Lest anyone perceive the Rainfall to Groundwater approach to recharge as coming out of “left field”, without precedent, it seems appropriate to share the academic background that initially shaped my own perception/ paradigm.

Landscape architecture (my masters field) is poorly understood by the uninitiated, but FYI it encompasses far more than the garden design that comes to most minds when they hear the term. Without getting into detail, a fair synopsis is that the field encompasses the whole of human relationships with our landscapes – from regional and watershed to site scales and from ancient times through the present.

Among the tools students must acquire for landscape planning/ design is the Rational Formula (or Equation / Method) to predict the changes in anticipated stormwater runoff given changes in land cover, which typically occur through land development.

The Rational Formula is the simplest method to determine peak discharge from drainage basin runoff and has long been used by hydrologists and engineers, particularly for smaller drainages, as well as by landscape architects.

In its simplest form the Rational Formula is: Q=CiA
Where:
Q = Peak discharge, cfs
C = Rational method runoff coefficient
i = Rainfall intensity, inch/hour
A = Drainage area, acres/hectares, etc.

Slight variations on the basic formula have been developed by various agencies. In all cases, the runoff coefficient, “C” represents the ratio of runoff to rainfall. This ratio changes given changes in the land cover, so estimated changes in runoff from land development compare the C of existing land cover with that of the post-development land cover.

Practitioners have determined a range of presumed C values for different land cover types and conditions. The most detailed open access treatment I’ve seen is that in the 2005 Oregon Department of Transportation Hydraulics Manual, Appendix F. Regarding C, that document notes,

. . . It is the most difficult variable to estimate. It represents the interaction of many complex factors, including the storage of water in surface depressions, infiltration, antecedent moisture, ground cover, ground slopes, and soil types. In reality, the coefficient may vary with respect to prior wetting and seasonal conditions. The use of average values has been adopted to simplify the determination of this coefficient. . . .

Most users of this formula refer to a table of average values determined by land cover. Oregon DOT’s table defines three topographical variations per land cover type: flat, rolling and hilly, where

Rolling=ground slope between 2 percent to 10 percent
Hilly=ground slope greater than 10 percent

The Oregon DOT “C” values apply to storms of 10 or fewer years recurrence intervals and their formula includes a coefficient adjustment factor applicable to “less frequent, higher intensity storms”, but that’s more detail than needed for this discussion.

Since C is the ratio of runoff to rainfall, if follows that C for impervious surfaces like pavement and roofs is 0.9, or 90%, for gravel pavement it’s 0.85, or 85%.  C declines for more permeable surfaces and its lowest values correspond to vegetated areas. But even there it varies by general vegetation type. For example:

                                                       Flat              Rolling             Hilly
Woodland & Forests                  0.10              0.15                  0.20
Meadows & Pasture Land         0.25              0.30                  0.35

Note the 15% difference in runoff between those different general vegetation types. Such differences have long been accepted as “givens” among those who work with land planning, despite that they might not understand the various reasons why.

That aspect of my masters training is the reason I figured differences in runoff/ infiltration due to differences in vegetative cover would be obvious to hydrologists. So I expected those working on the Salinas River Project, mid-1990s, would understand my early input to their public planning process when I suggested that watershed restoration might at least help diminish the need for the “plumbing” alternative being advanced.

That was not the first time I was dismissed for advocating watershed awareness, but it was the first time (of several now) that my watershed context input to a public planning process was completely ignored. It actually happened several years before I (consciously) knew I would focus my doctoral work on Watershed Restoration for Baseflow Augmentation (my dissertation topic).

As it turned out, the Salinas River greater watershed/ catchment became the Central Coast “poster child” example of how watershed restoration could support habitat connectivity for the struggling steelhead population on that 100+ mile river, while also supporting human water needs.  A vast portion of that watershed, spanning Monterey and San Luis Obispo Counties, is clothed in nonnative annual grassland, with significant areas of oak woodland understories now dominated by the nonnative annuals.

