**1. Introduction**

Water management is a major and growing global issue for economic development and poverty reduction [1,2]. Water is essential for food and energy security [3,4], and water-related extremes of flood

and droughts have significant economic and social costs [5,6]. With increasing global population and economic development, demand for water and competition between uses and users are on the rise [7,8]. Global water consumption is estimated to have increased by 40 percent in the last four decades [8], mostly for irrigation, which represents 70 percent of total global water withdrawals [9]. While everyone depends on freshwater, the importance of groundwater is often overlooked; for example, groundwater provides drinking water for 1.5–3.0 billion people [10]. The current level of global water withdrawals is approaching a planetary boundary, which if crossed would take the Earth system outside a safe operating space for humanity [11]. As result of these pressures, an estimated 4 billion people experience severe water scarcity for at least one month of the year [12]. Local water availability constraints, rapid population growth and urbanization, inadequate infrastructure, and governance shortcomings [13] mean nearly 0.7 billion people lack access to a safely managed drinking water supply [14].

Accelerating climate change is perturbing the global water cycle [15], altering the average patterns of water availability and increasing the magnitude and frequency of water-related extremes in parts of the world. These changes, however, are uncertain and still poorly understood [15–18]. Climate change increases the uncertainty in projections of water supply and demand, and increases the uncertainty in feasibility and economic performance assessments of water infrastructure [19,20].

#### *1.1. Large River Basins—Character and Importance*

There is no widely accepted criterion for defining "large river basins", either in terms of drainage area or total discharge. Basins exceeding 100,000 km<sup>2</sup> in area—of which there are an estimated 130 or so globally (including 22 exceeding 1,000,000 km2)—could reasonably be considered large. However, rather than use an arbitrary criterion such, we adopt a looser definition that also includes geographically smaller basins where water management challenges are considered 'large', because of one or more of: (i) hydrologic complexity (high flows, hydrologic variability, non-stationarity, surface-groundwater interactions, multiple water sources—rainfall-runoff, snow, glacier melt); (ii) water management complexity (large population, supply-demand imbalance, inter-sectoral competition, rapid demand growth, pollution, high flood and erosion risk, climate change vulnerability),); and (iii) administrative complexity (transboundary coordination or conflict; federate-state-local coordination or conflict; governance complexity—intersecting legal, policy, regulatory frameworks).

Aside from remote and sparsely populated basins in northern Canada and Russia, most of the geographically largest river basins in the world are also international transboundary basins. A total of 286 river international transboundary river basins have been identified (spanning 151 countries and home to more than 40 percent of the global population); 80 percent of the total area and population of the transboundary basins is associated with the largest 156 basins [21]. Larger river basins tend to have a higher economic dependence on water resources, and the 14 basins with the greater economic dependence on water are home to almost 50 percent of the population of all transboundary basins—nearly 1.4 billion people [21]. In addition to international transboundary rivers, large basins within federal countries—such as the Murray–Darling in Australia—represent complex resource management challenges [22].

As well as the economic importance and the associated social values of large river basins, these systems are critical habitat for freshwater biodiversity. Rivers, lakes, and other 'wetlands' occupy just 0.8 percent of the Earth's surface but support 6 percent of all described species including 35 percent of all vertebrates [23], especially fish. Large rivers with higher flow volumes tend to support more fish species, and tropical rivers tend to have higher levels of species richness. The highest levels of riverine fish species richness are found in the Amazon, Orinoco, Tocantins, and the Paraná in South America; the Congo, the Niger Delta, and the Ogooue in Africa; and the Yangtze, Pearl, Brahmaputra, Ganges, Mekong, Chao Phraya, Sittang, and Irrawaddy in Asia [24]. Large rivers are also especially important for freshwater megafauna with slow life-history strategies and complex habitat requirements [25]. Globally, freshwater megafauna populations declined by 88% from 1970 to 2012, with mega-fishes exhibiting the greatest global decline (−94%) [25]. These major biodiversity declines highlight the

conflicts between economic development and environmental protection and conservation in large river basins.

Large river basins present particular challenges and opportunities for the use of emerging technologies in support of water resources management. Geographically, large basins typically traverse a wide range of hydroclimatic regimes, and processes that characterize basin-scale hydrological behavior take place at multiple scales. These give rise to technical challenges for the design and operation of hydrometeorological data collection systems, including the integration of ground-based and remote observations. Because they often span multiple jurisdictions—within and, or between countries—large basins have institutional complexities for coordinated data collection, sharing and analysis, as well as for decision making and for coordinated real-time operational management.
