*2.1. Literature Review*

### 2.1.1. Transboundary Water Cooperation

Various organizations have provided definitions of TWC. The UN-Water [8] considers TWC to be an "arrangement" established between transboundary basin countries, which may include bilateral or multilateral treaties or other formal arrangements. The European Union [9] believes that TWC and diplomatic issues are closely linked, aiming to urge countries to reach an agreement on the distribution and management of international shared water resources, and promote broader regional cooperation. The International Centre for Water Resources and Global Change [10] pointed out that although there is no singular definition of TWC, it can be regarded as a mutually beneficial exchange of two or more parties instead of competing for the same water resources. In academia, some scholars pointed out that water cooperation based on the signing of treaties is more effective. For example, Brochmann [11] and Dinar [12] emphasized the dominance of water treaties in TWC. Kistin [13] called on the academic community to go beyond the notion of cooperation as treaties and emphasized the important role of state and non-state actors. In general, there is currently no unified definition of TWC.

In this study, TWC, as the research object of the article, we believe that it should have four basic connotations. First, its essence involves the exchanges or mutual relations between multiple international political actors in the international community. Second, the actors include state actors and non-state actors, among which state actors occupy a dominant position. Third, the exchange or mutual relationship between actors is a cooperative relationship. Fourth, the object of cooperation is transboundary water resources, including two spatial forms that flow across national borders and form national borders by themselves. Therefore, we define TWC as "the actions of varying degrees of coordination, joint and mutual support between state actors, or between state and non-state actors, to ensure the realization of transboundary water resources development needs or related interest goals". In this article, unless otherwise specified, TWC between state actors will be our focus for discussion.

Compared with attempts to define the concept, there has been abundant empirical research on TWC. The existing studies are mainly distributed in the fields of political science and geography, and there are some connections and differences in paradigms and research methods. From the perspective of the research paradigm, political scholars dominate the discussion on this topic. In particular, international relations scholars analyze regional water cooperation cases from the perspectives of liberalism, functionalism, constructivism, and institutionalism [14–19], in order to clarify the broader mechanism of international cooperation behind them. Compared with political scholars, geographers pay more attention to the response of TWC under climate change and the ecological challenges [20,21] that river basins may face, and they also emphasize the geographical complexity [22,23] of water cooperation in the process. Some scholars conducted research from the perspective of water supply and demand, and pointed out that under the pressure of freshwater resources, the sustainable development of transboundary basins and regional water cooperation are facing great uncertainty [12,24–26]. Although there are differences in research paradigms here, geographers generally agree that a broader political and historical background needs to be fully considered in TWC research [27].

From the perspective of research methods, most of the current TWC studies focus on individual international freshwater basins and emphasize policy options for solving the challenges of the region, such as case studies from the Mekong [28–30], Indus [31,32], Nile [33,34], or La Plata [35] River Basin. Some scholars have developed concepts and research methods to evaluate TWC and explore its driving forces, such as integrated water resources management (IWRM) [36,37], water diplomacy [38–40], and water-energy-food nexus [41–43], which deepens our understanding of TWC from the perspective of social science research. Among them, the Transboundary Freshwater Dispute Database (TFDD) project developed by Wolf et al. [44,45] has provided reliable spatial data and events data for quantitative assessment of global risk basins and water cooperation and conflicts, leading the trend of quantitative research in this field. By applying different research perspectives to water events, the basins with the potential for political stresses or conflicting interests on a global scale have been identified, trends in hydropolitics of transboundary basins have been discussed, and the most concerning areas of water cooperation have been confirmed [46–48]. Nevertheless, there have been three characteristics in this field for a long time: the paradigm focuses on qualitative research from political science, while spatial analysis from geography is relatively limited; the spatial perspective focuses on the basin or regional scale, while some global scale evaluations also mostly use basins as the analysis unit; and compared with qualitative research or policy review, quantitative research is less and lacks analysis from the perspective of social networks.

With the state as the basic unit of analysis, this article attempts to explore the cooperative relationships behind the global TWC events in the Post-Cold War era from the perspective of space, network, and relations. Complementing related studies, this article focuses on more general answers to several key questions: what kind of spatial linkages

feature in global TWC, and what role do some key state actors play in it? With the profound evolution of globalism and interdependence, to what extent does the relationship between countries have an impact on TWC, and what mechanism does the process contain?