Salinas River Watershed

Click image to expand

These drylands have not historically been looked to for water resources so their flows have generally not been dammed. But recognize that they comprise the greatest potential source of natural groundwater recharge. Their catchment functions have been unwittingly degraded over centuries of human exploitation of these lands for other economic needs.  Restoration of those functions could help conserve what precipitation does come, routing it immediately to groundwater.  It just might also induce increased precipitation on those lands due to vegetative feedbacks with the regional climate system.

If one simply applies the Rational Formula to the case of the greater Salinas River watershed it should be immediately clear that more runoff would be produced from that “pasture” or rangeland cover type than would be the case from similar areas clothed in native oak woodlands or scrublands.

But, aside from the fact that hydrologists typically apply other formulas to such vast areas, eliminating the “interference” of biology that might mess up their clean, purely physical models, few such professionals have the interdisciplinary awareness to understand that the nonnative annual grasslands represent anthropogenically degraded watersheds/ catchments.

Then imagine applying the Rational Formula to to the Sacramento and San Joaquin River tributary watersheds/ catchments of the Delta and San Francisco Bay estuary complex, visible in the adjacent image.

Click image to expand

[For more on the anthropogenic nature of nonnative annual grasslands, please see California “Grasslands” vs. Altered State(s) and blog post 6. Ball and Chain & Other Links.]

In fact, most hydrological engineers have become so used to treating watersheds/ catchments as “plumbing” problems that they completely miss potential ecohydrological solutions. Most aren’t even aware that the history of scientific investigations supporting biotic influences on catchment functions is as old as the field of hydrology itself.

By not addressing the natural watershed/ catchment context of infiltration and percolation – enhancing/ restoring natural recharge by routing rainfall directly to groundwater, the engineers have missed the most natural way to enhance the cold water baseflows needed by anadromous fish, like steelhead and salmon. For a graphic depiction of baseflow, see the second set of images on Plants in an Ecohydrology Context or the newly refined, updated Rainfall to Groundwater Executive Summary.

The engineering approach to that problem has been to offer cold flows from reservoir bottoms – where feasible – forgetting that Nature had a better “design” for that, before we humans altered natural patterns. There are also the proposed “functional flows”, which may facilitate habitat creation, but will those or the flows from reservoir bottoms provide the timely cold water fish need as does the natural influx of cold water baseflows (from groundwater), late in the rainy season?

If applied hydrologists are aware of recent scientific documentation and discussions of ecohydrological functions appearing in their own journals, they have yet to apply those insights to real world problems, at least in California to date.

The authors of one recent review, “Macropores and water flow in soils revisited” (Beven and Germann 2013), published in Water Resources Research, state in the abstract, with respect to their title subject,

. . . the topic has still not received the attention that its importance deserves, in part because of the ready availability of software packages rooted firmly in the Richards domain, albeit that there is convincing evidence that this may be predicated on the wrong experimental method for natural conditions.

Here Beven and Germann revisit their 1982 piece, “Macropores and water flow in soils”, itself a review in the same journal, that was among the important sources cited in my dissertation and must surely be considered foundational in the field of ecohydrology. They note that the 1982 work,

. . . continues to be cited and is now one of the most frequently referenced papers in hydrology journals (Koutsoyiannis and Kundzewicz, 2007).

However, three decades later,

Well, there has certainly been lots of activity in the field but the dominant concept of soil physics in recent hydrological textbooks remains the Darcy-Richards equation [e.g., Brutsaert, 2005; Shaw et al., 2010]. The dominant concept underlying ‘‘physically based’’ hydrological models remains the Darcy-Richards equation . . .
(Beven and Germann 2013)

Awareness of the problem reaches farther back into the 20th century. Hewlett (1961) observed that the literature on the movement of moisture in the soil all rests on adaptations of Darcy’s law (1856) which was originally conceived to describe flows in saturated media. But until percolating soil water reaches the (saturated) water table, it flows through unsaturated conditions.

Hewlett (1961) devised an experiment “to test the existence of soil moisture gradients in sloping profiles and to point out their importance in small watershed hydrology”. Alas, despite that he conducted his work in the field, his experimental methods obfuscated the influence of soil macropores and “preferential flows” later to be elucidated by many others [Jigour 2008 (2011), Jigour (in preparation)].

