Exploring the interplay of new and young water fractions with hillslope topography in a subtropical headwater catchment

Abstract. Our study explores the intricate relationships between new water (F_new), young water (F_yw), and topographic influences within small catchments, utilizing the Height Above the Nearest Drainage (HAND) as a key metric. Analysis revealed that seepage water consistently contains higher fractions of young water than stream water, despite similar contributions of new water from precipitation, highlighting differential hydrological responses. The relationship between F_new and F_yw exhibited varying slopes for seepage and stream water, suggesting distinct hydrological sources and behaviors, with seepage water reflecting a “flashy” system and stream water indicating a “damped” system. A nonlinear dynamic was observed between HAND and transit time measures, with a notable threshold at approximately 10 m of HAND, beyond which the contribution of young and new water to streamflow declines, indicating a shift in hydrological pathways. This threshold behavior emphasizes the complex influence of topography on water movement and age within catchments. This research bridges gaps in our understanding of catchment hydrology, offering insights into the spatial and temporal dynamics of water movement and the pivotal role of landscape features in shaping hydrological responses.