Regional impacts of non-conventional water use: the role of water-systems thinking and modelling

Although the natural water system and urban water cycle are traditionally considered as separate domains, they are physically strongly connected. The hydrological system supplies water for anthropogenic use, and after use and treatment, wastewater is released from the urban water cycle back to the natural water system. Yet within the broad range of adaptation measures aimed at improving regional water availability, solutions that intentionally leverage these linkages remain underexplored. Exploitation of non-conventional water resources – such as industrial of domestic wastewater – is only recently being explored in temperate climates like in the Netherlands, as a complement to traditional groundwater and surface water supplies. Embracing such circular approaches, instead of the prevailing linear practice in which water is quickly discharged from an area, offers new opportunities for more balanced water allocation to protect the environment that depends so heavily on water resources.

We present examples in which the benefits and risks of cross-sectoral measures have been assessed. These include a brewery initiative in the southern Netherlands applying treated industrial wastewater for subsurface irrigation to reduce agricultural drought stress, the reuse of domestic wastewater for industrial applications, and water reuse for drinking water production. Such approaches have the potential to alleviate pressure on water resources which could benefit other water-dependent functions. Our findings show that a system perspective and clear evaluation criteria are essential to quantify the real potential of such (cross-sectoral) approaches and to identify the propagation of the effects of using non-conventional sources through the regional water system, including associated trade-offs. For instance, determining what proportion of residual water can be used or reused for agricultural drought mitigation requires assessing net effects on other functions, such as nature. Across the examples, we find that the propagation of quantitative effects – both positive and negative – remains insufficiently explored.

We further show that while dedicated models can effectively assess subsystem responses, they may be inadequate when a broader, integrated perspective is needed. Water systems thinking and modelling are increasingly used to analyse complex dynamic water systems, including groundwater systems, and are helpful in studying multiple water uses and planning strategies. Tools as Sankey diagram visualizations, related causal loop diagrams, and resulting system dynamics modelling frameworks help explore the regional feasibility of water (re)use, its potential to reduce groundwater and surface water demand, and the possible synergies and trade-offs between sectors. Additionally, these tools can serve as communication frameworks to engage stakeholders and support users/policy makers in understanding all the interlinkages, benefits and trade-offs of measures. We illustrate these insights with case-study implementations.