“Water Yield” vs Baseflow Augmentation
The U.S. Bureau of Forestry and the first forests it was charged with protecting within the Department of Interior in 1901 had only arisen after decades of political wrangling catalyzed by George Perkins Marsh’s Man and Nature (Marsh 1864). That work, while not scientific in terms of quantitative measurements, was based on direct observation of degraded lands and desertified watersheds documented as fertile and mesic during Biblical times. A more recent synopsis is available online in W. C. Lowdermilk’s [1942 (1948)] Conquest of the land through seven thousand years.
From at least the time of the transfer of forest lands and establishment of the Forest Service within the Department of Agriculture in 1905, some had questioned the scientific veracity of the forest conservation movement, despite that its strongest proponents were men of a variety of scientific disciplines (Hays 1959 (1975)), congealing around a holistic, ecological understanding of the interrelationships between water resources and land degradation — what John Harte coins as a Darwinian perspective (Harte 2002). [The Harte 2002 piece once was, but alas is no longer freely available online.]
Not surprisingly perhaps, the strongest objectors to forest management for water conservation were of what Harte (2002) describes as the Newtonian worldview. And far from simply scientific debate, the story is fraught with political strategy, intrigue and personality conflicts. It must be acknowledged that, in order to justify public financing of scientific investigation, politics must be a huge player. But interagency political power plays are also clear in the history.
The orientation of the U.S. Army Corps of Engineers was adamantly linear and surficial, concerned solely with enhancing navigability of streams and not seeing any connection between headwaters and downstream conditions that engineering couldn’t remedy. The Weather Bureau had its obvious, rather narrow at the time, province of data collection. The USGS had a more expansive, yet predominantly physical science orientation, with some notable exceptions. Standing alongside Gifford Pinchot, first chief of the Forest Service, in advocating forest protection were irrigation agencies of the (dryer) western states.
Western irrigators pioneered in the theory that watershed vegetation directly affected their water supply. Forests, they argued, absorbed rainfall, retarded stream run-off, and increased the level of ground water; forests retarded snow melting in the early months of the year, reduced spring floods, and saved water for summer use when supplies ran low; forests retarded soil erosion and silting in irrigation ditches and reservoirs. Private power and water supply corporations, as well as municipal water departments, joined with irrigators in presenting these arguments. They opposed commercial use of the watersheds; they fought to prevent lumbering in the forests and grazing on the mountain ranges. They centered their fire especially on sheep, which cropped vegetation close to the ground and, they argued, vastly accelerated erosion.
[Hays 1959 (1975) p 23 see original for three footnoted sources]
Wagon Wheel Gap Experiment
Scientific research was part of the Forest Service mission and they commenced their task in 1909 with a paired catchment study on two adjacent drainage basins, each approximately 200 acres and draining to the Rio Grande in southern Colorado, near Wagon Wheel Gap (Bates and Henry 1928). The catchments bore the same geology and were physically very similar, with the exception of shape and elevation ranges—one being more elongate and extending to a higher elevation.
Their elevations are between 9,000 and 11,000 feet, whereas the areas in Colorado producing living streams extend mainly from 8,000 to the highest peaks, some of which are 14,000 feet in altitude. These watersheds should be average or only slightly below in water-yielding capacity. (Bates and Henry 1928)
The results of the second phase of the experiment were published in the Colorado Monthly Weather Review (Bates and Henry 1928) and reviewed by then-former Forest Service Chief of Research Raphael Zon in the Journal of Forestry (Zon 1928), who provides some details missing from the original, along with his commentary.
One striking thing about the description of site selection, which, according to Schiff (1962) was done by Forester Carlos G. Bates, is the reference to living streams. Doubtless tied to the country’s political perspective invested in the Army Corps’ efforts toward navigable streams, this emphasis on surface waters is a factor that biases the experiment from the start, though the foresters did not realize it, even when the results were published (e.g., Zon 1928). Again, this issue of surface waters relates to the elevation—at lower elevations the drainages presumably become intermittent as far as surface features go, but that happens in semiarid regions because of increasing depths of alluvium buildup with decreasing elevation. The stream is still there, but just subsurface. (See U.S. Inception of Groundwater Study, below)
Following an eight-year calibration period, vegetation on one catchment was cut and measurements of stream flow made for seven years thereafter. The scrubby aspen resprouted and were on their way to reforestation within that time, among the numerous factors that cloud this experiment. To encapsulate, the results showed some, though limited relative slowing of floodwaters by the untreated vegetation and small but meaningful increases in even summer flows from the cut-over drainage. Various problems were acknowledged at the time, but the experiment was the first, and thus the best of its kind in the country at the time and it did yield some provocative results.
