[A-HW28] Hydrology and Water Environment

[AHW28-P22] Trade-off analysis of drought-resistant water supply strategies and allocation measures using multi-objective interbasin net-flow simulation-optimization model

*Chien-Lin Huang¹, Gene J.-Y. You¹, Nien-Sheng Hsu¹ (1.National Taiwan University, Taiwan)

Keywords:Drought-resistant trade-off analysis, interbasin water resource system, network flow model, water supply allocation, simulation optimization model, multi-objective operation

This study aims to analyze the water shortage trade-offs of the optimized water supply-allocating patterns under unevenly distributed hydrological drought conditional scenarios to propose formulated methods for various drought scenarios under supportable supplied measures in diverse water demand nodes and discuss the optimal drought-resistant adaptive operational strategies. The established methodology consists of the following topics: (1) Drought annual minimum low-flow frequency analysis with various durations; (2) Designing drought low-flow hydrograph using Alternating Block Method; (3) Multi-objective simulation optimization model construction for diverse water supply and allocation; (4) Scenario simulated trade-off analysis of water supply optimization for industrial and livelihood tap water and agricultural irrigation water; (5) Fallow strategy simulation optimization under cross-watershed supply support in designed drought low-flow event; (6) Trade-off analysis of optimized fallow modes under cross-regional water diversion and drought-resistant supply support measures from reclaimed and hyporheic water. To describe the unsteady extreme events between flood peaks and drought low-flows with spatiotemporal uniform water demand spectrum, this study improves the objective function of the optimization model from the traditional annual lumped water shortage index (SI) to the refined ten-days-interval network-distributed modified shortage index (MSI), which is to minimize the shortage rate of industrial and livelihood tap water and agricultural water demand nodes after optimized supply allocating operations. The decision variables include the quantity of water released from dams, intake or supply of industrial and livelihood tap water, irrigation water intake, and inter- and cross-regional water diversion for supply support. The constraints include the water flow mass conservation continuity equation and physical limit for various nodes of dam/weir, reservoir storage, irrigation/tap water demands, water diversion/intake, lateral confluence, and junction. This study constructs the water supply allocating simulation optimization model using the General Algebraic Modeling System (GAMS), solved by the constrained nonlinear programming quasi-Newton method. The cross-watershed supply network flow system across northern Taiwan incorporates the water resources of Shimen Reservoir, Feitsui Reservoir, and Baoshan II Reservoirs. Simulations from 2020 to 2022 show that opening the Taoyuan-Hsinchu tap water supporting pipe can reduce the shortage rate in the Hsinchu supply area by 4.16%-5.58%. If the irrigation area is not fallow in the 50-year return period drought event, the average tap water shortage rate in Hsinchu, Taoyuan, and Banxin supply areas will reach 29.43%, 18.13%, and 12.58%, respectively. Results show that the optimal ten-day tap water shortage rate simulated from August 2020 under fallow in the Taoyuan, Banxin, Taipei, and Hsinchu supply areas can all be less than 0.11%. However, under closing and opening the Tao-Chu support pipe, Hsinchu, Shimen, and Taoyuan irrigation areas need to operate fallow by the water shortage rate of 17.14%-20.54%, 21.99%-23.04%, and 49.53%-51.89%, respectively, which optimized supply can reduce the shortage rate of Shimen and Taoyuan irrigation areas by 35.39% and 28.41% compared with historical operations, respectively. Only the Hsinchu tap water area experienced shortage with an average rate of 13.81%-14.48% during the 200-year return period with a 12-month design drought flow event under the Tao-Chu connecting pipe closed. To achieve a shortage rate of less than 3% in tap water supply areas, Shimen, Taoyuan, and Hsinchu irrigation areas need to be left fallow during the 12-month design drought event in the return period from 50 to 200 years with shortage rates increased from 35.26%, 71.05%, and 33.61% to 50.39%, 95.67%, and 48.60%, respectively. After adding reclaimed water to support supply, the upstream Shimen and downstream Taoyuan irrigation areas can reduce the water shortage rate by 1.86%-9.18% and 0.35%-3.33%, respectively under various long-duration and high-return period drought scenarios.