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The intermittent drought we experience in California, which may be part of a regional “megadrought” with anthropogenic signals (Williams and colleagues 2020), has us imagining that this is among the more impacted regions on the planet.  Who here would guess that one location even more acutely attuned to how weather/climatic patterns combined with human land uses impact water resources would lie within the Tropic of Cancer?  

That would be the Republic of Panama and, despite its relatively moist environment, water resources are the “lifeblood” of the country’s economic existence.  That is because a mind-boggling amount of freshwater is needed to operate the lock systems of the Panama Canal.  So that entire enterprise is dependent on the sound functioning of the catchments/ watersheds feeding the canal system.

That fact came to my attention via serendipity.  A few months ago I felt exhausted one evening and needing to veg out, I settled on watching a video I knew I had viewed before, but it was the only one calling out to me at the time – the 2020 PBS 3-part series The Age of Nature.

In the first episode, titled “Awakening”, the second case example concerns the Panama Canal watershed/ catchment.  I recalled that the first time I watched this I had made one of the multitudinous notes that ideally serve as memory-extensions, to look into that subject.  I’m sure I’ll run across that note later, but this time I couldn’t wait.  Despite feeling tired, I had to pause the video at the end of that segment and see what more I might quickly learn about the Panama situation before continuing on.

In the video, Dr. Stanley Heckadon-Moreno of the Smithsonian Tropical Research Institute discusses the concerns, first broached in 1978, over land use changes potentially impacting the enormous quantities of fresh water needed to power the canal’s “colossal”, as narrator Uma Thurman describes them, locks.  That water, drawn from the Chagres River, appeared to be threatened by increasing deforestation of the watershed/ catchment – particularly for conversion to pasture, just as has happened all over the world, including California, BTW.  

In The Age of Nature. Episode 3, “Understanding”, Professor Tom Crowther discusses global forests as carbon sinks, considering the potential for reforestation.  He observes that “before the expansion of human civilization there [were] almost twice as many trees on the planet as there are today”.  Here in California, it’s generally more accurate to speak of woodland or scrubland, rather than forest conversion, though type conversion can and has happened in all cover types here.  Back to Panama …

Panama Canal Trip 1994  “Family collection of Infrogmation of New Orleans”., CC BY-SA 4.0, via Wikimedia Commons

 Dr. Heckadon-Moreno observes in the film that,

Every ship that goes through the Panama Canal requires about 50 million gallons of fresh water from the Chagres – every ship. And there are 40 ships a day so you multiply that and it’s an astronomical quantity of fresh water. It’s the energy, the water of the Chagres that makes possible the Panama Canal.

50,000,000 gal. = 153 acre-feet x 40 = 6,120 acre-feet/ day x 365 days = 2,233,800 acre-feet/ year to power the Panama Canal, perhaps even more now that new channels have been added, per Panama Canal on Wikipedia.

The video goes on to recall that during the 1970s, the government wanted the land to be made more productive, so they encouraged people to clear pastures, perhaps not dissimilarly to what our California state government was doing in the 1960-70s, as documented in my blog post #6. Ball and Chain & Other Links.

Heckadon-Moreno came to Panama in 1979 to study the anthropology of the region and was shocked by the ongoing forest conversion there. In the process of clearing land with fire, escapes were inevitable, taking out even more forest than intended. “What used to be forest, now it’s like a desert”, he observes. Landscape-scale erosion ensued.

Initially, concerns  over the viability of the Chagres River and Panama Canal focused on sedimentation.  Given that forest conversion to pasture was the suspected root of the problem, Hecadon-Moreno’s team began studying the forest’s ecology, relative to cleared sites.  He recalls, “Gradually it dawned on me how important a role in the cycle of water trees have.”  He recognized that the organic matter of the forest had a role in storing water in the soil and that when the dry season comes that water is released to the creeks. 