U.S. Inception of Groundwater Study
Hired by the USGS in 1907, Oscar Edward Meinzer was named acting chief of the Division of Groundwater in 1912, becoming permanent chief the following year. When Meinzer joined USGS, the division consisted of himself and three others, partly due to attrition of older geologists, including his predecessor as chief, W. C. Mendenhall (Maxey 1979). By his retirement in 1946 the division had grown to more than 80 staff (Fryar 2007). Meinzer is credited with recognizing: 1.) based on his field experience, the need for resource evaluation studies throughout the country, 2.) the need to organize disparate earlier studies and methodologies and to go forward on that basis, 3.) groundwater hydrology as multidisciplinary, 4.) the need for a systematic approach to problem solving using multidisciplinary teams, and 5.) the need to promptly provide the public with accurate information (Maxey 1979).
Meinzer’s own technical publications numbered 110 (Fryar 2007 citing Deming 2002). Probably best-known was Geological Survey Water-Supply Paper 489, “The occurrence of ground water in the United States; with a discussion of principles” (Meinzer 1923a), for which Meinzer was awarded his Ph.D. by the University of Chicago in 1922 (Fryar 2007). Also published that year, the Water-Supply Paper 494, “Outline of groundwater hydrology, with definitions” (Meinzer 1923b) offers a concise overview of the subject, but is more companion than summary of the larger work. These two works “literally became textbooks that were used internationally by nearly all workers in the field. They constituted a ‘state of the art’ review of the groundwater field and gave clean and concise definitions, many of which persist to the present (Maxey 1979). “Plants as indicators of ground water” (Meinzer 1927b) is itself an indicator of Meinzer’s early ecohydrological insights, though it does seem obvious that anyone interested in groundwater would want to understand such relationships. “Large springs in the United States” (Meinzer 1927a) bears mention partly for the striking subject it covers, namely springs flowing at rates primarily greater than 100 cubic feet per second (cfs). Meinzer puts that rate in perspective, using the apparent original term,“second-feet”.
… in many parts of the United States a spring that discharges 1 secondfoot— that is, 1 cubic foot a second, or 48 gallons a minute, would be regarded as a remarkable spring. Such a spring would fill about a dozen barrels in a minute. In the entire country, however, there are thousands of springs that yield 1 second-foot or more, and hundreds that yield 10 second-feet or more. Moreover, according to the incomplete data summarized in the following pages, there are 65 springs in the United States that have an average yield of 100 second-feet or more, and several springs or groups of springs that have an average yield of 500 second-feet or more.
A second-foot of water is equal to about 646,000 gallons a day.
Meinzer’s 1923 water-supply papers laid out the fundamentals, with the larger work including such major chapters as “kinds of rocks and their water-bearing properties” and “structure of rocks and its influence on ground water”. Contemporary hydrogeologists presumably have their own favored sources for such information and, as Fryar (2007) points out, Meinzer’s studies predated understanding of plate tectonics, yet this work remains an informative reference for interdisciplinary learners, including hydrologists seeking holistic understanding of the systems they work with.
Aftermath of Wagon Wheel Gap Experiment
Subsequent to publication of the Wagon Wheel Gap results, in 1934 a spirited “discussion” of those results was published in Transactions, American Society of Civil Engineers, leading to the conclusion by primarily eastern U.S. engineers that removal of forests would result in increased “yields” of water for human uses, despite the published objections, with documentation of erosion and sedimentation problems, by western U.S. engineers with knowledge of forests and similar studies in California.
Click images to enlarge
Hydrology as a New U.S. Discipline
There was no scientific organization of hydrologists in the United States until 1930 when a Hydrology Section was organized within the American Geophysical Union (Meinzer 1942).
In 1940 it had 741 members, and including that year it [had] published in the Annual Transactions of the American Geophysical Union about 600 technical papers and reports, covering about 3,000 pages of printed matter.
