Dr. Ciaran J. Harman is an Associate Professor in Environmental Health and Engineering at Johns Hopkins University.
What does ecohydrology mean to you?
Someone once described ecohydrology to me (Paul Brooks I think) as the study of how plants muck up a perfectly good physics problem. I think there’s some humorous truth to that. It is possible to do a lot of interesting hydrology without really thinking about plants and life in general, and considering only the fluid mechanics. You can even go a long way understanding the role of plants in evapotranspiration as merely a kind of ‘wick’ that allows soil water to be drawn into the atmosphere. The result of that approach is inevitably that plants (and ecosystems in general) are reduced to a set of empirical parameters (rooting depth, max stomatal conductance, canopy roughness, etc.) that we can measure and catalog, and which must be estimated in any given applied situation.
That way of doing things has proved to be something of a dead-end. Rarely can these parameters be predicted with fidelity (and their fidelity is rarely checked). Their variability is overwhelming, even within a single plant species sometimes. And it is difficult to anticipate how these parameters will change under changing conditions.
To me, ecohydrology is the project of going beyond that reductive approach and instead engaging with plant physiology, evolution, and ecosystems more deeply. It is the search for a deeper understanding of how life on earth has evolved complex adaptations that enmesh it with its thermodynamic and material environment. It seeks to understand the role of water in how life reacts to its environment and manipulates it to its own ends.
On a more personal level, ecohydrology is a dimension of my 20-year fascination with the coevolution of hydrologic systems and the broader landscapes that they are part of.
What are your undergraduate and graduate degrees in?
I have a B.Eng in Environmental Engineering and a B.A. in Asian Studies from the University of Western Australia. I graduated in 2002. I went to the University of Illinois for grad school in 2005, starting with a Masters in Geography (2008), and a PhD in Civil and Environmental Engineering (2011).
How did you arrive at working in/thinking about ecohydrology?
It was something I was exposed to in grad school, particularly though my advisor, Siva Sivapalan, and through some influential peers – particularly Sally Thompson. Stan Schymanski’s PhD work on stomatal optimization was particularly formative – it helped me understand how the water balance was linked to evolutionary demands and plant physiology at a deep level.
I muddled into doing ecohydrology myself mostly through work on the water balance. Peter Troch and Paul Brooks helped a group at the NSF synthesis summer school in 2010 use the MOPEX dataset to better understand the imprint of ecohydrology on the water balance, showing how annual ET responded to variations in annual precip in a surprisingly consistent way across many watersheds. I’ve also spent a LOT of time thinking and talking about the Budyko curve, much more than appears in publications (which is probably for the best).
More recently I have been returning to ecohydrology along different avenues. I’m having a wonderful time interacting with Kanishka Singh at Cornell who is doing some very cool experiments looking at tracer transport through trees. I think his results are going to be transformative. My own PhD student David Litwin has been working on ‘hydrologically-explicit’ landscape evolution and is starting to think about how water balance partitioning might play an important role in that. It has been a great opportunity to dust off some ideas in that area that have been laying around for too long.
What do you see as an important emerging area of ecohydrology?
I feel like I have such a limited view – there’s a lot of cool things out there, and it is hard to keep up with more than the small niche I’m invested in!
Something that I do think is important is the increasing understanding of how vegetation interacts with the shallow subsurface. There was a paper by Blair McLaughlin last year that showed nicely how subsurface architecture controlled variations in plant responses to drought within and across species. I’m part of a critical zone project at the moment that is trying to understand this kind of issue using geophysical imaging. There is also a lot of really cool work being done by geomorphologists and geochemists interested in understanding how vegetation drives the evolution of the critical zone. I think so much of what happens above ground is linked to things underground, but our limited ability to ‘see’ underground has resulted in these interactions being discounted.
Do you have a favorite ecohydrology paper? Describe/explain.
Cripes, this is a hard one. OK I’ll go with favorite of the moment – ask me again tomorrow and I’ll say something different.
I really like this paper by Jochen Schenk presenting “The Shallowest Possible Water Extraction Profile:
A Null Model for Global Root Distributions” (https://acsess.onlinelibrary.wiley.com/doi/10.2136/vzj2007.0119).
It is such an elegant model that makes startlingly good predictions about ecosystem-level root depth distributions from climate alone. Rumor has it that Schenk came up with the idea in grad school but his advisor didn’t like it, and so he published it on his own. I think it is terribly underappreciated. Ying Fan was inspired by it for this excellent paper (https://www.pnas.org/content/pnas/114/40/10572.full.pdf) though ultimately used a more sophisticated resistance formulation.
At the moment we are using Schenk’s model to capture the effect of root water uptake from the vadose zone on landscape evolution. This allows us to avoid having to specify a rooting depth, and instead assume a climate-adapted one. I sat down and worked out analytical solutions for the root depth distribution a few months ago, though I have no idea what to do with them. If anyone thinks they’ll be useful let me know!
What do you do for fun (apart from ecohydrology)?
I’m at that stage of life where I don’t have much time outside of family and work. I have cycled through a LOT of different hobbies over the years though, including photography, swing dancing, rock climbing, choral singing, roller derby, recreational math, and gardening. I have pretty severe ADHD, and switching hobbies a lot is common for people like me. Hopefully I’ll have more time soon and can take up a new hobby – any suggestions?
