</sup> and the annual average quantity of wastewater is 5.98 billion m<sup>3</sup> . HRB belongs to an area with an extreme water shortage. The total amount of inter basin water transfer in HRB was 3.51 billion m<sup>3</sup> (including the amount of water diverted from Yangtze River and Yellow River). The total water resources of Luanhe River, Beisanhe River, Yongding River, Daqing River, Ziya River, Zhangweinan Canal, and Tuhai-Majia River are 4.67, 4.38, 2.49, 4.65, 6.36, 4.41, and 3.32 billion m<sup>3</sup> , respectively. The average annual runoff in HRB into Bohai Bay was 3.51 billion m<sup>3</sup> . The annual average runoff totals of Luanhe River, Beisanhe River, Daqing River, Ziya River, and Tuhai-Majia River into Bohai Bay are 0.92, 1.05, 0.64, 0.34, and 0.54 billion m<sup>3</sup> , respectively. It is obvious that PLB has far more water resources than HRB, especially the amounts of water delivered to the outlets. Such difference might be explained by the lesser amounts of precipitation, the larger area of plain terrain, and the larger demand for water supply in HRB, leading to the lesser NH<sup>4</sup> + -N loads delivered to the outlets compared to PLB. The summary of water resources of the main rivers in PLB and HRB is in Tables S1 and S2.

### 2.1.1. Nutrient Sources

Point source data in 2017 of NH<sup>4</sup> + -N in PLB and HRB were provided by Chinese Research Academy of Environmental Sciences and Ecological Environment Monitoring and Scientific Research Center of Haihe River Basin and Beihai Sea Area, respectively. The load from point sources involved municipal and industrial wastewater treatment plants. The median annual load in PLB from point sources was 125,744 kg/year of NH<sup>4</sup> + -N, with a range of 4479–1,568,808 kg/year. The median annual load in HRB from point sources was 42,084 kg/year of NH<sup>4</sup> + -N, with a range of 0–2,952,298 kg/year.

Farmland, woodland, grassland, and residential land were considered as non-point sources of nutrients [48]. Land use data in 2015 were acquired from the Data Center for Resources and Environmental Sciences of Chinese Academy of Sciences (https:// www.resdc.cn/data.aspx?DATAID=184, accessed on 22 December 2020) and used to label farmland, woodland, grassland, water body, residential land, and barren land. In PLB, there were 24.60% farmland, 67.15% woodland, 4.20% grassland, 1.98% water body, 2.04% residential land, and 0.01% barren land, as shown in Figure 2a. In HRB, there were 49.03% farmland, 19.16% woodland, 18.45% grassland, 2.44% water body, 9.74% residential land, and 1.18% barren land, as shown in Figure 2b. It is clear that the areas of woodland and grassland in PLB were almost equal to those in HRB; and that the areas of farmland and residential land in PLB were far smaller than those in HRB.

#### 2.1.2. River Network and Nutrient Load Estimates

The river network was obtained from the Digital Elevation Model (DEM) data (90 m × 90 m) of PLB and HRB by using ArcHydro tools; 85 and 310 stream reaches were delineated, respectively. The DEM data were downloaded from the Geospatial Data Cloud site, Computer Network Information Center, Chinese Academy of Sciences (http://www.gscloud. cn/sources/accessdata/306?pid=302, accessed on 28 December 2020).

Streamflow data in 2017 were available from only Qiujing, Meigang, Wanjiabu, Hushan, Waizhou, Dufengkeng, and Lijiadu stations in PLB, which were provided by the Jiangxi Academy of Environmental Sciences. Streamflow at ungauged watersheds in PLB was estimated by using a GWLF model [49]. The version of the GWLF model used in this study was ReNuMa version 2.2.2 [50]. Streamflow data were available from 203 stations in HRB, which were provided by Ecological Environment Monitoring and Scientific Research Center of Haihe River Basin and Beihai Sea Area. Streamflow at ungauged watersheds in HRB was simulated by interpolation analysis using the GIS platform. Water quantity station distributions are shown in Figure 3a,c.

**Figure 2.** Land use distributions in (**a**) PLB and (**b**) HRB. **Figure 2.** Land use distributions in (**a**) PLB and (**b**) HRB.

**Figure 3.** Water quantity station distribution (**a**) and Water quality station distribution (**b**) in PLB, Water quantity station distribution (**c**) and Water quality station distribution (**d**) in HRB. (http://data.cma.cn/data/cdcdetail/dataCode/SURF\_CLI\_CHN\_MUL\_DAY\_V3.0.html, **Figure 3.** Water quantity station distribution (**a**) and Water quality station distribution (**b**) in PLB, Water quantity station distribution (**c**) and Water quality station distribution (**d**) in HRB.

Water quality data in 2017 were provided monthly by the Jiangxi Academy of Environmental Sciences from 58 sites for PLB, and by Ecological Environment Monitoring and

where is the Annual nutrient load of reach *i*, is simulated by the GWLF model (or interpolation analysis) as above, and represents the mean annual in-

The model's land-to-water delivery variables were mainly determined by the spatial attribute data. Annual average precipitation, annual average temperature, and slope were used in model calibration. The data of precipitation and temperature were downloaded from China Meteorological Data Service Centre

= × (1)

stream NH4+-N concentration.

