Water is an essential natural resource for human survival as well as the health of the entire ecosystem, including agriculture. It is fundamental for long-term sustainable growth of economies and societies globally and locally. Water resources, which cross political boundaries, are vital for both the environment and human populations. Transboundary water management refers to the cooperative process of managing shared water bodies across political boundaries, ensuring the equitable and sustainable use of water resources by riparian states (Bernauer & Böhmelt, 2020). Cooperation and operational arrangements on transboundary water management among riparian countries are necessary for sustainable development in transboundary waters (Bernauer & Böhmelt, 2020; Yalew et al., 2021). 

One important tool in this context is River Basin Management Plans (RBMPs), which help riparian countries coordinate water policies, integrating water quality, quantity, and environmental protection measures (Yalew et al., 2021). The United Nations Water Conference 2023 highlighted for measures to improve stakeholder cooperation and emphasized the need for tangible initiatives to support integrated water resources management in order to achieve the Sustainable Development Goals (SDGs). Given that water does not adhere to political boundaries, data and information sharing across these borders is crucial. 

However, one of the key challenges for transboundary water management is data and information exchange across borders. Mianabadi et al. suggest that challenges such as mistrust and a lack of cooperation arise from the reluctance of riparian states to share accurate and timely data, which poses a significant barrier to both operational and strategic planning in transboundary water management (Mianabadi et al., 2020). Although mutual data exchange across borders is a recognized principle of international water law, only a limited number of transboundary water treaties explicitly enforce it (Leb, 2020). For instance, the 1997 UN Convention on the Law of Non-Navigational Uses of International Watercourses requires the regular exchange of data and information, such as hydrological, meteorological, and water quality data, to ensure effective watercourse management (United Nations, 1997). Furthermore, the EU Water Framework Directive (WFD) emphasizes the importance of cooperation among member states in managing transboundary water bodies, with a focus on achieving a "good status" for all water bodies by requiring joint monitoring, data exchange, and coordinated management plans. This framework directive underscores the necessity of mutual data exchange as a critical element in effectively managing shared water resources, aligning with the principles of international water law. The United Nations' Sustainable Development Goals (SDGs), especially SDG 6.5, also emphasize the need for countries to collaborate on shared water resources, fostering cooperative agreements and joint management to improve water use efficiency and ensure access to water for all. Sadly, only thirty to fifty percent of transboundary agreements across all continents have a direct information-sharing mechanism (Gerlak et al., 2011).

Important parameters in transboundary water management

To provide an overview of the critically important variables for monitoring in a transboundary setting, relevant variables and indicators were elicited for joint monitoring, data exchange, and coordinated management plans. The following provides a selection of the most important ones. One of the most critical variables in transboundary water management is river flow, which includes parameters like discharge, water level, and sediment transport. Discharge must ideally be monitored continuously through flow meters, remote sensing, and/or hydrological models to inform equitable water sharing and mitigate risks of flooding or drought (Akpoti et al., 2024). Sediment transport is equally important, as it affects water quality and can lead to downstream issues like siltation in reservoirs, impacting storage capacity and water flow (Estigoni et al., 2017). Moreover, Water temperature, pH, dissolved oxygen, and nutrient concentration should also be monitored regularly. However, poor or no data sharing between countries jeopardize this action and lead to gaps in understanding the overall health of share water resources. 

Furthermore, dams play a significant role in transboundary water management as they affect water availability and flood risks on the downstream country. Thus, monitoring the amount of water released from dams is crucial particularly during dry spells or in water scarce regions. Additionally, in the event of a dam failure, it is critical to have accurate data on the potential areas that could be flooded. Monitoring dam safety and performance is essential, and this can be facilitated through a combination of ground-based sensors and satellite technology which allows for the monitoring of structural integrity and water levels in real-time (Adamo et al., 2021). Additionally, the health of ecosystems in transboundary water systems is critically important as it provides valuable services such as water filtration, flood control, and supporting biodiversity all of which are crucial for the resilience of transboundary water systems (Anghileri et al., 2024)). Therefore, the use transboundary agreements and cooperative frameworks are vital to overcoming challenges related to water release management, dam safety, and ecosystem health, and ensuring that shared water resources are managed sustainably for the benefit of all stakeholders.

The use of EO for transboundary water management: 

This article aims to provide an extensive summary of space technology applications for transboundary water management. Table 1 provides an overview of available satellite missions, the sensors mounted, their spatial and temporal resolutions, as well as the parameters they measure in the context of numerous applications relevant for transboundary water management. Satellite missions such like Landsat, Sentinel-1, 2, and 3, GRACE-FO are also important for decision-making in transboundary water management and newer missions such as SWOT will be in the future. These missions provide critical data for monitoring land use, surface water, groundwater, floods, and environmental impacts across borders, offering continuous and reliable information essential for managing shared water resources.