Someone once described ecohydrology to me (Paul Brooks I think) as the study of how plants muck up a perfectly good physics problem. I think there’s some humorous truth to that. It is possible to do a lot of interesting hydrology without really thinking about plants and life in general, and considering only the fluid mechanics. You can even go a long way understanding the role of plants in evapotranspiration as merely a kind of ‘wick’ that allows soil water to be drawn into the atmosphere. The result of that approach is inevitably that plants (and ecosystems in general) are reduced to a set of empirical parameters (rooting depth, max stomatal conductance, canopy roughness, etc.) that we can measure and catalog, and which must be estimated in any given applied situation.
That way of doing things has proved to be something of a dead-end. Rarely can these parameters be predicted with fidelity (and their fidelity is rarely checked). Their variability is overwhelming, even within a single plant species sometimes. And it is difficult to anticipate how these parameters will change under changing conditions.
To me, ecohydrology is the project of going beyond that reductive approach and instead engaging with plant physiology, evolution, and ecosystems more deeply. It is the search for a deeper understanding of how life on earth has evolved complex adaptations that enmesh it with its thermodynamic and material environment. It seeks to understand the role of water in how life reacts to its environment and manipulates it to its own ends.
On a more personal level, ecohydrology is a dimension of my 20-year fascination with the coevolution of hydrologic systems and the broader landscapes that they are part of.
What are your undergraduate and graduate degrees in?
I have a B.Eng in Environmental Engineering and a B.A. in Asian Studies from the University of Western Australia. I graduated in 2002. I went to the University of Illinois for grad school in 2005, starting with a Masters in Geography (2008), and a PhD in Civil and Environmental Engineering (2011).
How did you arrive at working in/thinking about ecohydrology?
It was something I was exposed to in grad school, particularly though my advisor, Siva Sivapalan, and through some influential peers – particularly Sally Thompson. Stan Schymanski’s PhD work on stomatal optimization was particularly formative – it helped me understand how the water balance was linked to evolutionary demands and plant physiology at a deep level.
I muddled into doing ecohydrology myself mostly through work on the water balance. Peter Troch and Paul Brooks helped a group at the NSF synthesis summer school in 2010 use the MOPEX dataset to better understand the imprint of ecohydrology on the water balance, showing how annual ET responded to variations in annual precip in a surprisingly consistent way across many watersheds. I’ve also spent a LOT of time thinking and talking about the Budyko curve, much more than appears in publications (which is probably for the best).
More recently I have been returning to ecohydrology along different avenues. I’m having a wonderful time interacting with Kanishka Singh at Cornell who is doing some very cool experiments looking at tracer transport through trees. I think his results are going to be transformative. My own PhD student David Litwin has been working on ‘hydrologically-explicit’ landscape evolution and is starting to think about how water balance partitioning might play an important role in that. It has been a great opportunity to dust off some ideas in that area that have been laying around for too long.
What do you see as an important emerging area of ecohydrology?
I feel like I have such a limited view – there’s a lot of cool things out there, and it is hard to keep up with more than the small niche I’m invested in!
Something that I do think is important is the increasing understanding of how vegetation interacts with the shallow subsurface. There was a paper by Blair McLaughlin last year that showed nicely how subsurface architecture controlled variations in plant responses to drought within and across species. I’m part of a critical zone project at the moment that is trying to understand this kind of issue using geophysical imaging. There is also a lot of really cool work being done by geomorphologists and geochemists interested in understanding how vegetation drives the evolution of the critical zone. I think so much of what happens above ground is linked to things underground, but our limited ability to ‘see’ underground has resulted in these interactions being discounted.
Do you have a favorite ecohydrology paper? Describe/explain.
Cripes, this is a hard one. OK I’ll go with favorite of the moment – ask me again tomorrow and I’ll say something different.
I really like this paper by Jochen Schenk presenting “The Shallowest Possible Water Extraction Profile:
A Null Model for Global Root Distributions” (https://acsess.onlinelibrary.wiley.com/doi/10.2136/vzj2007.0119).
It is such an elegant model that makes startlingly good predictions about ecosystem-level root depth distributions from climate alone. Rumor has it that Schenk came up with the idea in grad school but his advisor didn’t like it, and so he published it on his own. I think it is terribly underappreciated. Ying Fan was inspired by it for this excellent paper (https://www.pnas.org/content/pnas/114/40/10572.full.pdf) though ultimately used a more sophisticated resistance formulation.
At the moment we are using Schenk’s model to capture the effect of root water uptake from the vadose zone on landscape evolution. This allows us to avoid having to specify a rooting depth, and instead assume a climate-adapted one. I sat down and worked out analytical solutions for the root depth distribution a few months ago, though I have no idea what to do with them. If anyone thinks they’ll be useful let me know!
What do you do for fun (apart from ecohydrology)?
I’m at that stage of life where I don’t have much time outside of family and work. I have cycled through a LOT of different hobbies over the years though, including photography, swing dancing, rock climbing, choral singing, roller derby, recreational math, and gardening. I have pretty severe ADHD, and switching hobbies a lot is common for people like me. Hopefully I’ll have more time soon and can take up a new hobby – any suggestions?
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