Preferential flow and transport in soil: progress and ... - CiteSeerX

European Journal of Soil Science, February 2008, 59, 2–13

doi: 10.1111/j.1365-2389.2007.00991.x

Preferential flow and transport in soil: progress and prognosis B. E. C LOTHIER , S. R. G REEN & M. D EURER Sustainable Land Use Team, The Horticultural and Food Research Institute of New Zealand Ltd, PB 11-030, Palmerston North 4442, New Zealand

Summary Soil is the first filter of the world’s water; its buffering and filtering determine the quality and quantity of our reserves of subterranean and surface water. Preferential flow can either enhance, or curtail, the capacity of the soil to buffer and filter, and it can compromise, or boost, other ecosystem services. We ask ‘when do preferential flow and transport matter?’ We identify 12 of 17 ecosystem services that benefit from preferential flow and three that are affected detrimentally. We estimate by simple arithmetic the value of preferential flow to ecosystem services to be globally some US$304 billion (109) per year. We review the 1989 Monte Verita` meeting on preferential flow processes and summarize the 2006 presentations, some of which are published in this issue of the Journal. New technologies and innovative experiments have increased our understanding of the conditions that initiate and sustain preferential flows. We identify contemporary exigencies, and suggest avenues for their resolution. We are progressing through observation-led discovery. Our prognosis is that new data will enable us to develop better models, and more aptly to parameterize existing models, and thereby predict the impact, benefits and detriments of preferential flow in soil.

Introduction The world’s consumption of water is doubling every 20 years, which is more than twice the rate of our population increase. New sources of water are becoming scarcer, or more expensive to treat and remediate. It is no wonder that water is being increasingly referred to as ‘blue gold’ (Barlow & Clarke, 2002; Vidal, 2002). Soil is the first filter of the world’s water. The soil’s ecosystem services of buffering and filtering are crucial for establishing the quality and quantity of our subterranean and surface water reserves. Land management can affect the ability of the soil to protect receiving waters by altering our soil’s ability to buffer and filter. Some 70% of the world’s freshwater abstracted from such reserves is consumed by agriculture, and intensification of land use is deemed necessary if we are to feed the world’s burgeoning population. We must maintain the value of the natural capital stocks of the world’s soil and water in the face of such intensification. Through unsustainable intensification of land management, we could exceed temporarily the carrying capacity of the earth and so put our natural capital in decline, for as noted by Hawken et al. (1999) ‘. . . the ability to accelerate a car that is low on gasoline does not prove the tank is full’. We need to Correspondence: B. E. Clothier. E-mail: [email protected] Received 11 January 2007; revised version accepted 29 August 2007

2

ensure that the natural capital value of the earth’s skin, our soil, is maintained. International conventions exist for monitoring the environmental performance of countries (see Organisation for Economic Co-operation and Development). Within these there are measures of soil quality that are used to raise awareness of the value of the soil’s natural capital and its ecosystem services, so that the performance of land-management practices can be tracked, and so that policies for sustainable management can be developed. Various frameworks for assessing indicators of quality have been developed. Andrews et al. (2004) considered that for the three land-management goals of productivity, environmental protection and waste recycling, there were six supporting soil functions, or ecosystem services: nutrient cycling, water relations, physical stability and support, filtering and buffering, resistance and resilience, plus diversity and habitat. They used 12 indicators to monitor the integrity of these soil functions, or soil-ecosystem services. Two of their indicators involved preferential flow and transport processes, namely macro-aggregate stability and bulk density. Sparling & Schipper (2002) reported on soil quality at the national scale for New Zealand. Their 13 indicators contained two that would pre-condition the soil to preferential flow and transport processes: bulk density and macroporosity. Their macroporosity # 2007 The Authors Journal compilation # 2007 British Society of Soil Science

