Decoupling of precipitation and streamflow trends induced by forest expansion: Evidence from time-varying Budyko analysis

Study region56 watersheds distributed across the island of Taiwan.Study focusUsing a time-varying Budyko framework, this study aims to: 1) quantify shifts in catchment water storage capacity (represented by the landscape parameter, m[jls-end-space/]); 2) decompose runoff variations into climate-induced and landscape-induced components to identify the drivers of streamflow reduction; and 3) assess the implications of using time-invariant versus time-varying landscape parameters for future water resource projections.New hydrological insights for the region: 1)This study provides the first quantitative evidence of a widespread “decoupling” between precipitation and streamflow in Taiwan; while 79.6% of watersheds show upward precipitation trends, only 60.7% show upward runoff trends, with many showing significant landscape-driven reductions; 2) Decomposition analysis reveals that enhanced watershed water storage capacity (an increasing m[jls-end-space/]) has offset climate-driven runoff gains in the majority of watersheds. This increase in storage capacity is strongly correlated with forest expansion, which acts as a primary driver for reduced runoff despite increasing rainfall.; 3) The assumption of stationarity in the Budyko landscape parameter leads to significant biases. Using a time-invariant parameter results in an overestimation of future runoff by up to 240 mm under RCP8.5 scenarios, highlighting the necessity of incorporating time-varying parameters for accurate hydrological projections in changing environments.