Is earlier always better? A comparative assessment of rainfall replenishment timing for multiyear drought mitigation

Abstract. Multiyear droughts (MYDs) are recognized as severe drought events, with especially profound impacts on both human activities and ecosystems. However, the optimal rainfall replenishment timing (t_optimal) for MYDs mitigation remains insufficiently understood. With that in mind, we conducted a retrospective analysis of historical MYDs based on the Palmer Drought Severity Index (PDSI) in China during 1961–2020, and the calibration period was set to 1961-1990. We performed a series of numerical experiments involving precipitation gradient increases for 351 selected MYDs, distributed across 199 grids (2°×2°), from 1991 to 2020, and developed a drought mitigation quantitative model (DMQM). In addition, a key coefficient (k) derived from DMQM was defined to quantify the mitigation efficiency, and t_optimal was then identified as the timing corresponding to the maximum k (k_max). Overall, drought severity exhibits a nonlinear response to increased precipitation. k_max occurred most frequently in the first month of drought onset (t₁), accounting for 58.79 % of all grids, while the second (t₂) and third (t₃) months were also non-negligible, accounting for 22.11% and 11.06 %, respectively. Compared to the humid river basins in southern China, the arid and semi-arid northern regions had a higher probability for k at t₂ or t₃ to exceed k at t₁. Drought duration (DD) was identified as a key factor, as longer DD was associated with a greater likelihood of t₂ or t₃ being the t_optimal, evidenced by R² values of 0.526 and 0.578, respectively. These findings contribute to ensuring timely and regionally appropriate MYD mitigation strategies and interventions.