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What does a state do when it learns its tightly controlled management of freshwater isn’t meeting its historically touted benefits for salmonds?  Add another layer of control – truck the fish up to suitable habitats above the dams.  ‘Gotta say, it suggests a dream-like excursion for the fishes once they get to the promised land.  It’s certainly a stopgap approach.  But is there really nothing more we can do besides allowing greater environmental flows from the dams?

Catchment a.k.a. watershed restoration as advocated here was conceived as something that might be done for salmonids (and humans) despite the dams.  The greatest expanses of lands with historically degraded catchment functions largely lie downstream of the dams, nevertheless their groundwater is all headed toward the surface in the mainstem and/or its mouth.  Virtually all that water in the mountains and foothills – the subsurface water that feeds springs and vernal pools – is following the irresistible call of gravity toward the nearest outlet to the ocean.  So enhancing baseflow on tributaries does the same for the mainstems.  Catchment restoration enhances baseflows. 

Technically, these subsurface flows constitute “runoff” in hydrologic terms.  However, they are overlooked in standard hydraulic models, precisely because hydrologists have yet to be able to model the behavior of water moving through the vadose zone.  See the paraphrase atop Surface-Groundwater Systems in a Holistic Water Cycle, along with a bit about the vadose zone.  This disparity is the reason Beven and Germann (2013) revisited their topic thirty years after introducing it to the Water Resources Research community.

As a consequence, water agencies have tended to think primarily in terms of surface water flows, more recently groundwater, but absolutely not about catchments (over the past century, at least), which are the source of the majority of rainfall that ever hits the ground, as pointed out in my blog post #10. How Does Groundwater Get There?  Some Basics.  That’s a huge missing piece of the water resources puzzle, yet the water agencies pretend it doesn’t exist because they don’t know how to model it.  [There are some water balance equations that allow for catchment “wetting” (detention storage), but my understanding is that they are not in wide usage, especially among hydraulic engineers.]

Catchment restoration as advocated here promises to augment cold groundwater influxes to tributaries and mainstems coursing through the hotter inland regions of northern and central California – specifically the issue brought up in the below paper, which could hardly have provided more crucial information for our times.

Willis, A. D., R. A. Peek, and A. L. Rypel. 2021. Classifying California’s stream thermal regimes for cold-water conservation. PLOS One 16:e0256286.   

Excerpt from the abstract:

… Several salient findings emerge from this study. Groundwater-dominated streams are a ubiquitous, but as yet, poorly explored class of thermal regimes. Further, flow regulation below dams imposes serial discontinuities, including artificial thermal regimes on downstream ecosystems. Finally, and contrary to what is often assumed, California reservoirs do not contain sufficient cold-water storage to replicate desirable, reach-scale thermal regimes. …

See also the authors’ blog post:

“Dammed hot: California’s regulated streams fail cold-water ecosystems”. Posted on August 29, 2021 by UC Davis Center for Watershed Sciences by Ann Willis, Ryan Peek, and Andrew L. Rypel

And the thing is, the expansive opportunities for catchment restoration cumulatively add up to a lot of potential storage – just not the surface storage the “More Water Now” folks were demanding.  Subsurface storage – see Retention vs Detention Storage  And that helps humans, along with many other species.

These opportunities lie in the nonnative annual grasslands that are the manifestations of anthropogenic catchment degradation.  In terms of ecological succession they represent a sort of “dead end” or “stable state” – they do not readily progress to another ecological state in the absence of conscious human intervention now (e.g., Stromberg and Griffin 1996).

I emphasize restoring native woody plant species, especially oaks, because this is the catchment component that has been most diminished through human land uses.  But in recent years even the hardy blue oaks – mainstays of the interior rangelands, whose natural regeneration has been of long-standing concern – appear threatened by climate change and, IMO, the combination of that with ongoing impacts of the nonnative annual grass cover changing the hydrology of their root zones.  

Continuity of tree roots with bedrock aquifers has been brought to recent attention by the following publication:

McCormick, E. L., D. N. Dralle, W. J. Hahm, A. K. Tune, L. M. Schmidt, K. D. Chadwick, and D. M. Rempe. 2021. Widespread woody plant use of water stored in bedrock. Nature 597:225–229.

[Note that, to facilitate review by all, where applicable I will provide URLs herein for free downloads of literature I’m citing here, as is the case for the second URL, above.]

