Carbon balance zoning-informed multi-objective water allocation under the water-carbon nexus in economically developed regions

Focusing on water supply-demand and carbon balance challenges in economically developed regions, this study develops a systematic framework of “carbon balance analysis→ water demand prediction→ water-carbon nexus→ multi-objective optimization→ scheme evaluation”. The InVEST model estimates carbon sink capacity and identifies regions with severe carbon imbalance (Suzhou, Wuxi, Changzhou, Zhenjiang, Nanjing) based on carbon supply-demand ratios. Current development and low-carbon scenarios are set considering population, urbanization, and industrial structure. A multi-objective optimization model is constructed to minimize water deficit and carbon emissions, solved by NSGA-II. An entropy weight-TOPSIS method integrated with carbon zoning results is applied for scheme screening. Results show: (1) Carbon sinks exhibited a “high north, low south” pattern with declining totals from 2005 to 2020, while emissions concentrated in the south and nearly doubled. (2) Under 2030 P = 75% inflow frequency, water shortages occur under both scenarios; however, the low-carbon scenario significantly improves the shortage rate from 13.90% to 6.07%. (3) Carbon emissions are reduced by 13.02 Mt. and 11.17 Mt. under 50% and 75% frequencies, with the five imbalanced regions contributing 56.85% and 62.37% of total reductions under low-carbon scenario. The five carbon-deficit regions serve as the primary contributors to these reductions, highlighting the spatial effectiveness of low-carbon water allocation. This study reveals water-carbon synergistic regulation patterns and provides quantitative decision support for sustainable water management under the “dual carbon” goals.