**1. Introduction**

Water pollution control and sustainable development of international river basins have been a major challenge in the current ecological research. Yet, due to the difficulty of data acquisition, reluctant cooperation across boundaries, and the absence of exchange mechanisms [1,2], there are few successful cases. Also arising from unbalanced development along rivers, the trans-provincial river management is a regional form of trans-boundary problem. However, provinces (or states) are more willing to cooperate in controlling pollution than countries given sovereignty and territorial integrity. Successful cases include the Murray–Darling Basin in Australia [3], Delaware River in the United States [4], and Xin'an River in China [5]. Therefore, the management of trans-provincial river basins in water ecology can provide valuable information for solving trans-boundary ecological issues.

Ecological compensation (hereinafter "eco-compensation") has long been considered an effective economic instrument for controlling water pollution in trans-regional basins [6]. Eco-compensation, also known as "payment for environmental services", was proposed in the late 1990s for watershed management to address the environmental problems caused by the Industrial Revolution [7]. At present, eco-compensation has captured increasing

**Citation:** Wan, W.; Zheng, H.; Liu, Y.; Zhao, J.; Fan, Y.; Fan, H. Ecological Compensation Mechanism in a Trans-Provincial River Basin: A Hydrological/Water-Quality Modeling-Based Analysis. *Water* **2022**, *14*, 2542. https://doi.org/ 10.3390/w14162542

Academic Editors: Dengfeng Liu, Hui Liu and Xianmeng Meng

Received: 4 July 2022 Accepted: 15 August 2022 Published: 18 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

attention nationwide and is seen as a promising complementary method to alleviate the identified contradiction among stakeholders [8]. The form of eco-compensation measures varies by country. In China, the central government has launched a series of explorations and research projects for eco-compensation since 1999. One typical scheme is the nationalscale Grain for Green Program, which compensates rural households for converting sloping croplands to forests or grasslands to reduce soil erosion [9,10]. At the local administrative level, the first trans-provincial eco-compensation pilot scheme is the Xin'an River scheme, launched in 2011. This is an eco-compensation agreement between Anhui and Zhejiang provinces, targeting pollution control from the upper regions in Huangshan city, Anhui, and thereby maintaining high water quality in the lower reaches, Qiandao Lake in Hangzhou city, Zhejiang [11]. The list of other pilot eco-compensation schemes in China is found in Wang et al. [12]. These studies are crucial in clarifying the implications of watershed eco-compensation and establishing typical methods and standards. How to reasonably account for compensation and what measures should be taken to effectively protect transregional watersheds from water pollution depend largely on the accurate mapping of compensation stakeholders [13]. A solution generally lies in addressing the four questions below. First, what are the major water pollutants and where do they come from? Second, how do pollutants change throughout their journey from upstream to downstream, and are they accumulating or separating? Third, what actions could be taken to reduce the load across regional boundaries? Last, but not least, what is the cost of pollution control and how should it be compensated?

Despite a number of studies on these questions, there is still a lack of systematic quantitative analysis on how eco-compensation responds to the dynamic change in pollutants. Previous scholars from different disciplines have investigated the effectiveness of eco-compensation and measures of reducing water pollution from different perspectives. For research on eco-compensation, current studies primarily focus on policy analysis involving game theory [6] and water-related regulations [11]. However, such methods are mostly static and disregard the responses to hydrology and water quality and, thus, cannot adequately address non-point-source pollution.

In pollution control, the wide range of substances that may pollute water bodies leads to a variety of options for reducing pollution [14,15]. In China, more than 2600 lakes, including the nation's largest freshwater lake, Poyang Lake, have been subjected to high loads of nitrogen (N) and phosphorus (P) [16,17]. Agricultural operations, including crop fertilization and livestock farming, are one of the major sources of N and P pollution to surface water. For example, Boesch et al. [18] and Reckhow et al. [14] found that high nutrient leaching from farmland into the US Chesapeake Bay is the foremost water quality concern for the waterway, and thus nearby agricultural land needs to implement the best management practices. Successful tracking of pollution control strategies relies upon the careful modeling of non-point nutrient fluxes, and distributed process-based models are then introduced [19–21]. These models include, but are not limited to, Soil and Water Assessment Tool (SWAT) [22], Integrated Valuation of Environmental Services and Tradeoffs (InVEST) [23], and Annualized AGricultural Non-point Source (AnnAGNPS) [24]. Among these, SWAT is arguably the most widely used model, especially at the watershed scale [25,26]. In summary, the above scattered studies reveal a fragmented and insufficient link between non-point-source simulation and eco-compensation, thereby failing to support effective policy formulation.