The case studies illustrated in this article further underscore key applications of space technologies in real-world scenarios. For instance, the use of EO for dam and reservoir monitoring, river flow and aquifer management, and water quality monitoring that all highlight the importance of continuous and reliable monitoring for effective management and cooperation in shared water resources. 

River flow

Accurate monitoring of river flow measured in terms of volume, velocity, and seasonal fluctuations is essential for effective management of shared watercourses. Technologies like remote sensing enable continuous monitoring of flow (Junqueira et al., 2021). River flow data is especially vital for managing large transboundary rivers where seasonal changes in discharge influence agricultural and urban water use downstream. However, without real-time data on flow variations, riparian states may struggle to achieve equitable water distribution and ensure sustainable usage. Space-based technologies like SWOT and GRACE-FO provide precise measurements of river flow dynamics and operational planning (Ndehedehe, 2023).

Water level

Satellite radar altimetry has immense potential for monitoring surface water resources and predicting the intra-seasonal, seasonal, and inter-annual variability of inundated surface water over large river basins. Over the last two decades, satellite missions such as the Environmental Satellite (ENVISAT), European Remote Sensing (ERS), Topography Experiment (TOPEX)/Poseidon, and Joint Altimetry Satellite Oceanography Network (Jason) have been launched to monitor water levels in lakes. Altimetry-based water levels have been utilized to monitor transboundary river basins such as the Brahmaputra (Maswood and Hossain, 2016), Amazon (Silva et al., 2012), Orinoco (Frappart et al., 2015), and Congo rivers (Becker et al., 2014) through the observations of hydraulic variables such as cross-sectional area, width, slope, and surface water level (Gleason and Hamdan, 2017).

Dam and reservoir monitoring 

There are various ways to select a dam or reservoir site. Unfortunately, most traditional methods are costly and time-consuming. Advances in space technologies, however, allow to save on both, time and cost. Satellite remote sensing enables the continuous monitoring of key parameters such as land use/land cover (LULC), vegetation density, soil erosion, and water quality. By analyzing LULC and water quality data, it’s possible to assess the sedimentation risk. For instance, a dam near poorly vegetated, bare land will likely accumulate higher sediment and debris, affecting storage capacity and performance (Zhao et al., 2017). Landsat and Sentinel allow for real-time updates on these environmental factors, which can help predict sedimentation and inform maintenance needs. Additionally, satellite-based hydrological modeling can predict inflow and outflow patterns, assisting in optimal water release schedules (Shen et al., 2022). By utilizing outputs from those datasets, dam managers can make informed decisions on dam lifespan and enhancing operational efficiency.

Dam Site Selection

A well-selected site for a dam may provide optimum benefits with minimal problems related to performance and environmental impact issues. The use of EO allows for a comprehensive analysis of natural landscape features such as terrain stability and water availability that are critical for sustainable project planning, reducing risks of costs of environmental degradation and operational challenges. The engineering and construction of a dam can be complex, as it involves persons from many disciplines based on requirements dictated by the landscape or other environmental factors as well as social rules. Remote sensing can help provide basic data on natural factors such as elevation, terrain, or slope, which would otherwise have to be obtained by costly and time-consuming traditional methods. The diversity of satellites and sensors available today offers powerful tools for informed decision making, ensuring that developments are compatible with both natural landscape and social considerations. However, decision making remains fundamentally a human driven process, balancing technical insights with broader project requirements. 

Dam Safety

Dam safety is crucial for protecting downstream communities and infrastructure (Wishart et al., 2020). One major concern for dam stability is subsidence, which can be caused by various factors such as groundwater extraction, mining, and geological shift. Subsidence can cause cracks or shifts in the dam, affecting its integrity and increasing the risk of failure. Monitoring dam safety using satellite-based tools helps detect early signs of ground movement, allowing timely interventions to maintain dam safety (Lumbroso et al., 2019).

Figure 1 illustrates the first comprehensive multi-sensor cumulative deformation map for the dam generated from space-based synthetic aperture radar (SAR) measurements, which reveals that parts of the dam are undergoing rapid subsidence. Deformation was rapid during 2004–2010, it slowed in 2012–2014, and has increased since 2014. Negative values indicate motion away from the satellite, consistent with subsidence. 