A few excerpts from McCormick and colleagues (2021) are appropriate here:

Abstract [with endnotes omitted]:

In the past several decades, field studies have shown that woody plants can access substantial volumes of water from the pores and fractures of bedrock. If, like soil moisture, bedrock water storage serves as an important source of plant-available water, then conceptual paradigms regarding water and carbon cycling may need to be revised to incorporate bedrock properties and processes. Here we present a lower-bound estimate of the contribution of bedrock water storage to transpiration across the continental United States using distributed, publicly available datasets. Temporal and spatial patterns of bedrock water use across the continental United States indicate that woody plants extensively access bedrock water for transpiration. Plants across diverse climates and biomes access bedrock water routinely and not just during extreme drought conditions. On an annual basis in California, the volumes of bedrock water transpiration exceed the volumes of water stored in human-made reservoirs, and woody vegetation that accesses bedrock water accounts for over 50% of the aboveground carbon stocks in the state. Our findings indicate that plants commonly access rock moisture, as opposed to groundwater, from bedrock and that, like soil moisture, rock moisture is a critical component of terrestrial water and carbon cycling.

Other germane excerpts [with endnotes omitted]:

… The circulation of near-surface water by plant roots has consequences for a large number of Earth-system processes, including landscape evolution, ecosystem carbon storage and nutrient delivery to streams.  …


… For example, our deficit analysis suggests that in California alone, 20 km3 (16.2 million acre-feet) of water can be extracted from bedrock by woody plants annually. This is approximately equal to the volume of water stored in all of the state’s reservoirs combined, and about three times the state’s annual domestic water use. …


Given that the dynamics of rock moisture have the potential to regulate the timing of groundwater recharge and runoff, bedrock water storage may be critical to water resource planning. …


Thus, bedrock water storage dynamics are likely key to understanding the sensitivity of carbon, water and latent heat fluxes to changes in climate.

(McCormick and colleagues 2021)

Presumably because the data was not readily accessible by McCormick and colleagues, they missed this early source:

Lewis, D. C. and R. H. Burgy. 1964. The relationship between oak tree roots and groundwater in fractured rock as determined by tritium tracing. Journal of Geophysical Research 69:2579-2588.

In any case, attitudes about plant ecological relationships have changed to some degree in the intervening nearly six decades.  I can nevertheless imagine how some readers might respond to this news – perhaps not too dissimilarly to Lewis and Burgy’s (1964) response to the results of their research that showed that blue oaks are absolutely accessing bedrock moisture, in the following excerpt from their Conclusions:

… To the hydrologist, who studies the occurrence and distribution of water, the trees are analogous to a number of pumps spread uniformly over an area, all operating to depress the water table. In the situation which has been studied, soil moisture sampling cannot measure summer transpiration by trees. Furthermore, the analogy helps to explain the mechanism of response to treatment in watershed management programs which include killing or removal of oak trees. When the direct transpiration of groundwater is eliminated (the pumps are permanently removed), groundwater levels will rise and be maintained at higher levels by natural recharge.

(Lewis and Burgy 1964)

That interpretation of their data exemplifies their time – the mid-20th century view of trees as “thieves” of water humans might otherwise use.  This paradigm inspired a century of “experiments” to show how removing trees, then shrubs, would yield more water for humans.  It was essentially a case of mass “hallucination” or confirmation bias.  And some, perhaps many (?) among us have still not let go of that antiquated perspective.  But the overall attitude had transmuted into state policy in California by the time of Lewis and Burgy’s study, as documented in my blog post #6. Ball and Chain & Other Links.  I offer a counterpoint to that paradigm on the Alternate Paradigms page, “Water Yield” vs Baseflow Augmentation.

[BTW, “confirmation bias” only came to my attention through a 2017 news story about a near-miss landing of a jet atop others on the taxiway at SFO.  Near the bottom of the page on Surface-Groundwater Systems in a Holistic Water Cycle, a lavender box encloses the title and link to, along with a spare sampling of excerpts from a July 31, 2017 Mercury News article detailing the apparent reasons the pilots thought everything was just as it should be.  Rather shocking to learn that confirmation bias is a “thing” in aviation, eh?]

Furthermore, while the (especially) physical scientists in the U.S. were drawn to the concept of improving water yield by removing plants, “like moths to a flame” (and most biologists just accepted their paradigm without question; some still do), there was an alternate paradigm running through the work of primarily forest hydrologists over the same early decades of the 20th century.  Near the last in this line of government publications was:

Lassen, L., H. W. Lull, and B. Frank. 1952. Some plant-soil-water relations in watershed management. Circular 910. U.S. Dept. of Agriculture Division of Forest Influences, Washington, D.C.

