Plants in an Ecohydrology Context

At the cusp of the 20th Century, western U.S. irrigators, farmers and ranchers correctly viewed forest cover in their watersheds as conserving and protecting their water resources.   [Hays 1959 (1975)].  

But similar claims along the eastern U.S. coast applied to floodplains far removed from their newly protected watersheds were found to stretch the truth, prompting rejection of that entire perspective.

As the century progressed, with strictly physical scientists leading the political charge, the viewpoint shifted to one of plants as “thieves” of waters we might otherwise use.  “So let’s remove as many plants as possible and ‘reclaim’ that water for ourselves”  became the buzz.

 

“Historical approaches to vegetation manipulation for water yield”, an appendix in the forthcoming book, Rainfall to Groundwater: History of the Science (Jigour in preparation), offers a  synopsis of how this paradigm shift transpired.

“Water yield” was the paradigm that dominated water supply thinking throughout the 20th Century, despite that a century of research aimed at proving the concept offered only nebulous results.

In many ways this (his-)story exemplifies a political Coup d’état of the dominator paradigm, over the early 20th Century progressive conservation movement.

It also illustrates how science, no matter how empirical and rational we attempt to make it, remains subject to not only the disciplinary paradigm(s) it emerges from, but to the paradigm that prevails politico-socio-culturally in any given time period.

Doubtless today, many among us still perceive plants as those greedy neighbors sucking up all the water.  This viewpoint arises from knowledge of a factoid – that plants transpire (release water vapor to the air) – isolated from its environmental and ecosystem context.

Yes, of course plants release water vapor to the air.  If they didn’t, we wouldn’t be here.

But what about their other roles in the hydrologic cycle?  It’s not just a one-way process!

Plant roots and their decay products create channels through which water moves into and is detained in the soil.  Along with their soil ecological associates, they establish macropores that carry precipitation deep into the vadose zone, thence to bedrock aquifers, thence to downstream aquifers, baseflows and finally the ocean.

 

Click images to enlarge

GeorgialhVadose Zone, Labels by V. Jigour, CC BY-SA 4.0

See Surface-Groundwater Systems in a Holistic Water Cycle

Many of us have long been aware that tropical deforestation can impact global climate functions, so we donate to save the rainforests, etc. But those same climate-mediating functions operate everywhere, not just in rainforests!.

OK, so what if you could care less about global climate scenarios?  Say, you’re just concerned that riparian vegetation is sucking up all the streamflows that might otherwise feed the groundwater resources you depend on for irrigation?  Consider:

Transpiration is wholly dependent on relative humidity, more specifically vapor pressure deficit [also see here].

Transpiration by massed plants, such as occur in riparian areas, raises the local humidity,  lowering the local vapor pressure deficit so that plants actually take up less water than would a single, isolated tree under the same environmental conditions.

You can feel this effect when traversing a riparian zone.  You feel the higher humidity.  

Significant portions of this palpable moisture condense on surfaces each night, not even leaving the local system (depending on local/ regional climate and weather patterns, of course).

And those riparian root zones, along with the soil ecosystems and structure they engender, detain stormwaters just like their upland counterparts – functioning as “sponge” to mediate flood flows, sustain summer baseflows and thwart seawater intrusion. This is referred to by hydrologists as “streambank storage” or “bank storage”  (Kondolf and colleagues 1987, Ponce 1989, Winter and colleagues 1998).

Click images to enlarge

Hence the irony of the public agency response to the 1995 flooding on the Pájaro River coastal floodplain, where all riparian trees were removed except for one individual every 70+ feet.  Refer to “Stuck in the Mud: the Pajaro River in Peril” (Robin 2006) for video documentation of the aftermath.  Seawater intrusion is among the threats on the Pájaro, yet this action degraded one of the best bulwarks against it.

Where were the scientific underpinnings for that decision???  

Just do something, even if it’s wrong???  

How did that help the farmers impacted by the flooding?  Psychologically?  Is that enough?

Blaming the trees for the flooding completely overlooks the condition of the watershed uplands from whence most precipitation drained.  [Just look up at the hillsides above Chittenden Pass, for example.  Hint:  Nonnative annual grasslands.]

Again, plants were deemed “the enemy” in this case.  Apparently more enlightened perspectives have prevailed over the past two decades.

