Anne’s picks of the June literature: Watershed Hydrology

ResearchBlogging.orgA post by Anne JeffersonIt starts when when a water molecule in precipitation lands on the ground, and it ends when that same water molecule leaves the watershed as streamflow. In between, that molecule may move over the land surface, through the soil in big holes (macropores) or in tiny spaces between grains in the soil, through the bedrock as groundwater, or any combination of those pathways. How long it takes for the water molecule to make its journey, what hydrologists call the transit time, depends on the flow paths that it takes. And that transit time, in turn, affects biogeochemical processing and contaminant persistence. Inversely, if hydrologists can measure the distribution of transit times for a particular watershed, they can infer things about the storage, flowpaths, and sources of water in the watershed. Thus, transit time distributions help us peek into the hidden inner workings of the watershed….if we understand what we are really measuring and what those measurements are really telling us. And that topic is one of lots of active research in the community of watershed hydrologists, and its the subject of a number of recently published papers.

In what seems to be an annual tradition, Hydrological Processes has devoted their June issue to topics relating to catchment hydrology and flowpath tracers. This year, the focus is Preferential Flowpaths and Residence Time Distributions and it’s edited by Keith Beven. It’s the sort of issue that makes me want to go over to the library stacks and spend the day in a comfy chair reading and enjoying the journal from cover to cover. While all of the articles in this special issue make my pulse race a little, here are a couple that really strike my fancy:

McDonnell, J., McGuire, K., Aggarwal, P., Beven, K., Biondi, D., Destouni, G., Dunn, S., James, A., Kirchner, J., Kraft, P., Lyon, S., Maloszewski, P., Newman, B., Pfister, L., Rinaldo, A., Rodhe, A., Sayama, T., Seibert, J., Solomon, K., Soulsby, C., Stewart, M., Tetzlaff, D., Tobin, C., Troch, P., Weiler, M., Western, A., Wörman, A., & Wrede, S. (2010). How old is streamwater? Open questions in catchment transit time conceptualization, modelling and analysis Hydrological Processes, 24 (12), 1745-1754 DOI: 10.1002/hyp.7796

In this invited commentary, McDonnell and 28 colleagues lay out the definition of transit time and the current limits of our understanding on its controls in watersheds and its relationship to hydrograph characteristics, groundwater, and biogeochemical processing. They then provide their research vision for pushing past these limits, through a combination of field research and advances in modeling.

Kirchner, J., Tetzlaff, D., & Soulsby, C. (2010). Comparing chloride and water isotopes as hydrological tracers in two Scottish catchments Hydrological Processes, 24 (12), 1631-1645 DOI: 10.1002/hyp.7676

Oxygen isotopes of water and chloride concentrations have been widely used to estimate watershed travel times. They are generally regarded as conservative tracers, but they are not perfect. Here Kirchner et al. compare the time series of the two tracers for a pair of Scottish catchments and show that while both tracers exhibit strongly damped signals relative to precipitation, the travel times calculated using oxygen isotopes were 2-3 times longer than for chloride. So it seems that both tracers are telling us similar things about the ways that catchments move and store water, but that quantitative estimates of travel time are going to be tricky to compare across tracers.

Stewart, M., Morgenstern, U., & McDonnell, J. (2010). Truncation of stream residence time: how the use of stable isotopes has skewed our concept of streamwater age and origin Hydrological Processes, 24 (12), 1646-1659 DOI: 10.1002/hyp.7576

