Why do streams respond so quickly when storm flow is so old?

A stream rises within minutes to hours of heavy rain, yet chemical and isotope tracers show that much of the storm runoff is "old" water that fell long before. How do catchments release old water so fast, and how long does water spend underground?

Question

Why do streams respond so quickly to precipitation when storm flow consists largely of old, pre-event water? And what is the distribution of transit times, the time between water arriving as precipitation and leaving the catchment, in the terrestrial water cycle?

Background

With the question on preferential flow, it addresses the distribution and nature of flow paths in the twenty-three unsolved problems in hydrology. The authors note that the community still has a long way to go to include velocities, celerities and residence-time distributions in models of the catchment hydrograph [1]. Kirchner called the rapid release of old water a "double paradox" of catchment hydrology and geochemistry [2].

What is known

• Celerity, the speed at which a pressure disturbance propagates, exceeds velocity, the speed at which water itself moves; catchment models had been evaluated only against celerity [3].
• Seasonal oxygen-isotope cycles from 254 watersheds show that about a third of global river discharge fell as precipitation within the previous two to three months, although typical mean transit times are years to decades [4].
• Mean transit times estimated from seasonal tracer cycles can be wrong by several hundred percent in heterogeneous catchments, whereas the young water fraction (transit times under about 0.2 years) is estimated accurately [5].

What would count as progress

No single result settles a question like this; contributions like these would move it forward:

• Open, high-frequency isotope records of rain and streamflow for many catchments, from which time-variable transit time distributions can be estimated.
• A catchment model that reproduces both the hydrograph (celerity) and tracer breakthrough (velocity) at a benchmark site, with parameters fixed in advance and tested on a held-out period.
• Cross-catchment analyses that explain what controls young water fractions and transit times, such as topography, geology and climate.

References

1. Blöschl, G. et al. (2019). Twenty-three unsolved problems in hydrology (UPH) – a community perspective. Hydrological Sciences Journal 64(10), 1141–1158.
2. Kirchner, J. W. (2003). A double paradox in catchment hydrology and geochemistry. Hydrological Processes 17(4), 871–874.
3. McDonnell, J. J. & Beven, K. (2014). Debates—The future of hydrological sciences: a (common) path forward? A call to action aimed at understanding velocities, celerities and residence time distributions of the headwater hydrograph. Water Resources Research 50(6), 5342–5350.
4. Jasechko, S., Kirchner, J. W., Welker, J. M. & McDonnell, J. J. (2016). Substantial proportion of global streamflow less than three months old. Nature Geoscience 9(2), 126–129.
5. Kirchner, J. W. (2016). Aggregation in environmental systems – Part 1: Seasonal tracer cycles quantify young water fractions, but not mean transit times, in spatially heterogeneous catchments. Hydrology and Earth System Sciences 20(1), 279–297.

Source

Imported from Twenty-three unsolved problems in hydrology, question 8, curated by Blöschl et al. Listed as open in the source (2019) and still an active research question as of 2026-10-20. The statement above is written in our own words; see the source for the original wording, history and discussion.