Impressive as that growth may be, it serves to illustrate that there simply were not a lot of U.S. hydrologists around during the 1930s and 40s and all of them were learning as they went. Some of these professionals were federal foresters studying hydrology on experimental catchments whose establishment had begun in the late 1920s and early 1930s. It was a nascent discipline without well established principles, which, to a certain extent, remains evident today through ongoing controversies within the discipline (Jigour in press).
1937 Symposium on Watershed Management
The late Luna Leopold (1970) documented, after the fact, the emergence of watershed science directed at water yield at the 1937 symposium on watershed management. Prior to that time, he tells us,
The early use of experimental watersheds was nearly without exception directed at evaluating the effect of forest and grazing practices on sediment production and floods.
Presenters at that 1937 symposium included “many of the men prominent in the field” (Leopold 1970) and their papers were published in the Journal of Forestry. According to Leopold, a standout among them was Robert E. Horton who presented his new concepts for stabilizing stream flow by increasing infiltration (Horton 1937). Leopold pointed out that Horton was a “private engineer”, though he had been a civil engineer during his early career, including “becoming New York District Engineer of the U.S. Geological Survey in 1900” (Paynter Undated).
Horton’s paper begins by summarizing the three approaches to stabilizing stream flow:
One school, comprised mainly of engineers, has held that stabilization can best be accomplished by surface storage reservoirs. … The second school … comprises mainly foresters and so-called conservation enthusiasts [whose] credo … has been that forests are natural stream flow regulators.
Horton summarizes the benefit-cost issues regarding reservoirs but, not surprisingly for his time, does not mention the ecological costs. He traces the conservation orientation to the influence of President Theodore Roosevelt.
Groups supporting this [second school] claim have been active ever since his time, and the idea of the beneficial effects of forests as stream regulators has become so well established that it is given as an asserted fact in many high school textbooks on science.
Horton elaborates on the error of theory crediting leaf litter as the source of stream stabilization on forested watersheds, stating that: 1.) litter is so coarse-textured as to have little capillary power to hold water, 2.) once saturated it contributes little or no effect thereafter, and
3.) leaf litter, particularly from broad-leaved trees, is likely to become packed and wet, particularly when it has lain under a cover of snow, and thereafter may greatly obstruct the entrance of water through infiltration to the soil.
Horton may have been referring to Zon or W. C. Lowdermilk, who also spoke at that symposium (Leopold 1970), but if he was, he hadn’t been doing his homework, as especially Zon [1927 (1912)], along with Lowdermilk (1930), offer hard scientific evidence for the contributions of leaf litter. Or Horton’s comments may have been directed at the work of visionary forest proto-ecohydrologist Charles R. Hursh, whose “Litter keeps forest soil productive” (Hursh 1928) had been published in Southern Lumberman, and with whose published insights on catchment hydrology Horton’s writings seem in greatest direct conflict.
Again, more on this apparent rivalry is covered in the forthcoming book (Jigour in press), considered in the context of the evolution of hydrology. While Horton may have been/ seemed a luminary to hydrologists, he was weak in interdisciplinary perception. However, he apparently exuded a charismatic sense of self-confidence that impressed fellow hydrologists. Some might call it chutzpah, given his carelessness about backing up with evidence some of his statements of so-called “fact”, though he did offer data on other subjects.
… even after allowing for water which runs down the trunks and so reaches the ground, tree crowns intercept an average of 15 to 25 percent of the rainfall; the percentage is close to 100 percent in light showers but is relatively small in heavy rains. This water is directly evaporated and does not reach the ground. It has virtually the same effect as the reduction of rainfall [in] the region by an amount equal to the loss by interception. While there is also some interception by growing grass, crops or other vegetation aside from trees, it is also true that any beneficial effect of forests in stabilizing stream flow is accompanied by a serious loss or reduction of total run-off. (Horton 1937)
So darn those pesky trees, anyway! But please note that even precipitation subject to interception/ evaporation is not lost. It simply returns to the greater hydrologic cycle, with corollary benefits for the region it rose from.