My original of that was a photocopy, but some years ago I discovered it is available as a small, inexpensive hardback through a publisher in India.  (Since it was a government document it was not copyright and we are the beneficiaries in that this publication reads better in book form.)  As clear from the above URL, it is now available on Google Books, the pdf there a bit nicer than my original photocopy retrieved via Interlibrary Loan long ago.

Contrast their take on trees as “pumps” with that of Lewis and Burgy (1964):

In this respect plants may be likened to pumps emptying out a reservoir.  The more water that is pumped out, the more space is available in the reservoir for precipitation.  (Lassen and colleagues 1952)

So what do trees and other woody plants do for catchment functions that the nonnative annual grasses don’t – besides tapping bedrock aquifers?  Echoing Lassen and colleagues, think of that tapping as a two-way street.  Especially once those roots die, the channels left by their decomposition serve as macropores – providing pathways, or conduits through the vadose zone.  

Gerald M. Aubertin (1971) was another forest hydrologist who serves as sort of a missing link between the former “Forest Influences” forest/ catchment hydrology folks and contemporary hydrology, in that his paper was cited by Beven and Germann (1982):

Aubertin, G. M. 1971. Nature and extent of macropores in forest soils and their influence on subsurface water movement. USDA Forest Service Research Paper NE-192, Northeastern Forest Experiment Station, Upper Darby, PA.

Rainfall that infiltrates, then percolates through the vadose zone preferentially flows through such macropore conduits at rates far faster than it does through the surrounding soil matrix.  Macropores serve preferential flows laterally through the vadose zone, but since we know that trees tap into fractured rock aquifers, it follows that macropores also bring preferential flows down to bedrock aquifers, following the paths of former root channels.  

However, the evidence suggests that such soil structural features as macropores degrade over time when woody and other perennial plant species supporting that structure are gone; even more readily when subjected to soil compaction.

Click image to expand

Restoration: Nurse Plants vs Nonnative Annual Grasses

I was tempted to diverge here into concerns about blue oak sustainability, especially given climate change, but concluded that the subject is big enough to merit its own post – next time.  Suffice it to state that blue oak woodlands are the general restoration target most appropriate for the greatest expanse of Rainfall to Groundwater opportunities – namely, the existing nonnative grasslands surrounding the Central Valley.  Closer to the coast other oak species and their associates become more important.  As noted in my last post, I envision shrub and subshrub “nurse species” as restoration facilitators, especially given the existing nonnative annual grasses that are notoriously difficult to manage.  

I’m fairly certain my first awareness of the concept of “nurse plants” was initiated by Ragan Calloway’s work (Callaway and D’Antonio. 1991, Calloway 1992).  The earlier paper documents improved survival of coast live oak seedlings associated with native shrubs.  The latter suggests that blue oak seedlings benefit from shading by shrubs.  

Another way to think of “nurse plants” is “facultative mycotrophs” – a term I first heard decades ago from Ted St. John, a southern California plant/restoration ecologist with expertise in the significance of mycorrhizal fungi to ecological restoration.  Facultative mycotrophs are plant species that can form mycorrhizal relationships but don’t require them to get a foothold in a disturbed site.  Thus, they can serve as hosts to get those mycorrhizal relationships re-started in locations where they’ve been lost.  While nonnative annual grasses may support some species of mycorrhizal fungi, the soil ecology of this cover type has presumably been altered significantly from that under its original cover type.  In any case, the idea is to use the tenacity/ vigor of early seral species (in a natural ecological succession following disturbance) to jump-start the restoration. 

This kind of restoration strategy is necessary when you are faced with whole landscapes that have been overtaken by persistent, nonnative annual grasses, including some really noxious weedy species among them.  Very smart people have been throwing considerable resources at attempting to eliminate or even just control some of these invasive species while their ranges have only expanded over the decades.

A case in point is medusa head, which I summarize on California “Grasslands” vs. Altered State(s), under the subsection, Noxious Weeds as Side Effect of the Grassy Rangelands Paradigm, near the bottom of the page.  Its current Calflora distribution, as of Feb 5, 2022, indicates populations in San Diego County that I either missed when I wrote up that page in 2018, or perhaps represent new invasions – ?

