The contextualization of macroplastics in a budget framework can help to improve our understanding of the physical processes determining macroplastic behaviour and impacts in rivers. Hence, better management practices can be adopted.

Microplastics pose numerous threats to aquatic environments, yet understanding their transport mechanisms remains limited. Drawing from natural sediment research provides valuable insights to address this knowledge gap. One key dimensionless number used to describe sediment transport is the transport stage, referring to the ratio between the flow shear velocity and the particle settling velocity. However, variations in physical properties, such as shape and density, raise concerns about the applicability of existing sediment transport theories to microplastics. To address this challenge, we employed a physical modeling approach, examining 24 different nonbuoyant microplastic particles in a turbulent open channel flow. Utilizing 3D particle tracking, a total of 720 trajectories were recorded and analyzed. Microplastic particles exhibited transport modes akin to natural sediments, including rolling/sliding, saltation, and suspension. The transport stage strongly correlated with these modes, as well as with the mean forward velocity and mean position in the water column. Notably, particle shape emerged as a critical factor influencing transport dynamics. Due to their lower settling velocity, fibers tended to stay closer to the water surface with lower forward velocities compared to spheres. Based on the laboratory results, a new phase diagram for microplastics is introduced analogous to an existing diagram for sediments.

This paper shares an early-career perspective on potential themes for the upcoming International Association of Hydrological Sciences (IAHS) scientific decade (SD). This opinion paper synthesizes six discussion sessions in western Europe identifying three themes that all offer a different perspective on the hydrological threats the world faces and could serve to direct the broader hydrological community: “Tipping points and thresholds in hydrology”, “Intensification of the water cycle”, and “Water services under pressure”. Additionally, four trends were distinguished concerning the way in which hydrological research is conducted: big data, bridging science and practice, open science, and inter- and multidisciplinarity. These themes and trends will provide valuable input for future discussions on the theme for the next IAHS SD. We encourage other Early-Career Scientists to voice their opinion by organizing their own discussion sessions and commenting on this paper to make this initiative grow from a regional initiative to a global movement.

Jordan’s water scarcity prompted a national plan whereby treated wastewater is utilized to amend agricultural irrigation water so as to reallocate freshwater to urban/domestic uses. The policy, however, has engendered farmers’ resistance in the Northern Jordan Valley (NJV), causing a stalemate in putting new infrastructure into operation. This research investigated the socio-economic causes of farmer resistance and contestation, and examined the government’s institutional approach to overcome the challenges. We found that the perceived risks of wastewater reuse such as salinization and restrictions from international markets figure prominently in the farmers resistance. As yet, farmers have managed to avoid the shift to treated wastewater use by using the political agency of elite farmers who control the Water Users Associations. These same farmers have adopted informal water access practices to overcome freshwater shortages. At the same time, small producers who don’t have possibilities to access extra water and with less political clout seem more willing to irrigate with treated wastewater. We conclude that understanding the heterogeneous context in which the envisioned wastewater users operate is key to predicting and solving conflicts that arise in treated wastewater reuse projects.

Recently, the rapid increase in global plastics production has caused various ecological and economic issues, worsened by poor material and waste management. Among the market-based instruments that could help mitigate the environmental impacts of plastics throughout their life-cycle, we evaluate the advantages and limitations of incorporating a cap-and-trade (CAT) system into future policy mixes. Our aim is to inspire further investigation of CAT’s feasibility rather than presenting it as the ultimate solution. Drawing from past CAT implementations in domains such as water resource management and carbon emissions, we outline three key policy design considerations: (1) material and target group identification, (2) cap establishment and permit allocation, and (3) development of a competitive market environment. We explore a three-tiered approach with global, national, and sectoral caps covering the plastic lifecycle from cradle to grave. While there are viable reasons to consider a plastics CAT, significant challenges persist, which may ultimately limit its implementation. In the context of ongoing UN Plastics Treaty negotiations or future policy developments, this evaluation of CAT can be beneficial for assessing when and how this tool can address the negative externalities of plastics.

Microplastics pose numerous threats to aquatic environments, yet general understanding of the mechanisms governing their transport remains limited. Drawing upon research on natural sediment provides a valuable resource to address this knowledge gap. One key dimensionless number used to describe sediment transport is the transport stage, referring to the ratio between the fluids shear velocity and the particle settling velocity. However, differences in physical properties (e.g., shape, density) raise concerns regarding the applicability of existing sediment transport theories to microplastics. To address this challenge, we employed a physical modelling approach to examine the trajectories of 24 negatively buoyant microplastic particles varying in size, shape and density. Utilizing a 3D particle tracking setup, we captured the movement of the particles in turbulent open channel flow. A total of 720 trajectories where recorded and analysed. The results revealed a strong correlation between the transport stage and the mean forward velocity of the particles, as well as their mean position in the water column. Notably, particle shape emerged as a critical factor influencing particle transport dynamics. Fibres showed a tendency to be transported closer to the water surface while experiencing slower mean forward velocities compared to spheres. The microplastic particles exhibited rolling/sliding, saltation and suspension as transport modes, comparable to natural sediment. The results show a strong correlation between the transport stage and the percentage of time in which microplastics experienced a certain mode of transport. Based on the laboratory results, a new phase diagram for microplastics is introduced, analogous to an existing diagram for sediments.