Note:  The familiar “sponge effect” is referenced here as the phenomenon that keeps rivers flowing through the dry season.  In truth, while perhaps a helpful metaphor on the path to understanding, that must be recognized as somewhat of a conceptual oversimplification because natural vadose zones, unlike sponges, are structured by the plant roots and their soil ecological associates that course through that zone.  It is a subterranean, multispecies collaborative (mutualistic), nonlinear “architecture” and communication/ exchange system – memed as the “wood-wide web”.

Aerial view of the Province of Chiriqui, Republic of Panama Nov. 22, 2014   FranHogan, CC BY-SA 4.0, via Wikimedia Commons

Note:  The aerial photos shown here are from western Panama, not within the Chagres River watershed, but are the only ones I found of the region and offer some sense of land cover features there.

Then, in 1983, severe drought struck, threatening the canal, economic engine of the republic, and the concerns went all the way to Panama’s president.  Hecadon-Moreno recalls him saying, “Dr. Heckadon, this is a national security issue, and we have to stop it.” 

Chagres National Park was established in 1985 to protect the river and by extension, Panama’s entire economy.  The park protects 129,000 hectares (320,000 acres) of watershed/ catchment forests.  According to the video, 25% of Panama now comprises national parklands. [Are they looking to achieve 30 X 30 – ?]

Heckadon-Moreno speculates that if the team’s proposal had not been taken seriously back then, there would be no Panama Canal today as we know it.  

I think each country comes to a point [where] it has to make decisions.  What is the best, not in the short term, but what is the best in the long term for the most people?

At the time I was thinking, gee, it sounds like it was fairly easy for Heckadon and colleagues to convince the Panamanian government.  They didn’t even have that much scientific evidence at that point.   Why the heck am I having such a hard time convincing the State of California to even consider this???

Upon later reflection I realized that one thing the Panama situation had going for it was the relatively recent land conversion that enabled observers to readily make the connection between that and the channel-choking sedimentation and water degradation.

In contrast, despite being expedited by state agency actions for a time in the mid-20th century, anthropogenic land cover type conversion had been ongoing in California over more than two centuries by then (I include prehistoric indigenous peoples’ burning in that) and the landscape-scale changes here were gradual enough to be missed by most –  to the point that we began viewing the annual greening up, then drying to straw-color of the hills as the “natural” order of things here.

Aerial view of the Province of Chiriqui, Republic of Panama  Nov. 22, 2014  FranHogan, CC BY-SA 4.0, via Wikimedia Commons

That night of my second viewing of the Age of Nature series I wanted to learn more about Panama’s situation but was trying not to burn too much energy to scratch that itch.  So my first stop was Wikipedia’s page on Chagres National Park, where one finds a limited amount of information – in the English version, at least.  But, of the three footnoted references there, the first being to, I found this second more helpful reference was freely available and at least gave me an overview:

Condit, R., W. D. Robinson, R. Ibáñez, S. Aguilar, A. Sanjur, R. Martínez, R. F. Stallard, T. García, G. R. Angehr, L. Petit, S. J. Wright, T. R. Robinson, and S. Heckadon. 2001. The status of the Panama Canal Watershed and its biodiversity at the beginning of the 21st century: Long-term ecological studies reveal a diverse flora and fauna near the Panama Canal, harbored within a corridor of forest stretching from the Caribbean to the Pacific, but deforestation, land degradation, erosion, and overhunting remain threats.  BioScience 51:389–398.[0389:TSOTPC]2.0.CO;2  

I was unable to find an online version of the third footnoted reference on that page:

Wadsworth F.H. 1978. Deforestation: death to the Panama Canal. Pages 22–24 in Proceedings of the US Strategy Conference on Tropical Deforestation. Washington (DC): US Department of State and US Agency for International Development.

