**1. Introduction**

An excess of nutrients, which are generated from point and non-point sources and are eventually transported to water bodies, has led to severe eutrophication throughout the world in recent years [1]. Eutrophication of water bodies causes toxic algal blooms, oxygen depletion, loss of biodiversity, and thereby, the degradation of water quality and aquatic ecosystem services [2]. Ammonium nitrogen (NH<sup>4</sup> + -N), which naturally arises from the decomposition of organic substances through ammonification, is a critical nutrient produced by human activities such as fertilizing, livestock breeding, and municipal wastewater treating. Synthetic nitrogen fertilizers are slightly absorbed by crops (about 10%), while large quantities of synthetic nitrogen are exported to aquatic systems through surface runoff and decomposed to NH<sup>4</sup> + -N [3]. Domestic and industrial wastewater are also important manners, by which NH<sup>4</sup> + -N enters into aquatic systems. Excessive NH<sup>4</sup> + -N leads to eutrophication, endangering aquatic species and polluting water sources [4]. Since NH<sup>4</sup> + -N has a tremendous influence on local water quality, it is vital for water quality protection to assess the amount, sources, and transport of NH<sup>4</sup> + -N [5].

Previous research has focused on the processes of NH<sup>4</sup> + -N stream transport. For instance, Jin et al. employed the Integrated Nitrogen Catchments (INCA-N) model to link

**Citation:** Yin, J.; Chen, H.; Wang, Y.; Guo, L.; Li, G.; Wang, P. Ammonium Nitrogen Streamflow Transport Modelling and Spatial Analysis in Two Chinese Basins. *Water* **2022**, *14*, 209. https://doi.org/10.3390/ w14020209

Academic Editor: Karl-Erich Lindenschmidt

Received: 20 November 2021 Accepted: 7 January 2022 Published: 11 January 2022

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upstream processes to downstream water quality of NH<sup>4</sup> + -N in the Hampshire Avon catchment [6]. Ervinia et al. applied the INCA-N model to identify the source and processes of NH<sup>4</sup> + -N in the Jiulong River Watershed (JRW) [7]. Zhang et al. employed QUAL2K model to explore the transport of NH<sup>4</sup> + -N in a creek watershed with sparse data in southeast of China [8]. Xue et al. simulated the land surface hydrological runoff and routing processes in the Xiaoqing River Basin and analysed the concentration of NH<sup>4</sup> + -N temporally and spatially, using the Soil and Water Assessment Tool (SWAT) model and HEC-RAS model [9]. Dai et al. assessed the sources and transport of ammonium nitrogen in a karst basin using the SPAtially Referenced Regressions on Watershed attributes (SPARROW) model [5].

Complex models, such as Agricultural Non-point Source (AGNPS), Hydrological Simulation Program FORTRAN (HSPF), INCA-N, and SWAT, which have been developed to evaluate water quality and sources of nutrients [10], have hefty data requirements [11] and are time-consuming processes [12]. Hybrid empirical and mechanistic models based on regression, such as the SPARROW model, can be used to conduct studies at different spatial scales on nutrient transport with smaller input datasets, such as data on nutrient load, nutrient sources, and landscape properties. Since it was first established by the U.S. Geological Survey (USGS) [13], SPARROW has been extensively applied in North America [14–24], Asia [25–30], New Zealand [31,32], Spain [33,34], and Brazil [35] with satisfactory performance. SPARROW has been employed in studies on regions of various sizes, from 153 km<sup>2</sup> [36] to 3.2 million km<sup>2</sup> [37]. Furthermore, the model performs well in both estimating the influences of human activities on the environment [38,39], and analyzing scenarios of climate change [40,41] or land use change [42,43].

Poyang Lake Basin (PLB) consists of five main river watersheds. Its water exchanges with Yangtze River after the streamflow of the five main rivers are injected into Poyang Lake, the largest freshwater lake in China [44]. PLB is a typical southern water basin in China, since it has a massive quantity of water and better water quality than northern water basins in China. Haihe River Basin (HRB), which contains seven major river watersheds, has a high population density and numerous large cities. Therefore, it plays an important role in the politics and economy of China [45]. The water quantity and quality of HRB is seriously affected by human activities, such as irrigation, fertilization, point sources, etc. The main purposes of this paper were to (1) establish SPARROW models in PLB and HRB and figure out the NH<sup>4</sup> + -N load and streamflow transport in these two basins; (2) compare NH<sup>4</sup> + -N load and streamflow transport between PLB and HRB and identify the distinctions between the two basins; (3) offer assistance in future control measures for better management in PLB and HRB.

### **2. Materials and Methods**
