Linking Extreme Rainfall Variability to Hydrological Alteration in a Tropical Sub-Watershed: Evidence from the Biyonga Sub-Watershed, Indonesia

Abstract

Understanding how extreme rainfall variability influences hydrological alteration remains an important challenge in tropical watersheds. While previous studies have largely focused on rainfall intensity as the main driver of hydrological responses, the influence of rainfall frequency and persistence has received less attention. This study examines the relationship between extreme rainfall variability and hydrological regime change in the Biyonga Sub-Watershed, Gorontalo Province, Indonesia, using daily rainfall and discharge records from 2007-2024. Fourteen extreme rainfall indices and 33 Indicators of Hydrologic Alteration (IHA) were analyzed using Principal Component Analysis (PCA), the Pettitt test, the Mann–Whitney U test, and Spearman rank correlation. A significant hydrological regime shift was detected in August 2013 (p < 0.001), followed by a moderate degree of flow regime alteration (39.86%). Significant changes were observed in 10 of the 33 IHA indicators (30%), particularly those related to seasonal flow, medium-term high flows, timing of minimum discharge, and flow dynamics. PCA identified two dominant dimensions of rainfall variability: rainfall intensity and rainfall frequency–persistence. Correlation analysis showed that hydrological alteration was associated with rainfall frequency and persistence (ρ = 0.472 for Date min; ρ = −0.476 for reversals), whereas rainfall intensity exhibited no significant relationship with the altered hydrological indicators. These results suggest that temporal rainfall characteristics exert a stronger influence on hydrological alteration than rainfall intensity alone, highlighting the importance of rainfall persistence in shaping flow regime dynamics within tropical sub-watersheds.