However, this excerpt from Condit and colleagues (2001) offers a clue:

… But deforestation has a second, more direct impact on water resources: It alters temporal patterns of flow.  We demonstrated this impact in a watershed at the boundary of the north end of Soberania National Park.  In a deforested catchment, 26% of incident rain entered streams almost immediately, while only 14% did so in an adjacent forested catchment matching in topography and geology (Ibáñez et al. 1999a).  As a result, stream flow during the wet season was higher in the deforested catchment than in the forested one, while the pattern reversed in the dry season. It is likely that further deforestation throughout the watershed would reduce dry season water supplies to the canal; a large-scale hydrological model is being developed that will predict this impact. Since dry season water supply is the major concern for canal operation—the only reason canal use has ever been limited—this issue appears to be far more important than the siltation issue. It is perhaps unfortunate that early papers warning about deforestation (Wadsworth 1978) focused on increased siltation instead of reduced dry season flow. [emphases added]

I was not able to find the Ibañez et al 1999a citation online. 

I was able to find this: Heckadon-Moreno, S., R. Ibañez, and R. Condit. 1999. La Cuenca del Canal: Deforestacion, Urbanizacion, y Contaminacion. Smithsonian Tropical Research Institute and Impresilibros, Panama.

But my Español is not great.  I find Google Translate very helpful at times, though it sometimes yields some rather hilarious results.  So I haven’t delved deeply into that one.

Aerial view of the Province of Chiriqui, Republic of Panama  Nov 22, 2014  FranHogan, CC BY-SA 4.0, via Wikimedia Commons

That evening’s immediate curiosity was mostly satisfied by the former paper.  Later on, in the course of catching up on research related to my topic in general, papers pertaining to Panama jumped out at me.  One I’ve had a chance to review is the following, author’s manuscript freely available at the second url below:

Litt, G. F., F. L. Ogden, A. Mojica, J. M. H. Hendrickx, E. W. Kempema, C. B. Gardner, M. Bretfeld, J. A. Regina, J. B. J. Harrison, Y. Cheng, and W. B. Lyons. 2020. Land cover effects on soil infiltration capacity measured using plot scale rainfall simulation in steep tropical lowlands of Central Panama. Hydrological Processes 34:878-897.

These authors explain the specific opportunity for their research as follows:

Land cover effects on hydrological processes in tropical catchments is a fundamental research component of the Agua Salud Project (ASP) study catchments located within the Panama Canal Watershed (PCW) (Stallard et al., 2010). The ASP infrastructure promotes land cover studies because soil parent material, bedrock, and topography are remarkably similar in ASP catchments, but stark contrasts in runoff efficiencies and flashiness exist between pasture, mixed land use, and forest catchment stream responses (Ogden et al., 2013; Cheng et al., 2018; Adamowicz et al., 2019).


Preferential flow pathways could hold the key to understanding contrasting runoff behavior across PCW land covers  …

They cite Beven and Germann (2013) as urging hydrologists to pursue research on preferential flow pathways (through the vadose or unsaturated zone).  That 2013 paper revisited Beven and Germann’s 1982 paper on the same topic, observing that hydrologists hadn’t really gotten far along that path, thirty years later.  Both those papers appear on Citations on this Site and the earlier one was among foundational literature in my doctoral dissertation.  It is appropriate to note here that from the start, i.e., Keith Beven’s doctoral work, he has been among a decidedly rare breed – catchment hydrologists.

Observing the limitations of earlier infiltration rate studies in the ASP, Litt and colleagues (2020) state that, 

The absence of a plot-scale infiltration investigation across multiple land covers in ASP partly motivated the present study.  Our study’s primary objective assesses bulk infiltrability using measurements at the plot scale across a range of humid tropical LULC’s [land use/ land covers]  within the PCW [Panama Canal Watershed].

