Coordinating systems headroom for more efficient multi-catchment water quality management at a critical checkpoint

Managing river water quality at critical checkpoints that have significant impacts on water use is important for sustainable catchment development. This requires managing the whole multi-catchment systems upstream of a critical checkpoint. Concepts of systems headroom (deficit below permit) and excess (extra above permit) have been used to distinguish sub-catchments’ roles in pollution contribution. Based on the concepts, we propose a three-phase management approach. In the first phase, we frame the headroom and excess at temporal, spatial, and source domains and evaluate them to investigate the systems mechanisms. The evaluation is by simulating physical processes a semi-distributed integrated model (CatchWat-SD). We apply the model to 12 sub-catchments that make up the Upper Thames river basin and validate it using monitoring data. In the second phase, we incorporate the evaluated headroom and excess in loads allocation to develop a strategy that coordinates systems headroom for more efficient and realistic interventions. In the last phase, we validate the strategies by simulating the scenarios that coordinate headroom at different domains and evaluating them in water quality improvement, efficiency, temporal steadiness, spatial homogeneity, and practical feasibility. Results show that dry seasons, downstream catchments and urban sources generally have more excess. Thus, more target loads reduction is allocated to dry season, downstream catchments and urban sources in the fully coordinated scenario. The higher degree of headroom coordination a strategy achieves, the better performance this strategy generally obtains in all five metrics. This study emphasises the need to incorporate headroom in loads reduction allocation, which helps to more efficiently improve systems water quality performance with a more realistic degree of intervention. The whole procedure can be further expanded to water quality management at multiple checkpoints for sustainable management in large catchments.