Preferential flow and transport in soil 3

measure (Klute, 1986) was chosen because it was considered to be a sensitive measure of soil quality. Thus pedologists consider there to be a direct relationship between the quality of a soil and those soil characteristics that would initiate and sustain preferential flow and transport through it. Conversely, to some soil physicists it almost seems a source of annoyance, and to others a challenge, that flow through soil is not the uniform phenomenon that would enable their theories to predict it easily. The ideal quality traits of a soil, for a soil physicist, we would wager, would be uniformity and isotropism. The preferential flow and transport that are commonplace in soil can either enhance, or curtail, the capacity of the soil to buffer and filter, with attendant consequences for the quality of receiving waters (Jarvis, 2007). Also, preferential flow and transport can compromise, or boost, a host of other ecosystem services such as gas regulation, waste treatment, provision of refugia for flora and fauna, and the damping of responses to environmental fluctuations (Costanza et al., 1997). Preferential flow and transport are the sine qua non of soils, and their study has a long history. In 1898, Colonel Moore of the Royal Engineers lamented that ‘. . . in undrained clay land, cracks one and two inches wide and five feet deep are sometimes met with, with the result that direct passage of sewage and surface water into them has occurred, so that the effluent is not purified as intended. It is thus very unsuitable for irrigation, unless the surface is specially prepared’ (Moore, 1898). More recently, in 1989, a workshop was held on Monte Verita`, Switzerland, to discuss field-scale water and solute flux in soils (Roth et al., 1990). One of the four Think-Tanks held at that meeting was charged with ‘Evaluating the role of preferential flow on solute transport through unsaturated field soils’. Roth et al. (1990) reported the findings of that Think-Tank (#4), and the others. Recently, in 2006, another workshop was convened on Monte Verita`, from which this and the following papers derive. It was wholly dedicated to preferential flow and transport in soil (Roulier & Schulin, 2006). That workshop, and this festschrift, recognize the incisive and innovative research of Hannes Flu¨hler, a leader in the study of preferential flow and transport processes in soil. The leadership of Hannes Flu¨hler is evident in the oft-cited paper on the susceptibility of soils to preferential flow of water by Flury et al. (1994). The breadth and depth of the footprint created by Hannes Flu¨hler’s research is measured in the ISI Web of Knowledge (http://portal.isiknowledge.com/), which records that his 116 papers have been cited more than 1850 times. We begin here with a definition and preview of ‘when does preferential flow and transport matter?’ Next, we make an estimate of the value that preferential flow and transport processes might have in the portfolio of the natural capital of our soils, and their ecosystem goods and services. Subsequently, we review the findings of Think-Tank 4 from the 1989 Monte Verita` meeting, and we recount their two-decades’ old advice for research to

enhance our ability to describe and predict preferential flow and transport in soil. Then we summarize the presentations, some of which are published in this issue. Next, we highlight the progress made by our most recent endeavours in addressing the lacunae that had been identified earlier in 1989. Finally, we identify contemporary exigencies, and in our prognosis we suggest profitable avenues for their resolution.

What is preferential flow and transport? Preferential flow and transport can be usefully defined as encompassing ‘. . . all phenomena where water and solute move along certain pathways, while bypassing a fraction of the porous matrix’ (Hendrickx & Flury, 2001). As noted by Hendrickx & Flury (2001), these phenomena occur at a wide range of scales, both spatially and temporally. Scale refers to a characteristic time, or length, of a process, measurement or model (Blo¨schl & Sivapalan, 1995), and it relates to the lifetime, periodicity or correlation length of the phenomenon or its observation. Preferential flow and transport phenomena can occur spatially at the pore scale of spatial order 103 m, at the core scale (101 m), in pedons (100 m), down hill slopes (103 m), though catchments (104 m), and across large regions of 106 m. Time-wise, these preferential processes can operate during fluid flows at the temporal order of 101 s, during hydrological events 101 hours, throughout seasonal changes 100 year, and across inter-annual variations of 101 years.

When and how is preferential flow and transport generated? Preferential flow and transport are ubiquitous. They matter when certain characteristics prevail at the soil surface, when there are heterogeneities within the soil profile and when flow instabilities occur. They arise through the functioning of plants and their roots, and occur when there are local sources, or sinks, across the landscape. We now briefly review these. An extensive review of the principles and controls on preferential flow and transport in soil has recently been published by Jarvis (2007).