This paper offers rich observations and since the author’s manuscript is freely available, it’s probably best to just share the abstract here:

Understanding land use/land cover (LULC) effects on tropical soil infiltration is crucial for maximizing watershed scale hydro-ecosystem services and informing land managers. This paper reports results from a multiyear investigation of LULC effects on soil bulk infiltration in steep, humid tropical, and lowland catchments. A rainfall simulator applied water at measured rates on 2 × 6 m plots producing infiltration through structured, granulated, and macroporous Ferralsols in Panama’s central lowlands. Time-lapse electrical resistivity tomography (ERT) helped to visualize infiltration depth and bulk velocity. A space-for-time substitution methodology allowed a land-use history investigation by considering the following: (a) a continuously heavy-grazed cattle pasture, (b) a rotationally grazed traditional cattle pasture, (c) a 4-year-old (y.o.) silvopastoral system with nonnative improved pasture grasses and managed intensive rotational grazing, (d) a 7 y.o. teak (Tectona grandis) plantation, (e) an approximately 10 y.o. secondary succession forest, (f) a 12 y.o. coffee plantation (Coffea canephora), (g) an approximately 30 y.o. secondary succession forest, and (h) a >100 y.o. secondary succession forest. Within a land cover, unique plot sites totaled two at (a), (c), (d), (e), and (g); three at (b); and one at (f) and (h). Our observations confirmed measured infiltration scale dependency by comparing our 12 m2 plot-scale measurements against 8.9 cm diameter core-scale measurements collected by others from nearby sites. Preferential flow pathways (PFPs) significantly increased soil infiltration capacity, particularly in forests greater than or equal to 10 y.o. Time-lapse ERT observations revealed shallower rapid bulk infiltration and increased rapid lateral subsurface flow in pasture land covers when compared with forest land covers and highlighted how much subsurface flow pathways can vary within the Ferralsol soil class. Results suggest that LULC effects on PFPs are the dominant mechanism by which LULC affects throughfall partitioning, runoff generation, and flow pathways.

Along with this bit from their Conclusions:

These field-based studies show that tropical land cover and land use influence soil infiltration rates and subsurface flow pathways, with implications for tropical land management aimed at the provisioning of hydrological ecosystem services. (Litt and colleagues 2022)

An even more recent paper that appears available upon request through Researchgate (I requested it but haven’t received it to date) is this, along with its graphical and standard abstracts:

Birch, A. L., R. F. Stallard, S. A. Bush, and H. R. Barnard. 2021. The influence of land cover and storm magnitude on hydrologic flowpath activation and runoff generation in steep tropical catchments of central Panama. Journal of Hydrology 596:126138.  


Click the above or any other image to expand.

Compare with the two slides on the R2G Front Page.  I concede my vadose zones look perhaps a bit deep (?) there but that was supposed to be bottom of slope and No Scale.  Certainly Birch and colleagues (2021) have more data.


Despite abundant research documenting that land use/land cover (LULC) have substantial impacts on the hydrology of humid tropical systems, field-based evidence for the physical mechanisms behind these impacts are still lacking. In particular, our understanding of the hydrologic flowpaths that generate runoff in these systems, and how they vary with respect to LULC is insufficient to inform both physically-based hydrologic modeling and land-use decision-making. In this study, we use end-member mixing analysis (EMMA) of stream chemistry, and hydrometric characterizations of hillslope soil moisture to identify hydrologic flowpaths in humid tropical steep-land catchments of varying LULC: mature tropical forest, young secondary tropical forest, cattle pasture. EMMA was applied to data from 14 storm events (six at the mature forest, five at the young secondary forest, and three at the cattle pasture) that were intensively sampled during the 2017 wet season representing a wide range of rainfall magnitudes and intensities. Additionally, volumetric-soil-moisture responses at multiple depths were characterized during and after 74 storm events occurring from 2015 to 2017. EMMA results indicated that lateral preferential flow within the top 30 cm of the soil profile was a dominant source of runoff generation at the two forested catchments, with the contribution of this flow path increasing with rainfall magnitude and intensity. This was corroborated by volumetric-soil-moisture data, that showed that a perched zone of saturation developed at 30 cm at the time of peak storm runoff during the largest events and lasted for the remaining duration of the event. EMMA indicated that runoff was a combination of infiltration-excess overland flow and lateral subsurface flow in the actively grazed pastoral catchment. There, overland flow contributed 62 % of runoff during the highest runoff rate sampled (35.3 mm/hr) and this contribution increased substantially with storm magnitude. This flowpath identification was also supported by volumetric-soil-moisture data at the pasture, with peak saturation at all depths during the largest storm events occurring up to 30 min after peak runoff. These results provide a mechanistic explanation for previously observed hydrologic differences among tropical LULCs. Additionally, the wide range of hydrologic conditions during these storm events provide a basis for understanding how future changes to this, and similar humid tropical regions will impact hydrological processes and water availability.

