Mitigation of anthropogenic climate change requires, among other measures, the development of low-emission and renewable energy sources. Although wind and solar energy are expected to have the largest potential contribution1, hydropower could still play an important role, if only because of its capacity to deliver electricity independently of weather conditions. (Fig. 1)

Fig. 1: Global distribution of feasible, profitable and currently developed hydropower potential, per continent (bar graphs).
figure 1

Individual countries are coloured by their unused potential, ranging from green (ample potential) to red (already exceeded). Figure adapted from ref. 5.

Currently, hydropower is the largest renewable source of electricity, generating more than all other renewable technologies combined: 54% of all renewable electricity; 15% of all electricity and 3% of all primary energy2. However, renewable energy is not by definition sustainable. Installations and infrastructure for solar, wind and hydropower require large quantities of minerals3, and solar farms and hydropower basins may compete for space with agricultural use.

The decision whether or not, or where, new hydropower systems are to be installed will only rarely be taken by policy makers in isolation, that is, without consulting both technical experts and societal stakeholders. To develop well supported policies, it is important to have a common understanding of the underlying assumptions and the impact of policy decisions. In that way, disputes about the underlying facts can be avoided and the discussion can be focused on (societal) values, costs and benefits. Information therefore needs to be credible (scientifically adequate), salient (relevant with regard to the need of decision makers) and legitimate (respectful of stakeholder’s divergent values and beliefs)4.

One consequence of this point of view is that assessments of hydropower potential should not only take technical constraints into consideration, but also social, economic and environmental. Now writing in Nature Water, Rongrong Xu and co-workers perform an exhaustive analysis of the global network of rivers to identify those hydropower locations that are still unused, and that have reduced environmental and social impacts5.

A common approach in exploring the options for future expansion for renewable energy is to analyse energy potential in terms of a series of constraints applied to a theoretical potential. In the case of hydropower, which is ultimately generated by gravity acting on an elevation head, this theoretical potential is defined by the potential energy of water with respect to a base level. In practice, however, almost all of this energy is lost to friction within natural river beds. Installations such as smooth pipes (reducing friction) and dams (increasing head gradients) are required to harness a meaningful fraction of this potential energy. The first constraint is thus a technical one.

A second constraint is an economical one: not all dams will be cost-effective. Costs can be related to dam construction (wide valleys requiring a large dam), accessibility, or land use. River valleys tend to be populated areas, so submerging the areas upstream from a dam location is not always a welcome or feasible option.

A third constraint deals with social and environmental impacts. Hydropower installations may require displacement of people, disrupt natural stream flow, sediment and nutrient supplies, and affect fish stocks and aquatic ecology6,7,8.

In a previous study, Gernaat et al.9 revolutionized the field of hydropower potential by developing a high-resolution assessment methodology, taking both physical and socio–economic constraints into account. The method involved a systematic scan of (almost) all rivers worldwide, using a high-resolution digital terrain model. Potential sites for hydropower stations were explored every 25 km along these rivers by analysing climate and terrain data and considering hydropower capacity and socio–economic costs as a function of station characteristics such as dam height. The cost model included costs for the construction of the dam itself, which is dependent on local topography, the technical installations (turbines, power lines, etc.) and the socio–economic costs related to the land use in the part of the upstream catchment that will be drowned by the dam reservoir. A cost optimization model was used to determine the trade-off between energy production and cost, prioritizing dams with the lowest cost per kWh. Finally, ecological constraints such as ecological flow restrictions and protected areas were considered.

The recent paper by Xu and colleagues5 improves upon this work9 by having a better geographical coverage (60˚ S to 90˚ N instead of 56˚ S to 60˚ N, effectively now including for example, Scandinavia), a higher resolution topography (3” instead of 15”), a denser search grid (4.5 km instead of 25 km) and more environmental and social constraints. For example, they directly exclude hydropower plant development in heritage areas, biodiversity hotspots, forests, peatlands, earthquake-prone zones, densely populated areas, and locations where dams/reservoirs already exist.

Their results suggest that the major share of the total theoretical potential for hydropower (58 PWh/year) is either already installed, not feasible or not profitable, leaving only 5.3 PWh/year as unused and profitable potential, mainly located in Asia (3.9 PWh) and, to a much lesser extent, in Africa (0.6 PWh). Europe and the Americas on the other hand have already over exploited their hydropower potential when both economic and environmental constraints are fully taken into account (Fig. 1).

Given the many controversies surrounding hydropower7,8, the added value of this study is that it allows for full accounting of social and environmental impacts of potential hydropower sites. This way, credibility, saliency and legitimacy of hydropower potential assessments are simultaneously enhanced, allowing for development of sustainable hydropower development strategies.