Analytical trade-off simulation-optimization of drought-resistant water supply allocation strategies under various demands using a multi-objective cross-watersheds network-flow model

This study analyzes trade-offs in water supply during shortages and droughts, focusing on sustainable measures at various demand nodes. It introduces drought-specific methods and optimal strategies. The methodology includes: (1) deriving an analytical solution for the water shortage index in a cross-watershed, network-flow, diverse supply system; (2) drought low-flow frequency analysis; (3) designing low-flow events using the Alternating Block Method; (4) a multi-objective simulation optimization model for resource allocation; (5) pattern analysis of water supply for industrial, livelihood, and agricultural needs; and (6-7) trade-off analysis of fallow strategies with cross-watershed diversion using recycled and hyporheic water. To address extreme events, the model's objective shifts from a yearly water shortage index to a ten-day modified shortage index (MSI), aiming to reduce tap and irrigation shortages. Decision variables include dam releases, tap and irrigation water supply, and regional diversion, with constraints on flow continuity and physical limits. The cross-watershed reservoir network-flow allocation model in Taiwan is developed using GAMS. Without agricultural fallow during the 2020 drought, tap water shortages would reach 29.43%, 18.13%, and 12.58% in Hsinchu, Taoyuan, and Banxin. Opening the Taoyuan-Hsinchu support pipeline reduces shortages by 4.16%-5.58% under non-fallow and fallow scenarios. Optimal fallow can cut shortages in Shimen and Taoyuan by 35.39% and 28.41%, respectively. During 200-year drought scenarios, shortages only occur in Hsinchu by 13.81%-15.32%, and pipeline operation reduces shortages to below 0.11%. To bring shortages below 3%, fallows are necessary across all areas during long-lasting, high-return droughts, where shortages maximum rise to 95.67%. Recycled water further helps reduce shortages in Shimen and Taoyuan by up to 9.18%.