This one is freely available at the second url below:

Birch, A. L., R. F. Stallard, and H. R. Barnard. 2021. Precipitation characteristics and land cover control wet season runoff source and rainfall partitioning in three humid tropical catchments in central Panama. Water Resources Research 57:e2020WR028058.

I’ve collected a few other papers regarding studies on the Panama Canal Watershed/ catchment, but the preceding are the most current and directly relevant to Rainfall to Groundwater.

I’m not sure specifically which Panamanian president it was that recognized watershed/ catchment degradation as a “national security issue” in the early 1980s – Panama had a series of relatively short-lived presidencies in the 80s but I say, Bravo!  Apparently progress can happen even while government endures tumultuous changes.

As a matter of fact, around the turn of the 20th century, many in the U.S. also viewed our diminishing natural resources as a national security issue – exemplified by the so-called “Progressive Conservation Movement, 1890-1920” (Hays 1959 (1975)) [See also McGee, W. J., editor (1909) both on Citations on This Site], especially prominent during Theodore Roosevelt’s presidency.

Since California’s water resources increasingly appear at risk due to anthropogenic climate change, we can only hope such foresight and leadership arises here soon.  Hasn’t happened lately.  And I’ve never been seeking something as extravagant as a new national park. 

Had DWR embraced the possibilities when I first submitted inputs about this in 2009 to the 2010 CA Water Plan then in progress, we might at least have some demonstration sites on which to assess the hydrological/ water resource implications of land cover changes.  Perhaps they just don’t like me as messenger, but alas, I appear to be the only one to have stepped up for this particular “assignment”.

A rainbow over the mountains off the coast of the Big Island of Hawaii March 20, 2007   IIP Photo Archive, CC BY 2.0, via Wikimedia Commons

Within the U.S. the place where studies of land use/ land cover effects on catchment/ watershed functions have caught on with greatest fervor is another tropical location – the Hawaiian Islands.  While that might seem surprising, again given the tropical climate, when you think about it, no pipeline from far away sources (Fiji water?) is likely to be supplying these islands with imported water any time soon, so they need to make the best of the islands’ water cycle, especially given climate change impacts.

In the case of Hawaiʻi, documentation of land use/ land cover relationships with catchment functions was made possible due to the Auwahi Forest Restoration Project on the southwestern slopes of Haleakala on the island of Maui.  The project was initiated in 1997 by United States Geological Survey biologist Art Medeiros, PhD, whom the Auwahi Wikipedia page describes as “retired” now.

Photo of the Auwahi Dryland Forest Restoration Project on the slopes of Hale’akala on the island of Maui, Hawaii  June 17, 2010  Arthur Medeiros, USGS Public domain, via Wikimedia Commons

Medeiros explains in a TEDx Maui talk his brilliant, IMO, rationale for establishing the project as a rectangle. It was to ensure that no one in the future, after he was gone, would mistake the project for a natural feature, but he goes into more detail about that in the video, as well as mentioning a bit of the cultural/ ethnobotanical significance of the forest restoration project.

So in this case, the forest restoration came first, enabling the subsequent hydrological investigations to compare the restored area with adjacent anthropogenically disturbed lands.

