 Environmental heterogeneity is common, but it is often ignored when studying environmental systems. This leads to errors in estimating the mean travel time of water through a system, which can be up to 100% higher than the actual value. A better way to measure the mean travel time is to look at the young water fraction, which is the fraction of runoff with transit times under two years. Seasonal tracer cycles can accurately predict this fraction. Even in highly heterogeneous catchments, where the mean transit time varies greatly. This article was authored by J. W. Kirchner.