Wastewater matters: incorporating wastewater treatment and reuse into a process-based hydrological model (CWatM v1.08)

Abstract. Wastewater treatment and reuse are becoming increasingly critical for enhancing water use efficiency and ensuring reliable water availability. Wastewater also significantly influences hydrological dynamics within urban watersheds. Although hydrological modeling has advanced to incorporate human–water interactions, large-scale and multi-resolution models often lack the comprehensive integration of wastewater treatment and reuse processes. This paper presents the new wastewater treatment and reuse module as part of the hydrological Community Water Model (CWatM) and demonstrates its capabilities and advantages in an urban watershed with intermittent flows. Incorporating wastewater into the model improves model performance by better representing low and peak flows during the respective dry and wet seasons. It allows for the representation of sectoral wastewater reuse, the exploration of different measures to increase wastewater reuse, and the examination of the effects of wastewater reuse on the water stress level. Modeling wastewater treatment and reuse is particularly relevant in regions with semi-arid or arid climates, rapid urbanization, or active policies promoting water reuse. The wastewater treatment and reuse module could be upscaled by minimizing the data requirements via simplified workflows. Combined with the availability of recent datasets on wastewater treatment plants and processes, a global application of the module is feasible. As current developments focus on water quantity, the water quality dimension of wastewater treatment remains a limitation. This opens prospects for incorporating water quality into the model and developing global input data for wastewater treatment and reuse.