Horton (1937) notes the complexity posed for comparative purposes by variations in vegetation cover and thus effective transpiration rates. Citing advances in hydrological sciences, Horton attributes the actual stream-regulating effects of forests to: 1.) their greater infiltration rates relative to adjacent similar cultivated land, “since the soil surface in the forest is undisturbed by cultivation, root, earthworm, insect and other perforations are much more permanent than on cultivated land” — perhaps, he proposed, more important in allowing air to escape, prerequisite for the filling of pore space with water, than for the entry of water itself; 2.) the greater permanence of surface irregularities on forest soils, which are lost on cultivated soils, except immediately after cultivation when infiltration rates can be very high;
3.) trees and their branches break the force of raindrops and prevent the packing of the soil surface, with consequent reduction of its infiltration capacity as compared with a cultivated soil directly exposed; 4.) the presence of leaf litter has a similar effect and at the same time reduces the velocity of overland flow, prevents soil erosion and in-washing of fine soil particles to the interstices of the soil and so maintains a higher infiltration capacity; (Horton 1937)
5.) while there are variations, evaporative losses from forest floor are typically less than from open areas; 6.) snow cover tends to remain longer and the depth of frost penetration is less in forests than open areas, allowing a longer melting, and hence infiltration period; and 7.) the hydrological differences between forested and non-forested lands are so numerous and complex as to evade generalizations about their effects on stream stability (Horton 1937).
Horton clearly got some things right, others not so much. As for that “third school” of thought, Horton attributes its rise to advances in scientific hydrology, though at that point in history we must question what “advances” he might have been referring to. He, rather haughtily IMHO, claimed,
There have always been scientifically minded men who have believed that the moot questions of forests and stream regulation could not safely be subjects of broad generalizations until the phenomena of the transformation of rainfall into run-off were better understood. Briefly the tenet of this school is that something may be accomplished in stabilizing stream flow if some practical means can be found to increase the infiltration of the soil. (Horton 1937)
1. Increase infiltration capacity of the soil. “There is no practicable way of permanently increasing the minimum infiltration capacity unless the soil texture or structure can be changed.” (Horton 1937);
2. Increase depression storage [One example is the small dikes created around cultivated plants to detain water, allowing it more time for infiltration (examples include terracing) – an ancient approach referred to as “runoff farming” and emulated with successful micro-catchments in Israel’s Negev Desert and elsewhere];
3. Decrease the rate of overland flow; and
4. Grow grass and close-growing grain crops (Horton 1937)
With respect to method 1., since soil texture – the percentages of particles classified by size as clay, silt and sand – cannot be readily changed unless the soil is completely removed and replaced, that leaves soil structure as the feature amenable to manipulation. Horton missed that point in this paper and later in his career. Note also that soil texture in a given soil horizon does change over time via ecological, as well as physical processes (Graham and Wood 1991). But soil texture is otherwise primarily a physical property. Soil structure, on the other hand, is absolutely influenced by soil bio/ ecology and thus is the property we can influence by restoring woody and perennial plant species, and their soil ecosystems, to lands whose watershed/ catchment functions have been degraded through anthropogenic land cover disturbance.
Although Horton had outlined this array of potential directions for future investigation, the fourth of his suggestions became the dominant approach over succeeding decades — manifested in efforts to reduce evapotranspiration by limiting vegetation height and volume, typically by outright removal of woody vegetation. Naturally, the U.S. Forest Service led the efforts. But University of California got into the action soon enough.
University of California Research on Water Yield & Paradigm Shift
Since blog post 6, Ball and Chain & Other Links, cites and includes quotations from What We Know About Brushland Management in California (Miller, editor 1949), along with documentation of other California efforts to remove oaks and other woody vegetation, some complementary excerpts are fitting here. F. J. Veihmeyer, for whom Veihmeyer Hall at UCD is named, offered, “Runoff, erosion and soil moisture from vegetated and burned plots in typical brush areas of California” (Veihmeyer 1949).
. . . Burning is the cheapest and probably will be the method most commonly used to get rid of the brush. Burning has been vigorously opposed by many because it is believed that soils subjected to burning will become less retentive of water; that the infiltration capacity of the soil will be decreased seriously; that water will not be conserved; and that excessive runoff and erosion will result. The preservation of vegetation on watersheds has been advocated so strongly and propagandized so widely, the public has been led to believe that its removal will result in disaster. An exhaustive review, however, of the material published up to 1935 on this subject by a member of the staff of the United States Geological Survey led to the conclusion, “there is insufficient evidence upon which to base a conclusion as to the influence of vegetation upon streamflow”.
The moisture properties of soils, such as the amount of water that can be stored in them and that which can be taken from them by plants, may be considered to be soil-moisture constants. The addition of organic matter in amounts greatly exceeding that likely to occur under natural conditions or even in agricultural practice will not materially affect the amount of water that can be stored in soil.