Beginning in 1845, Auwahi’s forest understory was destroyed by cattle grazing and burning [citation]. The native understory was replaced by extensive stands of the invasive shrub Ageratina adenophora [citation; note that this same species is an invasive pest plant in California, as well] that dominated until 1945 when eliminated by a biological control program coupled with severe drought (Medeiros et al. 1986). In response, ranchers planted slips of Cenchrus (Pennisetum) clandestinus [citation] [common name and hereafter referred to as kikuyu grass] throughout Auwahi ca. 1950 to enhance cattle pasturage and reduce erosion [citation]. By 1965, kikuyu grass had spread extensively, developed rank mats, and was regarded as a primary threat to forest health at Auwahi and responsible for a dramatic decline of native trees [citation]. 

(Medeiros and colleagues 2014)

Hawaiian Islands Banknote for 10 silver dollars c ,1860s – 1880s

As the National Park Service shares, with photos, on its Paniolo page, “Even before the mythology of the cowboy in the American “wild west” became popularized, Hawaiʻian cowboys (paniolo) were wrangling longhorn cattle on Hawaiʻi Island.”  That page offers an overview history, including the role of famous paniolos as a focus of national pride at a point “in the history of a nation that was mourning its stolen sovereignty”.  The photo there, “Taking cattle to steamer” shows the next stage in the process depicted below.

Loading Cattle at Kailua, Geography of the Hawaiian Islands (1908) by Charles Wickliffe Baldwin

Details of the restoration project appear here:

Medeiros, A. C., E. I. von Allmen, and C. G. Chimera. 2014. Dry forest restoration and unassisted native tree seedling recruitment at Auwahi, Maui. Pacific Science 68:33-45.

From the abstract:

“With less than 10% of their former distribution remaining, Hawaiʻian dry forests, though critically endangered, remain important biological and cultural refugia.”

A brief summary is offered here:, including, “Following 12 years of restoration, non-native species cover declined from 87% to 2%, while native species increased from 20% to 98%. “

In contrast with the nonnative annual grasses that have come to dominate California’s rangelands, the nonnative invasive grass of concern at Auwahi, kikuyu grass, is a creeping perennial grass of African origen that spreads vegetatively by rhizomes and stolons; the early cultivars that came to Hawaiʻi, in particular, are less prone to propagate via seed.  It’s probably been in California for a similar length of time as in Hawai’i, though has primarily spread along the coast, as affirmed by its Calflora distribution.

Kikuyu grass forms thick, dense mats,  It does have a lush feeling when mowed into lawns on the Big Sur coast.  Alas, it easily spreads into coastal scrub, where it becomes transformed into sometimes monstrous stalks and crowds out the natives. Given Hawaiʻi’s oceanic location and relatively high humidity, conditions there were ripe for this invader to take over, especially where woody plants had been removed for pasture expansion, including through burning.  Medeiros and colleagues (2014) observe that while it can be “useful as forage in marginal situations” it is invasive in Hawaiʻi and elsewhere.  In addition to the grass, there was the ongoing pressure from nonnative ungulates, requiring stout fencing,

An aspect of the restoration approach that wholly aligns with my own thinking on restoration of degraded catchments in California is that they used a “nurse” species, the hardy native shrub ‘A‘ali‘i (Dodonaea viscosa).  The native shrub nurse species approach is exactly how I envision “crowding out” nonnative annual grasses here.  ‘Truth is, Dodonaea viscosa, common name here is hopseed bush, is an attractive, drought tolerant introduced shrub species in California, with cultivars used broadly in landscape plantings throughout much of the state.  

