the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Sep 2025
Brief Communication: Investigating the invisible subsurface stormflow process through a thorough and systematic study across sites and scales
Abstract. Subsurface stormflow (SSF) is one of the least studied and therefore least understood runoff generation processes because detecting and quantifying SSF is extremely challenging. However, the impact of SSF on streamflow dynamics and water quality is much larger than commonly assumed. The hydrologic community should therefore not shy away from the challenge to monitor SSF, but should instead join forces, make creative use of novel sensing techniques and systematically tackle the investigation of this elusive process. While this endeavor is high in effort and risk, it also comes with the potential of high gain. We here describe the challenges and propose a possible way forward.
Competing interests: MW is a member of the editorial board of Hydrology and Earth System Sciences. TB is Chief Executive Editor of Hydrology and Earth System Sciences
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.- Preprint
(695 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 10 Nov 2025)
- CC1: 'Comment on egusphere-2025-4424', Keith Beven, 30 Sep 2025 reply
-
RC1: 'Comment on egusphere-2025-4424', Anonymous Referee #1, 13 Oct 2025
reply
I have read the manuscript multiple times, but I’m still not quite sure what to make of it. Not being a catchment hydrologist, I admit that I don’t see the complexities the authors do, and I have trouble seeing the points the authors hope to make. If I’m the only one missing the point, I’m happy to be ignored.
My understanding is that subsurface storm flow (SSF) is a useful concept to illustrate some middle member of a continuum of runoff depths and paths from a hillslope, from surface runoff, perched saturated flow (is this SSF?), and groundwater flow below the water table that discharges to local streams, and deeper flow bypassing the local stream and discharging in higher-order streams downgradient. If this is roughly correct, then the necessary condition for SSF to occur would be the formation of a temporarily perched zone of saturation above the permanent water table and a reasonably steep slope. As we have pretty good handle on the precipitation or snowmelt regimes, the bottle-neck would be to identify pedogenic or geologic strata that has lower permeability than above and below, to stop infiltration and to divert it laterally downslope. Candidates include soil B-horizons and fresh bedrock surfaces. In other words, knowledge of the critical zone (CZ) structure would be the top priority to nail down the places that may generate SSF. But I did not see any discussions on this obvious knowledge gap and the concerted effort of the past 25 years by the CZ community to do exactly this.
It is admirable that the authors have dug 12 large trenches to capture and monitor SSF, with example time series shown in Figure 2. However, it is not clear at what depths the flow is considered SSF, which must be prior knowledge in order to dig the trenches not too shallow or too deep. Is the “top flow” in Figure 2 considered SSF or the “bottom flow”, or both? What separates them? Is there a pedogenic/geologic boundary between? What are the logics for selecting the 4 catchments? Do they represent different snowmelt regimes, or pedogenic or geologic environments? Does each of them offer unique insights on the combinations of climate, vegetation, soil, geology, topography etc that can help us piece together a bigger picture of the places and times where SSF is an important flow path? Without such discussions, it is difficult to see why digging 12 large trenches in a narrow geographic range can pave the way forward.
I suggest that the authors sharpen their messages. First, an explicit definition of SSF is needed, i.e., what it is and how it differs from other flow paths, and how we know it is occurring and it is important. Second, present a synthesis of what we know on the site conditions where SSF has been observed and known to be important, highlighting the climate and substrate drivers of SSF. Third, what we don’t know and should know, e.g., is SSF only important in humid midlatitudes on shallow bedrocks? How about the tropics where the soil is deep? How about forested slopes/catchments vs. cropped or pastured land covers? And fourth, provide some pointers to move forward, e.g., select climate-substrate combinations across the wide world and get funding to instrument the slopes and catchments via an international consortium, followed by some community-level synthesis workshops, to evaluate the conceptual models such as fill and spill, and to come up with new models.
Citation: https://doi.org/10.5194/egusphere-2025-4424-RC1
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 356 | 31 | 9 | 396 | 2 | 3 |
- HTML: 356
- PDF: 31
- XML: 9
- Total: 396
- BibTeX: 2
- EndNote: 3
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Great stuff and an impressive program of experimental sites but your pictures suggest that the trenches are only on straight slope sections. Convergence and divergence of topography can be really important in how the subsurface responds (e.g. Beven, K.J. (1978), 'The hydrological response of headwater and sideslope areas'. Hydological Sciences Bulletin, 23(4), 419-437 - no nearly 50 years old!!) and changing connectivities to the stream, which might also depend on (preferential) connectivities in the bedrock (e.g. Scaini, A., Hissler, C., Fenicia, F., Juilleret, J., Iffly, J.F., Pfister, L. and Beven, K., 2018. Hillslope response to sprinkling and natural rainfall using velocity and celerity estimates in a slate-bedrock catchment.Journal of Hydrology, 558, pp.366-379).
So it will be very interesting to see, given such complicating factors, whether you can indeed "move beyond the uniqueness of place" - actually even in your 4 study catchments. Indeed, I wonder if that should be the aim at all, rather than actually accepting uniqueness of place and finding ways of embracing it in both analyses and models.
But good luck to the whole team for the programme - I shall very much look forward to seeing the results.
Keith Beven