The stable isotopes of water have a shelf life of about 5 years or less. It’s not that they break down (they are stable isotopes, after all); it’s that seasonal input signals get damped over time, so that ages greater than 5 years can’t be resolved. In contrast, tritium (the unstable isotope of hydrogen) has a half life of ~12.4 years. A few decades ago, water ages were estimated using tritium, which conveniently had a bomb peak that made a handy marker of recharge in the early 1960s. These days, water ages are usually estimated by the stable isotopes alone. In this paper, Stewart et al suggest that we are missing part of the story when we use just stable isotopes, because we effectively discount any contributions from water >5 years since it feel from the sky. Incidentally, those contributions that we have been neglecting? That’s the bedrock groundwater and it might be quite important to explaining the behavior of streams. Stewart et al. suggest that we return to embracing tritium as part of a “dual isotope framework” so that we can more accurately quantify groundwater contributions to streamflow. The issue of the shape of travel time distributions (are they exponential or fractal?) is explored in more detail in a paper by Godsey et al. in the same issue and Soulsby et al. explore how relationships between transit times and hydrograph and watershed characteristics might be used to estimate streamflows in data-sparse mountain watersheds.

Categories: by Anne, hydrology, paper reviews
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Comments (7)

  1. Do you happen to know who coined the term ‘time of concentration’, and why? It’s not exactly the most obvious thing to say.

  2. Anne Jefferson says:

    Hi Daniel – I don’t know the origins of ‘time of concentration’ and it’s not a term in much use by watershed hydrologists (though we periodically get in arguments about age versus residence time versus transit time). The idea of time of concentration is similar, as it’s the time for water to flow from the most remote spot in the watershed to the outlet, but it doesn’t refer to the water molecules travel time, but the pulse of water, which can be much faster. I see time of concentration used much more frequently in empirically-based, applied hydrology settings.

  3. Lab Lemming says:

    Is transit time easier to measure for ephemeral drainages?

  4. Anne Jefferson says:

    I have never seen transit time estimates for ephemeral drainages. Since transit time is most commonly calculated by comparing time series of isotopes (or chloride) in precipitation to those in streamflow over a period of several years, I imagine that it would be quite difficult to work with the broken record produced by ephemeral streamflow.

    That said, what an interesting question! If we approach the question theoretically though, we could probably make some hypotheses about the nature of the transit time distribution (TTD). In perennial streams, you can have significant groundwater efflux during baseflow periods. That influence on transit time would be missing in ephemeral streams….if we knew there wasn’t groundwater leaving the catchment underground. If we assume no groundwater, then ephemeral streams could have a very different shape to the TTD. Note though that you could still have water significantly older than the rain event that generates the streamflow, because infiltration from rain events displace older water stored within the soils, etc and pushes that old (or pre-event) water into the stream. So I wouldn’t necessarily say that the mean transit time of ephemeral drainages would be lower than perennial drainages. It would depend on the storage characteristics of the catchment. Finally, in ephemeral drainages, it is quite possible to have water flowing under the streambed, even when the stream is dry. Sampling and quantifying this would a real technical trick.

    Anyone want to give me some funding to do some real work on this problem? :-)

  5. Lab Lemming says:

    Um, can’t you just write on your calendar when the rainfall event occurs, then record the elapsed time for various positions downstream?

    Given that most of Australia’s drainages are ephemeral, there ought to be money here…

  6. Lab Lemming says:

    A flakier big picture question: Is there any way to tell what percentage of the world’s drainages are ephemeral through geologic time? If the modern world is cold and dry, does that make them more common or less so?

  7. LL, my guess is that a cold and dry climate would reduce the incidence of ephemerality over geological time. If I can get away with assuming the drainage network is in equilibrium with the hydroclimatic conditions over geological time, then I can disregard ephemerality due to environmental change (e.g., abstractions, stream capture, climate change). The remaining ephemeral streams I think must be associated with hydroclimatic conditions with very high variability and low mean flows – think semi-arid environments. But hydrological variability is tied to temp: the higher the temp the more moisture can be stored in the atmosphere, the greater the convective updrafts, and thus the greater the storm intensity. So, a colder dryer atmosphere would be less able to produce high flows. Add to this that fluvial incision goes to Q to a power greater than 1, and it looks like a cooler dryer climate has lower drainage density but more consistent flows.