Here in California, with a general restoration target of oak woodlands, native shrub and sub-shrub species would serve as the nurse crops – as they do in natural succession.  In Auwahi’s case, the nurse crop absolutely facilitated regeneration of other native forest species – a phenomenon also observed by Brennan and colleagues (2018) with respect to coyote brush in the Santa Monica Mountains. [ See Citations on this Site; an excerpt from the abstract appears on California “Grasslands” vs. Altered State(s), a little more than halfway down the page, adjacent an image of coyote brush. ]

But what we’re really here to consider is the results of the USGS hydrological analyses, reported in the following papers:

Perkins, K. S., J. R. Nimmo, and A. C. Medeiros. 2012. Effects of native forest restoration on soil hydraulic properties, Auwahi, Maui, Hawaiʻian Islands. Geophysical Research Letters 39:L05405.  

According to Hydrology at Auwahi:

The 2012 publication, funded in part by the County of Maui Department of Water Supply, was nationally recognized by the American Geophysical Union as documenting globally important findings and selected as an “AGU Research Spotlight” and the subject of a national USGS press release.

Perkins, K. S., J. R. Nimmo, A. C. Medeiros, D. J. Szutu, and E. von Allmen. 2014. Assessing effects of native forest restoration on soil moisture dynamics and potential aquifer recharge, Auwahi, Maui. Ecohydrology  

Perkins, K. S., J. D. Stock, and J. R. Nimmo. 2018. Vegetation influences on infiltration in Hawaiʻian soils. Ecohydrology 11:e1973.

Just a couple excerpts from the most recent publication should suffice:

… Vegetation can also modify soil properties in measurable ways over short timescales (Perkins, Nimmo, & Medeiros, 2012), influencing plant‐available water, evapotranspiration, and drainage (D’Odorico, Caylor, Okin, & Scanlon, 2007; Grayson et al., 2006; Nimmo, Perkins, et al., 2009; Sandvig & Phillips, 2006). …

From their Conclusions:

… Landscapes with trees and shrubs have the highest infiltration rates followed by grasses and then bare soil. As ecosystems are changed by human activity, they evolve and soils properties change thereby influencing how rainfall is partitioned into runoff, soil moisture, and groundwater recharge. …

Dodonaea viscosa-view Auwahi Kanaio boundary-Auwahi-Maui  May 22, 2004  Forest & Kim Starr, CC BY 3.0 US, via Wikimedia Commons

I’m pleased to note that that and related projects completed by this USGS team, based in Menlo Park, CA (by now probably fully moved to Moffett Field in Mountain View) apparently led to the following publication, which is free to read online:

Nimmo, J. R. 2021. The processes of preferential flow in the unsaturated zone. Soil Science Society of America Journal 85:1-27.  

One thing that struck me about these Auwahi USGS efforts is the employment of saturated hydraulic conductivity measures as proxy for flows through the unsaturated zone, which contrasts with other methods I’ve reviewed, such as those more recently used in Panama.  Initially I was resistant – we all have our built-up predispositions.  

But the more I considered it, I must say their methods make sense and they acknowledged excluding potential effects of throughfall from fog capture by forest canopies, which has been an important phenomenon there.   However, in Hawaiʻi, as in California, climate change may be reducing fog generation – here along the California coast and into the Central Valley.  At least reforestation has a chance of helping to prolong the persistence of coastal fog, especially in Hawai’i.  [I do fret about post-CZU fire regeneration of Big Basin SP redwoods for that reason of diminishing fog.]

Another thing that particularly struck me in these papers was the attention to hydrophobicity (within, not atop, the soil).  This is stated in the 2014 paper as “Hydrophobicity, hydraulic conductivity, and preferential flow are inherently linked properties”.  I had to say that, while I’d read about everything I could find about how macropores are apparently formed, hydrophobicity had not exactly jumped out at me, but after reading those papers, I realized it makes perfect sense as at least one of the drivers of preferential flow.  ‘Probably enough said on that subject for now in this venue.


