Water droplets on leaf
Ecohydrology is the study of the connections between living things and water resources Credit: Free Nature Stock (CC0 1.0)

As AGU celebrates its hundredth anniversary I would like to take this opportunity to highlight a century of ecohydrology research by AGU hydrologists. Wait! Back up. Isn’t ecohydrology a shiny new thing? The answer to this question depends on whether you define the science by its name (i.e., ‘shiny new thing’ + ‘ology’) or by the knowledge it accumulates through study or practice.

Let us start with the latter. The broadest definition of ecohydrology I have deduced from the literature is that it is the study of the connections between living things and water resources. Shiny perhaps. Not new. Consider, for example, Dixon’s theory on how plants lift water from the soil to their leaves and transpire this water out to the atmosphere to stay alive (Dixon & Joly, 1894). This classic work fits our definition of ecohydrology, and it is accumulated knowledge implicitly known to hydrologists because transpiration is a major component of the Earth’s water circulation system.

So what of the former idea of defining ecohydrology in terms of its name? Perhaps naming the ‘ology’ and establishing the bin in which to accumulate knowledge helps to promote the science and increase the rate of publications in it. That sounds like a potentially testable hypothesis.

Robert Horton, while reporting to the AGU’s “Symposium on relations of hydrology to other branches of geophysics,” called for hydrologists to pay closer attention to plant-water relations (Horton, 1933). Plants and water together, suggested by a hydrologist, and an engineer in fact! Horton had already published a manuscript where he quantified the role of canopy and leaf traits of different tree species on canopy interception of precipitation (Horton, 1919).

Between the 1930s and 1960s hydrologists developed clever devices to measure plant responses to soil water content, their interception of precipitation, and transpiration (Taylor et al., 1934; Richards & Russell, 1937; Veihmeyer, 1938; Veihmeyer & Hendrickson, 1955; Gardner & Ehlig, 1963; Skau and Swanson, 1963). Hydrologists were also interested in understanding how vegetation, and in particular its removal, affected runoff and water quality.

Apparatus for studying water relations in potted plants. Credit: Richards & Russell (1937), Fig 1

All of this research preceded the launch of AGU’s journal Water Resources Research, in 1965, and occurred six decades before ecohydrology became an official ‘ology.’

The earliest occurrence of the term ecohydrology I can find is a paper by Ingram in 1987. The term made its debut in an AGU journal in a paper by Caspary in 1990.

The earliest occurrence of the term ecohydrology I can find is a paper describing interrelated roles of water flow and peatland surface form on vegetation arrangement on the land surface (Ingram, 1987). The term made its debut in an AGU journal in a paper that proposed an ecohydrologic model of the interactions between plant roots, transpiration, stomatal conductance, soils and geology, and biogeochemical cycling (Caspary, 1990).

During the 1990s it was bandied about primarily in the context of aquatic systems (Grootjans et al., 1996; Zalewski et al., 1997; Baird & Wilby, 1999), with one cameo appearance as a keyword in a watershed modeling paper (Vertessy et al., 1996).

Ecohydrology was probably blessed with an ‘ology’ status after it was used by Rodriguez-Iturbe in 2000.

Ecohydrology was probably blessed with an ‘ology’ status after it was used by Rodriguez-Iturbe (2000) in his re-shaping of Peter Eagleson’s (1978, 1982) theory on the how vegetation affects soil water balance. For many hydrologists this was a “watershed moment” (sorry!) for ecohydrology, but to paraphrase Klemeš (1986) it was just one of many excursions into ecohydrology-related research.

Quantity of ecohydrology-related articles published in AGU journals, by year and journal, as a percentage of total number of papers published in the respective year and journal. For example, in 2017 about 20% of all papers published in Water Resources Research were related to ecohydrology. Letters indicate notable years: (a) in 1978, Peter Eagleson published his collection on soil-climate-vegetation; (b) in 1990, Caspary published his paper on ecohydrologic modeling; (c) in 2000, Rodriguez-Iturbe published his use of the term ecohydrology. Credit: D. Scott Mackay

Did ‘eco’ + ‘hydro’ + ‘ology’ result in a surge of publications on the subject?

Using the broad definition of connections between living things and water resources, I quantified the rate of publications in ecohydrology in AGU journals since Water Resources Research was launched.

I used the Web of Science and a fairly liberal set of search terms: biology, ecohydrology, ecology, vegetation, forest, ecosystem, wetland, hydrology, water, runoff, and transpiration.

I searched only in current AGU journals, placing the start date in 1965 and including publications through November 2018. A total of 5276 manuscripts were retrieved, most appearing in just six AGU journals (see right).

There was a low rate of ecohydrology-related activity in AGU journals between 1965 and 1990, and a pronounced increase thereafter. The introduction of JGR: Biogeosciences coincides with a small decline in ecohydrology-related work published in Global Biogeochemical Cycles, but ecohydrology’s share of Water Resources Research papers has grown almost linearly to about 20 per cent.

Recall our hypothesis was that naming the field promotes more publications in it. We can reject this hypothesis for ecohydrology. If naming the field mattered, then the year 2000 (or so) should have stood out as a critical point for the uptick in publication rate, a sort of Y2K of ecohydrology. But the critical point was actually around the year 1990. By Y2K, ecohydrologic research already accounted for 10 per cent of Water Resources Research publications, and interest in the area has grown to 20 per cent over 25 years in the journal.

Consequently, we may prefer to think of the field of ecohydrology in terms of the accumulation of knowledge from 100 years (or more) of study or practice, and its recent rapid growth a result of building a critical amount of knowledge.

The next 100 years of the field should be exciting.

The next 100 years of the field should be exciting. Based on the current rate of growth in ecohydrology publications, when AGU celebrates its 200th anniversary 100 per cent of Water Resources Research papers will be ecohydrology-related. Sorry, I couldn’t resist.

—D. Scott Mackay (email: dsmackay@buffalo.edu), Editor, Water Resources Research, and Department of Geography, University at Buffalo (SUNY)


Mackay, D. S. (2019), Ecohydrology: what’s in a name?, Eos, 100, https://doi.org/10.1029/2019EO123093. Published on 13 May 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.