One of the important applications of EO is monitoring the health of dams, water control structures, and landslides (Figure 2). New concepts for Areal Deformation Monitoring (ADM), namely the Space-Borne Interferometric Synthetic Aperture Radar (InSAR), and Global Navigation Satellite System (GNSS) are under development and investigated for use in monitoring dams.  These advancements including instrumental advances, sensor and process models, and data processing techniques require thorough validation and assessment before they can be safely applied in critical monitoring applications (Adamo et al., 2021). GNSS techniques have reached consolidated maturity for dam deformation monitoring, either for the periodical measurement of networks or for implementation into continuously operating systems (Scaioni & Wang, 2016). Research in data processing is ongoing, particularly aimed at improving the robustness and accuracy of geodetic networks. The development of sensor networks and methodologies for data integration has offered the opportunity to analyze different observations in a spatial and temporal context. In addition, Ground-Based Interferometric Radar (GBIR) can be used to derive differential deformation with very high resolution, particularly with respect to persistent scatterers. In recent years, some of these new technologies have been applied to dam deformation measurement, offering unprecedented opportunities for safety management and structural analysis. Among these are ADM techniques from Terrestrial Laser Scanning, Ground-Based InSAR, and spaceborne InSAR sensors, which provide the chance to extend the observed region to a large portion of a structure, instead of merely measuring a set of a few control points, in addition to GNSS techniques.

Water quality monitoring 

In transboundary water management, water quality is a critical concern, as downstream countries rely on receiving water that is safe for drinking, agriculture, and ecosystem health (Puri, 2004). Polluted water can carry significant risks, including ecosystem degradation, health hazards, and increased treatment costs for downstream nations. Large-scale dynamic and accurate water quality monitoring is necessary for the most urgent and successful actions. Water quality is the general term used to describe the chemical, physical, biological and thermal characteristics of water e.g., temperature, chlorophyll content, turbidity, and clarity among others (IOCCG, 2018). If you want to learn more about water quality aided by RS, read our article on water quality indicators monitored from space and our interview with Stuart Crane, Programme Management Officer at UN Environment.

Conventional approaches of monitoring water quality in situ often have difficulties in delivering the temporal and spatial coverage required for an effective assessment of big water bodies. On the contrary, the use of EO techniques is becoming more significant because they provide better temporal and geographical sample frequencies (Fouladi et al., 2022). Water quality metrics in EO are calculated by observing changes in the water's optical characteristics brought on by the presence of contaminants. Therefore, the estimation of water quality indicators has been a typical application of optical remote sensing. The best wavelength to monitor water quality via satellite depends on the chemical (the object of interest) to be detected. The visible and near-infrared radiation bands of the electro-magnetic radiation spectrum, with wavelengths ranging from 0.7 to 0.8 µm, were determined to be the most beneficial for tracking suspended sediments in water based on many in-situ analyses (Nazirova et al., 2021; Pereira et al., 2018). The optical characteristics of water obtained by remote sensing methods are subsequently transformed into water quality indicators through the application of physical models, radiative transfer functions, or empirical connections.

Researchers studying water resources and decision-makers can monitor water quality more successfully with the help of remotely sensed data (Gholizadeh et al., 2016). The ability and methods of remote sensing have been researched throughout the last few decades in order to monitor various aspects of water quality. The use of spaceborne sensors has the following drawbacks despite its great importance: 

• In some cases, spaceborne sensors are confronted with significant cloud limits. 
• Empirical and semi-empirical approaches in multispectral image analysis by spaceborne sensors may lead to overestimations at areas where there is a contribution of reflectance from the bottom to the water leaving reflectance. 
• Spaceborne sensors have a limited ability to cover the electromagnetic spectrum. The accuracy of the WQP estimation may be impacted by the absence of coverage in some bands, such as the middle infrared and the thermal bands.

Transboundary aquifer management using EO

Transboundary aquifers are aquifers that are located below the Earth’s surface of more than one state. They are crucial for riparian nations, as these resources often supply water for human consumption, agriculture, and industrial needs. Effective aquifer management requires an understanding of groundwater levels, recharge rates, and water usage patterns, which vary from region to region and impact all dependent countries. Hence, an improved awareness of the aquifer potential is essential for planning and long-term development. While test drilling and stratigraphy analyses are the traditional and effective techniques for identifying the locations of an aquifer, these processes are cost and time-consuming (Battaglin et al., 1993). 

In contrast, EO offers the advantage of covering large spatial scale, making it invaluable for depicting basin physiographic characteristics such as land use/land cover, slope, and drainage density, as well as structural features like fractures, faults, and cleavages (Derdour et al., 2022). Additionally, the Gravity Recovery and Climate Experiment (GRACE) and its follow-on mission GRACE-FO provide important data on changes in Earth’s gravitational field, which can indicate shifts in groundwater storage over time. This data is particularly suited for very large aquifer systems due to its spatial resolution of approximately 300 km, supporting regional groundwater management on a broad scale. GRACE-derived data is instrumental for large-scale transboundary groundwater assessments, especially in data-scarce regions (Gaffoor et al., 2021). 