Plants, more specifically shrubs, became “the enemy” on western rangelands throughout the history of both Spanish and Anglo cattle ranching.  Burning was favored to “enhance “ rangelands and during the first half of the 20th Century, State of California agencies promoted removal of even oaks, along with shrubby associates, to “improve rangelands”.

Over recent decades “shrub encroachment” became the meme among rangeland cognoscenti and numerous scientific papers addressed the phenomenon.  These researchers seldom questioned the utilitarian bias of the “shrub encroachment” theme.

Hydrologists got into the act also, advocating shrub removal to improve “water yield” by the mid-20th Century.  

A 2015 paper came to Verna’s attention wherein a hydrologic model for piñon-juniper removal was based on the water yield premise, demonstrating that, despite a century of inconclusive research, this (reductionist) water yield myth persists.

As for such models – garbage in, garbage out.  

Perhaps the most influential pivot point on the topic of “shrub encroachment” is, or should be, Wilcox and Huang’s (2010) “Woody plant encroachment paradox: rivers rebound as degraded grasslands convert to woodlands”.   

Especially in California, this notion that shrubs are somehow “encroaching” on/ invading lands that were naturally former grasslands is simply not applicable.  There is no evidence that California was home to extensive natural savannas – in the absence of human land management extending back through indigenous ancestral generations.

Really, when shrubs “invade” rangelands in California they are doubtless simply reclaiming former territory.  The only encroachment going on has been upon human economies that emphasize nonnative annual grasses.  And some of those didn’t fare so well during recent droughts.

Lost when woody and other perennial plants/ ecosystems are displaced by annuals are:

  • Extensive root systems and the old root channels resulting from their decay
  • Soil macropores arising from decay/ recycling of organics in those old root channels
  • Mycorrhizal fungal associations that significantly affect soil aggregation/ structure
  • Watershed/ catchment detention functions – the “sponge” – that would otherwise route significant portions of rainfall to groundwater, while reducing flooding

The previously noted fact that if plants didn’t transpire we wouldn’t be here is a tongue-in-cheek understatement of our dependence on plants.  Again, most agree that deforestation threatens global climate.

But subtler correlations exist between land cover and local/regional climate, with higher albedo (light colored) landscapes associated with reduced precipitation.  Such correlations can be especially pronounced for coastal areas like California due to direct oceanic influences.  

If we restore (lower albedo) native cover types to existing areas clothed with high albedo nonnative annual grasslands, will increased precipitation result?

We trust this synopsis may help shift awareness from the reductionist plants-sucking-up-all-our-water paradigm toward a more systemic, holistic, ecohydological understanding of plants’ complex roles in our water cycle and sustenance.

 

Further Exploration

Hays, S. P. 1959 (1975). Conservation and the gospel of efficiency: the progressive conservation movement, 1890-1920; with a new preface by the author. College edition. Atheneum, New York.

Jigour, V. (forthcoming) Rainfall to Groundwater:  1) History of the Science and 2) California and Beyond.

Kondolf, G. M., L. M. Maloney, and J. G. Williams. 1987. Effects of bank storage and well pumping on baseflow, Carmel River, Monterey County, California. Journal of Hydrology 91:351-369.

Ponce, V. M. 1989. Baseflow augmentation by streambank storage. Environment, Health, and Safety Report 009.4-89.13, Pacific Gas and Electric Company Department of Research and Development, San Ramon, California, USA.
http://ponce.sdsu.edu/baseflow_augmentation.html

Robin, Lois.  2006.  Stuck in the Mud: the Pajaro River in Peril.   http://www.loisrobin.com/Pajaro_River_film.html

Schiff, A. 1962. Fire and water: scientific heresy in the Forest Service. Harvard University Press, Cambridge, Massachusetts, USA.

Wilcox, B. P. and Y. Huang. 2010. Woody plant encroachment paradox: rivers rebound as degraded grasslands convert to woodlands. Geophysical Research Letters 37:L07402 doi:DOI: 10.1029/2009GL041929.

Winter, T. C., J. W. Harvey, O. L. Franke, W. M. Alley.  1998.  Ground Water and Surface Water: A Single Source.  US Geological Survey Circular 1139. https://pubs.usgs.gov/circ/circ1139/

 

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