But, as I wrote in the Conclusion on that page, if we stop trying to perpetuate grasslands over vast areas of the state that were likely not grasslands prior to anthropogenic impacts, we may finally get a grip on noxious rangeland weeds.  

Please don’t misunderstand me here – I consider California native perennial grasses (there are the atypical native annual grasses, as well) natural constituents of nearly all California native plant associations.  However, eliminating the nonnative annual grasses may require some interim period of predominantly woody or subshrub cover at each restoration site.  This should facilitate a subsurface transition to soil ecology and ecohydrology supporting perennial, over annual plant species.

In addition to ultimately growing tall enough to shade out the nonnative annuals, shrubby nurse species in particular can serve as physical barriers that can help reduce herbivore predation on desired species like oaks at restoration sites.  In fact, I would love to see trials comparing survival of acorns planted in hardwire cages with those protected solely by nurse plant species, perhaps some established in advance of the acorns.

Pathways to 30×30 Conservation Definition & Key Objectives

For whatever reason, I have long eschewed joining committees purposed with generating definitions;  ‘don’t have the requisite patience, or (?) I dunno, just not my thing.  Likewise, in the case of California Natural Resource Agency’s (CNRA’s) 30×30 effort I didn’t touch the call for a definition of conservation, despite that conservation ecology defined my doctoral work.  But I do find the draft definition in the Pathways to 30×30 draft report pretty solid – provided that inland waters are encompassed within the definition of “land areas”.  [And therein lies the rub about definitions – the inherent need for precision/ and or a definition of each word in a definition to ensure all are truly on the same page.  As a matter of fact, CNRA Deputy Secretary Norris shared during the Feb 1st public feedback virtual meeting linked below, that they’ve received a lot of feedback on the definition, noting “that lots of people read words differently”. ]

Conservation Definition for 30×30:

Land and coastal water areas that are durably protected and managed to support functional ecosystems, both intact and restored, and the species that rely on them.

Among key ideas there is “managed to support functional ecosystems” and I suspect few would argue the need for conserved lands management.  Yet it has long been my impression that state park lands have never had sufficient funding to allow staff to stay on top of maintenance, let alone pursue such niceties as ecological restoration, monitoring, etc.  

So during the Feb. 1st 30×30 Public Feedback Virtual Meeting, designed to receive oral comments on the Draft Pathways to 30 X 30 report, my ears perked up when one commenter identified himself as representing an organization of retired state park employees.  He may have said “Graybears” and a quick search suggests that is the group.  I’m really glad to know they will be submitting informed input on the draft report but also pretty curious about specifically what they have to say, aware that I may never know, since such comments are not shared publicly by CNRA.  

In the section of her introductory remarks for that meeting on “Advancing Strategic Actions”,  Deputy Secretary Norris offered some fairly impressive “Initial Investments” for implementing the 30 X 30 plan (at ~19:31 minutes into the recording), as follows: 

  • Nature-Based Solutions Set Aside – $758 Million
  • Coastal Resilience – $600 Million
  • Habitat Restoration – $645 Million
  • Wildlife Corridors – $105 Million

I do wonder whether the state intends to ensure sufficient funds so that state park lands can fully meet the 30×30 definition of conservation? 

Considering the apparent fiscal challenges of just meeting state park maintenance backlogs, it should be apparent that managing 30+% of California lands to support functional ecosystems will require a serious infusion of cash and land management talent.  And it seems a given that all conserved lands in California must be managed for resilience against wildfire, so there’s another layer of complexity.

My proposal for catchment restoration and management across vast rangelands assumes that water agencies will eventually want to pay for restoration and management (presumably via tailored conservation easements) of the catchments that serve them, once they (finally) figure out it’s more cost effective than the hard infrastructure (plumbing) approach that, at this point is grasping for mere leftover scraps.  That actually includes some state parklands that, in my view, should be supported in catchment restoration and management to the same extent as private rangeland owners.  In my first input to the Water Resilience Portfolio, resubmitted to the 30 X 30 process, I pointed to a specific park, best not named here, that could serve as an ideal demonstration site.

So it’s a good question why CNRA’s 30×30 program is apparently so resistant to an approach that could conserve (including restoration and adaptive management) vast acreages at limited, if any, direct costs to the state, given their Pathways to 30×30 Key Objectives (p 11), including protecting California’s unique biodiversity and “Conserve places that help California achieve carbon neutrality and/or build climate resilience”.   