Any surface treatment of brush-covered areas would not be expected to affect the water storage capacity of the soil provided the infiltration of water into the sol is not altered.”
While Veihmeyer doesn’t cite that USGS staffer, Verna has a pretty good idea who he was referring to and is omitting his name entirely from this discussion, as well, because that chap seems to have had a combative bias against “forest influences” and getting into all that would take up too much space here. If intrigued, consult Jigour (in press).
As for Veihmeyer’s statement about the “moisture properties of soil” as “constants” – pure conjecture, based on the idea of soils as solely physical phenomena, devoid of bio/ ecological influences. It’s kind of amazing how many such statements of “fact” got by unsupported by any data – solely assumptions – during the 20th century. Same applies to Veihmeyer’s last sentence quoted above. He may not have expected changes in soil water storage capacity with brush removal, but Verna would, and that partly depends on over what time period?
The problem with most such experiments is that final conclusions were generally drawn over observation periods of less than a decade. Alas, the same is generally true for much quantitative research that must be fit into the relatively brief time frames of individual students’ graduate school programs. In the case of the California programs summarized in blog post 6, Ball and Chain & Other Links, once the oak trees and other woody plants were killed, it would take a while for the old root channels they had engendered, along with their soil ecosystems to degrade. Generally the researchers were far away and onto other projects by that time.
Interestingly, Lewis was the lead researcher reporting 32 years later on results of a 17-year study that showed no significant difference in water yield after removing oaks in another Sierra Nevada study area. (Lewis et al. 2000). What a difference a few decades makes. But Verna gives Lewis credit for apparently keeping his mind open and/or shifting his previous paradigm.
I’d incorrectly assumed the same David Lewis was part of the team reporting in California Agriculture, “Research connects soil hydrology and stream water chemistry in California oak woodlands” (O’Geen and colleagues 2010). (So clearing that up here as an error on my part.) That report is freely available online so interested readers are encouraged to check it out. But a few pertinent excerpts are appropriate here:
. . . Physical (bulk density, water infiltration rates), chemical (nutrient enrichment, pH, cation exchange capacity) and biological (microbial biomass, soil respiration) properties are enhanced in soils under oak canopies (Dahlgren et al. 1997). In contrast, soils forming under annual grasses in the absence of oak canopy are less fertile, have higher bulk density and are more susceptible to surface-water runoff. . . .
. . . Under oak trees, A horizons are thicker (4 versus 2 inches), with higher organic matter and lower bulk density compared to those in open grasslands (Dahlgren et al. 1997). . . .
More than two-thirds of California’s drinking-water supply passes through or is stored in oak woodlands. . . .
In some instances, the variability of soil properties at SFREC and in the Sierra Foothill region exceeds our ability to document them at scales relevant to rangeland management. . . .
Currently, California’s soil surveys in rangelands are not mapped at scales fine enough to portray these soil features across landforms and their connectivity to streams. It is important that stakeholders communicate this concern to the U.S. National Cooperative Soil Survey to focus soil survey updates in these important landscapes. This level of detail is warranted to better understand patterns in oak regeneration, perennial grass restoration, forage productivity, rainfall-to-runoff characteristics and water-quality dynamics.
(O’Geen and colleagues 2010)
As for those reported increases in “water yield”, in virtually all cases, these were annual figures. Even in California, the “water yield” paradigm had become so congealed, no one questioned whether the increased water yield was coming at a time of year it might actually be captured and used for irrigation or other purposes. Obviously, when there’s a lot of rainfall the need for irrigation is greatly reduced, so just who would those increases in water yield benefit??? Who wants more water if it all hits the floodplain at once? No one was asking such questions.
The Problem with “Water Yield” – Annual Totals
As noted in the Rainfall to Groundwater Executive Summary section, “ ‘Water Yield’ Misconceptions”, and Ball and Chain & Other Links, the “water yield” experiments expanded from the initial forests to California oak woodlands and chaparral/ scrublands, along with southwestern U.S. piñon/ juniper and mesquite rangelands, reaping watershed/ catchment destruction in their paths.