While the Richards (1931) equation, incorporating Darcy’s law, was intended to apply to unsaturated or partially saturated conditions, it was based on experimental conditions that do not occur in real-world field situations.. Moreover, solving the Richards equation was speculative, at best, until modern computing power made it more accessible, as Beven and Germann (2013) point out.

Computational model codes applying the Darcy-Richards equation and variations upon it have become generally accessible in recent decades, however,

Arguably, the availability of such tools has diverted attention from more fundamental research on macropore and preferential flow, but there has been a wide range of experimental and modeling studies published on these topics since [Beven and Germann 1982].
(Beven and Germann 2013)

Beven and Germann (2013) go on to review a relatively thorough array of literature published since their 1982 paper, then offer “Appendix A: Preferential Flow as a Viscosity Dominated Stokes Flow and Kinematic Wave”. Interested readers are encouraged to consult their excellent, freely accessible 2013 work.

Just touching on other relatively recent pertinent research applied hydrologists should be aware of, one I’ve mentioned elsewhere on this site that was thankfully published prior to my finalizing my dissertation, is “Woody plant encroachment paradox: rivers rebound as degraded grasslands convert to woodlands” (Wilcox and Huang 2010). The freely accessible abstract offers a great summary.

Other noteworthy recent pertinent research includes: “The effect of trees on preferential flow and soil infiltrability in an agroforestry parkland in semiarid Burkina Faso” (Bargués Tobella and colleagues 2014), “Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics” (Ilstedt and colleagues 2016) and “The importance of tree cover for water resources in semiarid West Africa” (Bargués Tobella 2016), her doctoral thesis, which includes the previous two papers, along with other pieces and discussion.

– Verna Jigour, PhD.

References:

Bargués Tobella, A. 2016. The importance of tree cover for water resources in semiarid West Africa. Doctoral Thesis. Department of Forest Ecology and Management. Swedish University of Agricultural Sciences, Umeå, Sweden. http://urn.kb.se/resolve?urn=urn:nbn:se:slu:epsilon-e-3581 Permanent URL: https://pub.epsilon.slu.se/13553/

Bargués Tobella, A., H. Reese, A. Almaw, J. Bayala, A. Malmer, H. Laudon, and U. Ilstedt. 2014. The effect of trees on preferential flow and soil infiltrability in an agroforestry parkland in semiarid Burkina Faso. Water Resources Research 50:3342-3354. https://doi.org/10.1002/2013WR015197 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013WR015197

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. https://doi.org/10.1002/wrcr.20156 https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/wrcr.20156

Bruijnzeel, L. A. 2004. Hydrological functions of tropical forests: not seeing the soil for the trees? Agriculture, Ecosystems and Environment 104:185–228. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.543.1801&rep=rep1&type=pdf

Darcy, H. (1856), Les fontaines publiques de la ville de Dijon, Dalmont, Paris.

Hewlett, J. D. 1961. Soil moisture as a source of base flow from steep mountain watersheds. Station Paper 132,. USDA Forest Service, Southeastern Forest Experiment Station, Asheville, NC. http://cwt33.ecology.uga.edu/publications/863.pdf

Ilstedt, U., A. Bargués Tobella, H. R. Bazié, J. Bayala, E. Verbeeten, G. Nyberg, J. Sanou, L. Benegas, D. Murdiyarso, H. Laudon, D. Sheil, and A. Malmer. 2016. Intermediate tree cover can maximize groundwater recharge in the seasonally dry tropics. Scientific Reports 6:21930. https://doi.org/10.1038/srep21930

Jigour, V. M. 2008 (2011). Watershed restoration for baseflow augmentation. Dissertation. Interdisciplinary Studies: Arts & Sciences: Conservation Ecology. Union Institute & University.

Jigour, V. M. In preparation. Rainfall to Groundwater: History of the Science.

Richards, L .A. 1931. Capillary conduction of liquids through porous mediums, Physics 1, 318–333 https://doi.org/10.1063/1.1745010

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