Certainly catchment restoration would result in significant below-ground, as well as above-ground carbon sequestration (e.g. see Carbon Farming & Watershed Restoration).  While I don’t find it mentioned in the Climate Advisory Panel report, I recall that at some point during the associated panel discussion, it was acknowledged that very little has been documented to date with respect to soil carbon storage, which may be among the forms of carbon storage with greatest longevity.   

Furthermore, catchment restoration is a climate change resilience strategy that really should have been considered in a water resilience portfolio actually grounded in hydrology and in consideration of diminishing snowpack that we’ve counted on for detention storage in the past – see Retention vs Detention Storage..  It works for humans and many other species.  It’s not too late to consider it now, but the state’s ecohydrology is growing increasingly more dire as the water resources crowd dithers with finding new ways to employ “gray” infrastructure.  The early emphasis on “nature-based solutions” in 30×30 promised to get us into this territory but, with the exception of measures pertaining to coastal waters, the Pathways to 30×30 draft doesn’t seem to have come close to articulating nature-based solutions that weren’t already ongoing before they embarked on this 30×30 process.

The increased evaporative demand anticipated by some as a factor in atmospheric warming is apparently coming to pass, with the recent finding,

… that global land evapotranspiration increased by 10 ± 2 per cent between 2003 and 2019, and that land precipitation is increasingly partitioned into evapotranspiration rather than runoff …  (Pascolini-Campbell and colleagues 2021)

This would seem to make surface water storage and even surface conveyance subject to increased evaporative losses in a climate that appears to be warming in real time.

In contrast, the native California plants proposed for catchment restoration are preadapted to withstand longer and more intense periods of drought than we have yet to experience in the 21st century.  Various past research has employed tree ring chronologies to document long periods of drought in the American Southwest over the past few thousand years, but the most comprehensive and recent is that compiled by Williams and colleagues (2020) that documents prehistoric “megadroughts” – evidence that native plant species have had to adapt to much more challenging conditions than we’ve experienced so far in current times.  So native plant species are genetically preadapted to adjust to a possible anthropogenically-driven “megadrought” that we may have entered in this millennium per Williams and colleagues.  Furthermore, increased atmospheric carbon dioxide may be enhancing plants’ water use efficiency and has recently been documented in the American Southwest (Kannenberg and colleagues 2021).

Moreover, given that the Pathways … Key Objective #1 is, “Protect California’s unique biodiversity”, the state might step up with an acknowledgement of catchment restoration to support aquatic biodiversity, with salmonids as focal species.  (Note: “focal species” has meaning in conservation biology planning, whereas “iconic” does not.)  I consider California tiger salamander another catchment restoration focal species, in this case mostly off-stream – see What does Rainfall to Groundwater offer for vernal pools?

Adaptive Management by Whom?

In any case, somebody needs to adaptively manage those conserved lands and to monitor the restoration and management to ensure the catchment restoration goals and objectives are being met.  Boots on the ground and butts in the saddle.  It’s really hard to beat the place-based knowledge of those who come to know their lands intimately.  Ineffable awareness may come to those living close to the land, provided sufficient humility.  

Catchment restoration as proposed herein depends on stewardship of restoration efforts, as well as ongoing adaptive management to ensure catchment functions are achieved to the fullest extent.  Management for wildfire resilience is certainly a huge part of that.  Cattle are currently enlisted for fuel management on many public lands, as well as on the state’s much vaster private rangelands – how would that look under the Rainfall to Groundwater vision?

Permacultural Beef?

Truthfully, this was the last piece of the puzzle for me of how to make catchment/ watershed restoration work in California, given that the greatest expanses of rangelands are privately held.  I am proposing displacing the nonnative annual grasses with native woody and perennial species (and native annuals, of course). Don’t cattle require grass?  The large-bodied breeds imported from moister climes appear to require it.  But a precious few populations of cattle that came to the New World with Columbus and company, were left to their own devices for breeding by their indigenous human stewards, resulting in populations subjected to essentially “natural selection” on the American continents.

The page, Criollo Cattle? offers an introduction, with links for more info.  Scientists at the Jornada Experimental Range, a USDA Agricultural Research Service (ARS) site in Southern New Mexico, traveled to an area called the Sierra Tarahumara near Chinipas, Mexico to select and purchase such naturally selected cattle and brought them to Jornada to expand their numbers and study their potential as “desert-friendly cows”.  These animals, adapted over centuries here to browsing, as well as grazing – a necessity in the New World – so have become, perhaps, less “picky” eaters than the larger European domesticated breeds that have dominated markets for many decades.  In New Mexico they have appeared quite promising, though they are lucky that environmental conditions there have some similarities to the area from which the “criollo” stock were drawn.