Surface phenomena Many of the manifestations of preferential flow and transport we observe have their genesis at the soil surface. During rain, throughflow, or irrigation, at say rate vo(t), where t is time, whether or not there is a film of surface free-water at pressure head ho 0, is critical in determining how uniformly the infiltrating wetting front will enter the soil. If the soil surface ponds, at say time tp, then for t tp the surface film of free water can move rapidly across the soil surface to find local orifices, cracks or macropores down which to move preferentially (Figure 1). For a uniform rainfall rate that exceeds the hydraulic conductivity of the soil’s matrix very close to

# 2007 The Authors Journal compilation # 2007 British Society of Soil Science, European Journal of Soil Science, 59, 2–13

4 B. E. Clothier et al.

Figure 1 A schematic of the idealised stages of wetting of a macroporous soil that exhibits ephemeral hydrophobicity. Here tp (s) is the time-to-ponding, being the time at which the pressure potential head ho (m) at the surface first reaches zero, and tf (s) is the timeto-philicity when the hydraulic conductivity K(h) of the soil is at non-repellent maximum.

saturation, Ko, Perroux et al. (1981) developed an approximate expression for the time-to-ponding:

tp ¼

S2 ; 2vo ðvo Ko Þ

ð1Þ

where S is the sorptivity of the soil (Philip, 1957). So a key criterion for initiating preferential flow is the time to incipient ponding at the soil’s surface. This expression depends on the local inbound flux of water (vo) in relation to the capillary (S) and near-saturated conductive (Ko) properties of the soil. Clothier & Heiler (1983) revealed how preferential flow matters during sprinkler irrigation. For a given soil, when vo was increased from 4 to 100 mm hour1, so that tp became much less, there was a greater opportunity time, and greater volumes for macropore flow generated at the soil surface. Once surface ponding has occurred, the soil surface might locally accommodate fluxes up to the soil’s fully-saturated conductivity, Ks; otherwise larger-scale run-off will then occur. Increasingly, our observations are revealing that many, if not most, soils exhibit at some time, a degree of water repellency, or hydrophobicity. Dekker et al. (2005) have established that our awareness of hydrophobicity is increasing exponentially, as the cumulative number of publications has increased from around 60 in 1960 to more than 1200 in 2005. Hydrophobicity essentially reduces S to near zero, so that the time-to-ponding is often instantaneous and preferential flows are initiated immediately (Figure 1). After some time, say a time-to-philicity tf, the soil ceases to repel water, and the soil’s properties revert to their hydrophilic state. As suggested in Figure 1, flow and transport might, for t tf, become uniform (Clothier et al., 2000). Thus the length of a rainstorm, or irrigation event, t*, in relation to both tp and tf, which are characteristics determined to some extent by the soil, is critical in establishing thresholds for conditions at the soil’s surface, and controlling the timing and extent of preferential flow and transport (McGrath et al.,

2008). Thus the timing and extent of preferential flow and transport depend on complex interactions between the rainstorm’s properties, vo (t), t*, the soil matrix’s capillary and conductive properties, S, Ko, the soil’s saturated conductivity, Ks, plus the time-to-philicity, tf, the biophysical dependencies of which are largely unknown. Unravelling this complexity is grist for the mill of future research. Controlled experiments of preferential flow have in the past failed to mimic this complexity, such that their results have sometimes been artefacts of unnatural boundary and initial conditions. Thankfully, new field devices (Gee et al., 2003; Van der Velde et al., 2005) and large, undisturbed lysimeters (Durner et al., 2008) are providing us with more realistic assessments of when preferential flow and transport matters in the field. Ground-penetrating radar (GPR) is being used in conjunction with digital elevation models (DEM) to identify preferential funnel-flow pathways in fields, and these are being linked with observed patterns of soil-water content, as well as crop yield (Gish et al., 2005). Arrays of multiplexed TDR (Time Domain Reflectometry) probes, and cheap meteorological stations, are now capable of high-frequency sampling, and they can operate remotely to provide direct measures of the patterns of drainage that reflect preferential flow and transport. These are wirelessly allowing real-time interrogation and providing us with better and keener observations of the hydrological functioning of the surface layers of soil. Figure 2 shows our recent measurements of the detailed pattern of rainfall at a remote pastoral site, along with contemporaneous measurements of the soil’s water content, both from TDR rods inserted vertically down to 30 cm and ones inserted horizontally at 40 cm. Also, at this pasture site we had installed 24 fluxmeters, of a type similar to that of Gee et al. (2003). These fluxmeters, of radius 100 mm, had a core of depth 400 mm of undisturbed soil growing pasture connected to the underlying assembly which recorded, by a tipping spoon, water flow. The reservoir underneath stores the drainage water, so that samples could be



received run-on, for its drainage total of 320 mm was nearly twice rainfall. We have not yet analysed the leachate for solutes. