Young water fractions in spring discharge

Transit time distributions are fundamental to hydrologic research and practice, providing insights into how catchments store, release, and transport water and solutes. Mean transit time estimates, however, often suffer from significant aggregation errors in heterogeneous and nonstationary catchments. The young water fraction ( F yw ) provides a robust alternative that directly reflects the proportion of recent precipitation in discharge and enables consistent comparison between catchments. While numerous studies have focused on estimating F yw in rivers, little is known about its magnitude and variability in springs. To address this knowledge gap, this study quantifies F yw in spring discharge from a diverse set of 469 springs in Austria. Across the dataset, F yw is generally low and approximately log-normally distributed (mean 0.06), with variability among springs largely controlled by aquifer structure: karst springs display the highest fractions and greatest variability, talus springs intermediate values, and fracture and alluvial springs low fractions with limited variability. Evaluating the sensitivity of F yw to discharge reveals distinct responses to hydrologic forcing: karst springs are most sensitive, reflecting temporary shifts in relative flow path contributions, whereas fracture springs are least sensitive, reflecting conservative flow dynamics. Drawing on a dataset of 565 rivers compiled for comparison, springs exhibit substantially lower F yw , likely reflecting their predominantly slow subsurface flow paths. By analyzing F yw across a large number of springs, this study delivers the first systematic insight into young water contributions to spring discharge, informing conceptual models, water resource management strategies, and contamination risk assessments. • Large-scale study of 469 springs shows low young water fractions ( ). • Aquifer structure and hydrologic forcing control variability among springs. • Springs consistently contain less young water than rivers. • Young water fraction offers a simple, robust metric for alpine water management.