2.2.3. Land-To-Water Delivery Variables

Annual nutrient loads of these sites were assessed as follows:

Water quality data in 2017 were provided monthly by the Jiangxi Academy of Environmental Sciences from 58 sites for PLB, and by Ecological Environment Monitoring and Scientific Research Center of Haihe River Basin and Beihai Sea Area from 144 sites for HRB. Water quality station distributions are shown in Figure 3b,d.

Annual nutrient loads of these sites were assessed as follows:

$$Load\_i = Flow\_i \times Conv\_i \tag{1}$$

where *Load<sup>i</sup>* is the Annual nutrient load of reach *i*, *Flow<sup>i</sup>* is simulated by the GWLF model (or interpolation analysis) as above, and *Conc<sup>i</sup>* represents the mean annual instream NH<sup>4</sup> + -N concentration.

#### 2.1.3. Land-to-Water Delivery Variables

The model's land-to-water delivery variables were mainly determined by the spatial attribute data. Annual average precipitation, annual average temperature, and slope were used in model calibration. The data of precipitation and temperature were downloaded from China Meteorological Data Service Centre (http://data.cma.cn/data/cdcdetail/ dataCode/SURF\_CLI\_CHN\_MUL\_DAY\_V3.0.html, accessed on 19 June 2020). The slope data were extracted from DEM. The data of delivery variables were transformed to the final type of SPARROW model by using GIS platform.

### *2.2. SPARROW Model*

The total load leaving a given reach in the SPARROW model is considered as the sum of the load produced by itself and the load delivered from its upper stream [51]. The total load leaving reach *i* can be expressed mathematically as follows:

$$F\_i^\* = \left(\sum\_{j \in I(i)} F\_j'\right) \delta\_i A\left(Z\_i^S; \theta\_\mathcal{S}\right) + \left(\sum\_{n=1}^{N\_\mathcal{S}} \mathbb{S}\_{n,i} \mathbb{1}\_{\mathbb{N}} D\_{\mathbb{N}}\left(Z\_i^D; \theta\_\mathcal{D}\right)\right) A'\left(Z\_i^{\mathcal{S}}; \theta\_\mathcal{S}\right) \tag{2}$$

where *F<sup>i</sup> \** is the total load leaving reach *i* (kg/year), *F* 0 *j* is the load leaving upstream reaches from reach *j*, the set *J(i)* is all upstream reaches of reach *i*, *δ<sup>i</sup>* is the proportion of load delivered to reach *i* contributed by adjacent upstream reaches, *A Z S i* ; *θ<sup>S</sup>* is a function of first-order loss processes related to stream size, and *A* 0 *Z S i* ; *θ<sup>S</sup>* is the square root of *A Z S i* ; *θ<sup>S</sup>* .

*Sn,j* is the nutrient source n in reach *i*, *N<sup>S</sup>* is the total number of nutrient sources, *α<sup>n</sup>* is the coefficient of nutrient source *n*, and the land-to-water delivery term *D<sup>n</sup> Z D i* ; *θ<sup>D</sup>* is defined as follows:

$$D\_n\left(Z\_i^D; \theta\_D\right) = \exp\left(\sum\_{m=1}^{M\_D} \omega\_{nm} Z\_{m}^D \theta\_{Dm}\right) \tag{3}$$

where *Z D m i* is the land-to-water variable *m* within reach *i*, *M<sup>D</sup>* is the total number of delivery variables, *θDm* is the coefficient of delivery variable *m*, and *ωnm* is the delivery index for judging whether source *n* uses delivery variable *m* or not.

Stream delivery function, considered as an attenuation process acting on flux, is formulated by a first-order reaction rate process. The proportion of the load remaining after the delivery to the outlet of reach *i* is expressed as an exponential function:

$$A\left(Z\_i^{\mathcal{S}}; \theta\_{\mathcal{S}}\right) = \exp\left(-\sum\_{c=1}^{\mathbb{C}\_{\mathcal{S}}} \theta\_{\mathcal{S}c} T\_{c\
i}^{\mathcal{S}}\right) \tag{4}$$

where *T S c i* is the average travelling time of a stream in reach *i*, which is classified as instream decay class *c*. *c* is the number of in-stream decay class *c* streams, *C<sup>S</sup>* is the total number of in-stream decay classes, and *θSc* is the coefficient corresponding to average travelling time of stream. Two in-stream decay classes were used in the PLB and HRB SPARROW models.

The estimation method of the SPARROW model is a nonlinear weighted least squares (NWLS) algorithm performed by the SAS procedure PROC MODEL, based on Equation (2). NWLS, which is a robust technique to solve nonlinear problems, can be considered as an iterative linear estimation process, since it is related to the ordinary least squares [51]. Bootstrap analysis is used to validate the model and perform the uncertainty analysis. The bootstrap procedure is executed by randomly selecting with replacement monitored loads from the observations in the original calibration data set and fitting separate regression models to the resampled data [32].