Maʻalaea Bay, Maui, Hawaiʻi  August 14, 2005 Tony Webster, CC BY-SA 3.0, via Wikimedia Commons

Another recent study focused on East Maui dealt with somewhat different issues than those at Auwahi.  This ecohydrological economics analysis, funded by The Nature Conservancy in relation to a reserve there, emphasized invasion by nonnative tree species.

Bremer, L. L., C. A. Wada, S. Medoff, J. Page, K. Falinski, and K. M. Burnetta. 2019. Contributions of native forest protection to local water supplies in East Maui. Science of The Total Environment 688:1422-1432.  


In this article, we evaluate: 1) the potential benefits of watershed management actions which prevent non-native forest expansion in a forest reserve in East Maui in terms of groundwater recharge and reduced future costs to the water utility; and 2) the watershed management costs associated with protection of the native forest which provides these benefits. In so doing, our objective is to evaluate the potential for forest conservation to be financed (fully or partially) based on savings to the water utility associated with avoided replacement cost.

Excerpt from the (updated) abstract:

… we quantified the benefits of protecting native forest from conversion to non-native forest in East Maui, Hawai’i in terms of groundwater recharge, a highly valued hydrologic ecosystem service that water utilities increasingly seek to co-finance. Compared with two counterfactual invasion scenarios, the groundwater recharge benefits of planned conservation activities reached 50.9 to 122.3 million m3 over 100 years depending on invasion rate assumptions. This translated to 2.70 to 137.6 million dollars of cost savings to the water utility in present value terms (achieved through reducing reliance on more expensive water alternatives) under a range of discount rates and water scarcity assumptions.  Our results suggest that investing in native forest conservation provides an important hydrologic ecosystem service benefit that complements the range of benefits provided by these ecosystems.

In discussing costs of land management activities these authors mention fencing and nonnative ungulate removal, which I saw emphasized in at least one of the other papers I collected re what’s happening in Hawaiʻi around this general subject.  Another good point they make is the following:

Although there may be cases where it is indeed cost effective for water utilities and other water users to fully finance watershed conservation projects, in other cases co-financing through institutions that bring together additional stakeholders interested in other co-benefits, such as biodiversity, carbon sequestration, local livelihoods, and corporate social responsibility may be more feasible (Abell et al., 2017; Kroeger et al., 2019; Santos de Lima et al., 2019).

In California’s case there is CDFA’s Healthy Soils Initiative  of incentives funded by cap-and-trade that can be leveraged for catchment restoration.  But at present, if anyone is specifically targeting catchment restoration with these grants they must have thunk it up on their own, as there is no existing guidance to do that.  Also, there can be multiple reasons for awarding conservation easements while ranchers are free to continue to make a living, although R2G envisions some potential progressions there.  

But sociocultural issues have arisen in response to the new awareness precipitated by the Auwahi and other hydrologic/ water resource insights on the Hawaiian Islands.  In that context, the following paper perhaps comes off as a defensive response to these new awarenesses, but on the other hand, real, long-standing livelihoods and lifeways are at stake here, so there is good reason for concern on the part of some.

Bremer, L. L., N. Nathan, C. Trauernicht, P. Pascua, N. Krueger, J. Jokiel, J. Barton, and G. C. Daily. 2021. Maintaining the many societal benefits of rangelands: the case of Hawaiʻi. Land 10:764.

In their section on groundwater recharge they restate, with citations, some of the long-standing oversimplifications about evapotranspiration from grasses being less than from forests, as though ET is the only part of the water cycle that matters.   This restates a reductionist paradigm whose history dates back to the early 1930s, exemplified by my Alternate Paradigms page, “Water Yield” vs Baseflow Augmentation.  Specifically, they fail to view Plants in an Ecohydrology Context.  I could say more about that but am keenly aware that I’ve said more than enough for one post already.

And there is one thing I wholeheartedly agree with them on – the need for ongoing stewardship of rangelands – in Hawaiʻi, no doubt, and here in California.  More on that in my next post, upcoming soon.


Initial Citation

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.