Burcham (1957) offers a helpful map, “Figure 5.  Movements of livestock into the southwestern United States”, depicting historical movements of cattle from the Caribbean, northward through current Mexico and across the Gulf of California into Baja.  My thought upon seeing that was that cattle subjected to natural selection in Baja California would be the ideal stock to try out in California.  So I consulted the most current source I could find (Martinez 2011) and learned that no “Criollo” cattle untouched by human breeding efforts apparently occur there in contemporary times.  So if California ranchers wish to try out “Criollo” types, their best bet is likely to be the folks at Jornada Experimental Range.

The hope is that such cattle, smaller-framed and pre-adapted to environmental/ climatic patterns in North America, would be amenable to browsing native plant species on restored catchments and could thus serve fuel management, as well as food production objectives.  Even within the broad generalization of, for example, the class of blue oak woodlands, the nature of the California flora is that unique assemblages of native plant species will arise in each microclimate of the restored range, (re-)establishing the unique character of each region’s rangelands.  Since these would be perennial plant species, cattle could browse (and graze) year-round – the livestock version of permaculture, eliminating the need to ship the stock to out of state feedlots, as well as the fossil fuel intensive agriculture used to produce feedlot crops AND doubtless some significant portion of the infamous amounts of water required to yield a pound of beef.

A scenario of raising such stock on rangelands restored to their native plant cover would enable “range-fed”  (contrast with the currently common, “grass-fed”) beef to be marketed to consumers, highlighting the particular “terroir” of its local region that will arise with restoration of native plant species to the ranges, as suggested on my page, Livestock Appellations for California?  Excrement from cattle grazing on diverse, well vegetated rangeland ecosystems will be quickly recycled into the soil ecosystem, in contrast with the manure piles associated with feedlots.

So, in addition to the expanded soil carbon sequestration that will be engendered with restoration of native woody and other perennial plants (and their soil ecosystems) to the state’s rangelands, I have asked CNRA, in my comments on the Draft Natural and Working Lands Climate Smart Strategy, to please consider the Climate Smart benefits of managing California’s rangelands such that beef cattle stay within the state from birth to slaughterhouse, circumventing the need for fossil fuels to support shipping and out of state feedlot agriculture.  There’s some carbon math to consider.


Verna Jigour, PhD.

Citations not included within the text

Beven, K. and P. Germann. 1982. Macropores and water flow in soils. Water Resources Research 18 (5):1311-1325.  

Beven, K. and P. Germann. 2013. Macropores and water flow in soils revisited  Water Resources Research 49:3071–3092. 

Burcham, L. T. 1957. California range land: an historico ecological study of the range resources of California. California Division of Forestry, Sacramento. 

Callaway, R. M. and C. M. D’Antonio. 1991. Shrub facilitation of coast live oak establishment. Madroño 38:158-169.  

Callaway, R. M. 1992. Effect of shrubs on recruitment of Quercus douglasii and Quercus lobata in California. Ecology 73: 2118-2128.  

Kannenberg, S. A., A. W. Driscoll, P. Szejner, W. R. L. Anderegg, and J. R. Ehleringer. 2021. Rapid increases in shrubland and forest intrinsic water-use efficiency during an ongoing megadrought. PNAS 118:e2118052118.

Martinez, Jorge de Alba.  2011.  El Libro de los Bovinas Criollos de America.  Colegio de Postgraduados.  Texcoco, Estado de Mexico, Mexico.

Pascolini-Campbell, M., J. T. Reager, H. A. Chandanpurkar, and M. Rodell. 2021. A 10 per cent increase in global land evapotranspiration from 2003 to 2019. Nature 593:543–547.

Stromberg, M. R. and J. R. Griffin. 1996. Long-term patterns in coastal California grasslands in relation to cultivation, gophers and grazing. Ecological Applications 6:1189-1211.  

Williams, A. P., A. P. Williams, E. R. Cook, J. E. Smerdon, B. I. Cook, J. T. Abatzoglou, K. Bolles, S. H. Baek, A. M. Badger, and B. Livneh. 2020. Large contribution from anthropogenic warming to an emerging North American megadrought. Science 368:314–318.