Abstract: To evaluate the guarantee efficacy and inherent trade-offs of multi-source water sharing during sustained drought, a MILP allocation model was established for 32 county-level units in 6 cities in northern Henan, formulating staged coupling rules into preemptive priorities, cumulative tiers, and source-switching constraints with water deliveries as decision variables.Three scenarios—sharing Yellow River water（S1）, adding reservoir water（S2）, and further sharing South-to-North Water Diversion water（S3） —were evaluated. Results show that target shortage rates drop from 12.11% to 9.02% and 8.96%. Scenario S3 contributes an incremental shortage reduction of merely 2.65×10⁶m³, indicating that deficit-reduction gains saturate after incorporating reservoir water.Strict priority constraints induce rule-based shortages accounting for 65.83%～79.26% of the total deficit, with agriculture absorbing 55.25%～61.32% and the top five units concentrating 58%～72% of the deficit. Regional mitigation induces spatiotemporal and tier-level trade-offs: while essential domestic guarantees rise to 98.78%, S3 triggers a rebound in mid-drought shortages, postpones the peak dekad, and lowers semi-elastic and elastic guarantee rates. Relaxed experiments demonstrate that eliminating priority orders releases only 30.67% of target shortages, leaving the remainder limited by physical reachability. Enhancing intra-city sharing topologies entails trade-offs between macroscopic efficiency and spatiotemporal balance, requiring verification of key users and critical periods in drought relief management.