Spatiotemporal Dynamic and Drivers of Baseflow Index in Taiwan’s Catchments

The (sub-)surface hydrological processes vary over time and space, and the Baseflow Index (BFI) which is used to characterize aquifer discharge and river sustainability is no exception. BFI is defined as the proportion of stable baseflow come from aquifers to the total streamflow. Previous studies have constructed various models based on climatic and geomorphological characteristics to estimate the BFI, aiming to identify key driving factors and predict hydrological behavior in ungauged regions. However, these studies have two primary limitations: (1) they represent a catchment relied on a single long-term BFI, overlooking inter-annual variability, and (2) they utilized global models that assume constant responses of all catchments to factors. To address these limitations, this study compiled a panel data of 2,953 samples, comprising BFI, climate, and geomorphological characteristics of catchments from 60 gauging stations with more than 30 years of daily streamflow records in Taiwan. The identification of factors through correlation analysis and Geographically and Temporally Weighted Regression (GTWR) model. The results of the spatiotemporal dynamics revealed that the BFI of most stations exhibited a significant increasing trend, as well as notable positive spatial autocorrelation. Model comparisons indicated that the GTWR model outperformed the GWR and OLS models with lower AICc. Moreover, the inclusion or exclusion of spatiotemporal heterogeneity may lead to contradictory outcomes in driving factor identification. Correlation analysis without heterogeneity showed that elevation had the strongest negative correlation with BFI. However, the GTWR model indicated that almost all factors exhibited bi-directional influences on BFI varying across time and space. The importance of elevation was not significant in GTWR. Additionally, the rainfall intensity is the only one-way factor that had a negative influence on BFI. This study underscores the influence of climatic and geomorphological factors on BFI exhibits pronounced spatiotemporal heterogeneity. Neglecting spatiotemporal heterogeneity could lead to overestimation or underestimation of the importance of factors. The findings provide valuable insights into other hydrological processes and highlight the necessity of incorporating spatiotemporal heterogeneity analysis in catchment process and groundwater resource assessments.