One might think the combination of two publications, Hydrologic effects of changing forest landscape (National Academy of Sciences 2008) and “Woody plant encroachment paradox: rivers rebound as degraded grasslands convert to woodlands” (Wilcox and Huang 2010), would have sufficed to put the death nell in this kind of research, yet in 2015, Verna was aghast to see at least one scientific journal article about “water yield” from removing piñon and juniper from southwestern U.S. rangelands. And the term keeps popping up with respect to removing trees such as conifers from historic meadows, etc. It is a long-entrenched paradigm that seems to be dying a very slow death. So please check out these quotations and pass them on:
The potential for increasing water yields from forest management is low, which reflects that increases are less likely in seasons when water demand is high and increases tend to be much smaller in drier years.
(National Academy of Sciences 2008)
Woody Plant “Encroachment” Costing Water?
As for the long-held notion of woody plant “encroachment” onto rangelands and its uptake of water needed by humans – a notion that began in the U.S. southwest and spread to California, despite that our California “grasslands” were mostly not grasslands to begin with [see California “Grasslands” vs. Altered State(s)], check out this abstract:
The related phenomena of degradation and woody plant encroachment have transformed huge tracts of rangelands. Woody encroachment is assumed to reduce groundwater recharge and streamflow. We analyzed the long‐term (85 years) trends of four major river basins in the Edwards Plateau region of Texas. This region, in which springs are abundant because of the karst geology, has undergone degradation and woody encroachment. We found that, contrary to widespread perceptions, streamflows have not been declining. The contribution of baseflow has doubled—even though woody cover has expanded and rainfall amounts have remained constant. We attribute this increase in springflow to a landscape recovery that has taken place concurrently with woody expansion—a recovery brought about by lower grazing pressure. Our results indicate that for drylands where the geology supports springs, it is degradation and not woody encroachment that leads to regional‐scale declines in groundwater recharge and streamflows.
(Wilcox and Huang 2010, abstract)
Baseflow Augmentation & Enhancing Natural Recharge as Superior Paradigm
For California’s Mediterranean-type climate and doubtless other regions where water during the dry season is an economic issue, a much more useful framework than “water yield” is baseflow augmentation.
As noted in the Rainfall to Groundwater Glossary,
Baseflow: “the flow of perennial streams . . . , consisting of interflow and groundwater flow intercepted by the stream” (Ponce 1989b); “the fraction of streamflow that originates in ground water” (Ponce 2007). Baseflows are the low flows that sustain aquatic life like steelhead and salmon through the summer months in California and other regions with dry summer seasons. See What’s in it for steelhead & salmon? and What does Rainfall to Groundwater offer for vernal pools?
The basic idea behind Rainfall to Groundwater is to enhance natural recharge of groundwater by restoring the infiltration and percolation functions of anthropogenically degraded watersheds/ catchments in order to restore natural detention functions. [See also Retention vs Detention Storage.] By routing more precipitation into the ground right where it falls – Rainfall to Groundwater – baseflows will be naturally augmented.
Not only will this improve baseflow conditions for anadromous fish, other aquatic species and other groundwater dependent ecosystems, it will enhance the groundwater recharge needed by many California Groundwater Sustainability Agencies to bring their groundwater basins into compliance with California’s Sustainable Groundwater Management Act (SGMA) – all without the need for expensive new engineered structures.
Watershed/ catchment restoration will doubtless be less expensive to implement than engineered structures, but as these proposed natural systems mature they will become essentially self-sustaining, which cannot be said for the engineered structures, that require ongoing human maintenance – great for jobs, if that’s the objective, but hard on taxpayers/ ratepayers.
As noted elsewhere on this site, including blog post 5 DWR, Great Job on WAFR! Now Add R2G, engineered recharge structures require continuous maintenance to remove the buildup of fine sediments that clogs percolation functions.
In contrast, natural systems do that essentially free of charge – ecohydrological services. Restored/ naturalized floodplains flush such fine sediments into discreet pockets where they become substrate for riparian vegetation. Restored watersheds/ catchments can become largely self-sustaining, although some natural resource and adaptive management will be necessary. All far less costly to implement and maintain than the widely proposed engineered responses to SGMA, beginning with the requisite environmental review.
Can we please shift the prevailing paradigm from outdated notions of “water yield” and woody plant “encroachment” to baseflow augmentation/ natural recharge???
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Ponce, V. M. 1989b. Engineering Hydrology: Principles and Practices. Prentice Hall. Englewood Cliffs, New Jersey. Now, the Second Edition (2014) is online: https://ponce.sdsu.edu/enghydro/
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