To help decision-makers understand trans-regional non-point-source pollution issues from both environmental and economic perspectives, this study utilized the SWAT hydrological/water-quality model to simulate nitrogen/phosphorus cycling and their responses to eco-compensation strategies for an agricultural watershed, the Tangbai River Basin (TRB). As the TRB receives the most runoff in Henan province and joins the Han River, the longest tributary of the Yangtze River, in Hubei province, it is a representative trans-provincial river basin. A watershed eco-compensation mechanism is selected

as a water pollution control measure to provide a reference for trans-boundary water quality management. water pollution control measure to provide a reference for trans‐boundary water quality management.

responses to eco‐compensation strategies for an agricultural watershed, the Tangbai River Basin (TRB). As the TRB receives the most runoff in Henan province and joins the Han River, the longest tributary of the Yangtze River, in Hubei province, it is a representative trans‐provincial river basin. A watershed eco‐compensation mechanism is selected as a

*Water* **2022**, *14*, x FOR PEER REVIEW 3 of 21

#### **2. Data and Methodology 2. Data and Methodology**

#### *2.1. Study Area 2.1. Study Area*

The Tangbai River Basin (TRB, 31◦380–33◦43<sup>0</sup> N, 111◦340–113◦40<sup>0</sup> E, 24,190 km<sup>2</sup> , Figure 1) is a subbasin of the Han River Basin, which is the source of the central route of the South-to-North Water Transfer Project. The Tangbai River is formed by the convergence of two tributaries, the Tang River and the Bai River, flowing through Henan province and Hubei province, China (Figure 1b). The average flow rate at the basin outlet is 323 m3/s (1980–2012 time series). The two tributaries both originate from Nanyang city, the southwestern part of Henan province, and then flow into Xiangyang city of Hubei province and finally form the Tangbai River, which later joins the Han River. The Tangbai River Basin (TRB, 31°38′‒33°43′ N, 111°34′‒113°40′ E, 24,190 km2, Figure 1) is a subbasin of the Han River Basin, which is the source of the central route of the South‐to‐North Water Transfer Project. The Tangbai River is formed by the convergence of two tributaries, the Tang River and the Bai River, flowing through Henan province and Hubei province, China (Figure 1b). The average flow rate at the basin outlet is 323 m3/s (1980‒2012 time series). The two tributaries both originate from Nanyang city, the south‐ western part of Henan province, and then flow into Xiangyang city of Hubei province and finally form the Tangbai River, which later joins the Han River.

**Figure 1.** Geographic location (**a**) and 90 m digital elevation map (DEM, **b**) of the Tangbai River Basin, a shared river basin of Henan province and Hubei province. The curved path in (**b**) is the provincial boundary, and the blue lines are the two major tributaries constituting the Tangbai River. **Figure 1.** Geographic location (**a**) and 90 m digital elevation map (DEM, **b**) of the Tangbai River Basin, a shared river basin of Henan province and Hubei province. The curved path in (**b**) is the provincial boundary, and the blue lines are the two major tributaries constituting the Tangbai River.

The TRB is characterized by a dense population. Around 90,000 people live in the middle and upper reaches of the watershed, which is the famous Nanyang Basin, the na‐ tion's most productive agricultural regions. The excess use of fertilizers, traditional inad‐ equate farming techniques, and dramatic growth of industrial and decreasing runoff dur‐ ing the 1990s due to climate change have led to serious pollution and environmental deg‐ radation of the Tangbai River [27]. In recent years, the surface water quality has improved to a common Class IV status due to the long‐term regional cooperative actions and joint management between Nanyang (in Henan province) and Xiangyang (in Hubei province) [28]. However, such pollution control mainly focuses on point‐source pollution from man‐ ufacturing and runoff pollution from urban areas [29]. The agricultural non‐point‐source pollution in the rural areas of Nanyang city is still prominent. The rainfall‐ and snowmelt‐ The TRB is characterized by a dense population. Around 90,000 people live in the middle and upper reaches of the watershed, which is the famous Nanyang Basin, the nation's most productive agricultural regions. The excess use of fertilizers, traditional inadequate farming techniques, and dramatic growth of industrial and decreasing runoff during the 1990s due to climate change have led to serious pollution and environmental degradation of the Tangbai River [27]. In recent years, the surface water quality has improved to a common Class IV status due to the long-term regional cooperative actions and joint management between Nanyang (in Henan province) and Xiangyang (in Hubei province) [28]. However, such pollution control mainly focuses on point-source pollution from manufacturing and runoff pollution from urban areas [29]. The agricultural non-point-source pollution in the rural areas of Nanyang city is still prominent. The rainfall- and snowmelt-runoff processes affect the transfer of non-point-source pollution, and thereby disturb the availability of water resources between Nanyang and Xiangyang. As a typical trans-provincial watershed, managing non-point-source pollution in TRB remains complicated and difficult.