EO offers area-wide coverage and is transboundary in nature. However, EO of aquifers, usually works indirectly by means of proxy information or monitoring of secondary effects. Although groundwater cannot be observed directly by existing EO satellites, the location, orientation, and length of lineaments can be derived from EO data. These lineaments can then be used as inputs for studies of fractured aquifers, such as identifying sites for water harvesting. Despite groundwater being hidden up to over a kilometre below the surface, EO has demonstrated that observations from space of the Earth’s surface can provide useful and effective information to decision-makers and strengthen cooperative management of internationally shared aquifers. Thus, these techniques make it possible to study variations in groundwater in inaccessible areas or such for which no ground-based data are available. For further information on monitoring groundwater from space, please read our article on Space technologies in the detection, monitoring and management of groundwater here. While the aforementioned applications highlight the potential of EO in managing transboundary water management, it is also essential to consider the broader implications, including the benefits and risks associated with data sharing in these shared resources.

Benefits and risks of data sharing

Sharing data on transboundary water between riparian states is broadly regarded as essential in the history of cooperation in the context of water treaties. However, not all countries choose to participate in data sharing which can create challenges in transboundary water management. When countries do engage in information exchange, it helps establish trust among riparian countries and fosters cooperation (Mogomotsi et al., 2020).

According to the rational design approach in international relations¹, in the face of uncertainty, governments find it beneficial to pool their information and resources (Koremenos et al. 2001). In this way, data exchange on water resources can provide an initial step toward broader transboundary cooperation and agreement formation. Moreover, a solid information base is compulsory to delineate areas of disagreement and agreement and to inform and structure debate. During water negotiations in the Ganges Brahmaputra Meghna River system, a fundamental issue for states was the insufficient and unreliable data on river flows (Crow & Lindquist, 1990). Similarly, in the Euphrates–Tigris River basin, the absence of standardized institutions and comprehensive information, alongside major political challenges, hindered the riparian states from reaching an agreement (Nishat and Faisal 2000). The exchange of data and information can however form a basis for coordinated management, which at a basin level can improve water use efficiency and minimize the impacts of droughts, floods, and other extreme events (Grossman, 2019). This can be seen in the example of the Colorado River Basin, where collaborative efforts among state and stakeholders have led to enhanced water conservation strategies (Stern & Sheikh, 2022). 

While data and information exchange appear to offer significant benefits to states, it also presents several risks that stakeholders must consider carefully. States may be hesitant to participate in international cooperation due to concerns of a relative loss outweighing the benefit of any absolute gain, making cooperation detrimental to comparative power (Baldwin, 1993). It should also be acknowledged that sharing data during water treaty negotiations might be seen as weakening the bargaining position of one or more riparian states, which could lead to reluctance and potentially hinder progress in other areas of political agenda. Countries may be wary of sharing data that could expose unbalanced water usage, leading to diplomatic tension. There is also a concern that such data might be used to gain political leverage, influencing negotiations unfavorably for certain parties. Thus, data or information may be used to blame other parties for causing negative conditions in a shared basin (Timmerman and Langaas, 2005). Furthermore, the complexity of EO data can lead to misinterpretation, particularly among non-experts. Misunderstanding technical details may result in misguided policies, exacerbating existing tensions rather than resolving them. Ensuring that all stakeholders have the capacity to accurately interpret and utilize the data is crucial for effective collaboration (Solimini, 2016).

Conclusion

Cooperation and data sharing on transboundary water management among riparian states are crucial for sustainable development. To ensure effective watercourse management the most critical variables such as hydrological, meteorological, and water quality data needs regular exchange. Compared to field data measurements, Space-based technologies and applications are a cost and time effective approach that can significantly enhance transboundary water management efforts. EO provides various datasets such as Landsat, Sentinel, GRACE-FO, and VIIRS at respectively different spatial and temporal resolutions. The wide range of application of space technologies particularly in dam and reservoir monitoring, rifer flow and aquifer management, and water quality monitoring highlight the importance of continuous and reliable monitoring for effective management and cooperation in shared water resources. While transboundary data and information sharing lead to a better understanding of water resources in the region, help establish trust among riparian countries, and foster cooperation, it also comes with certain risks that each country has to assess unilaterally.

¹In the rational design approach of international relations, countries weigh costs and benefits when making decisions about cooperation. In situations of uncertainty, like managing shared resources, governments benefit from pooling information and resources to reduce risks and make better-informed decisions.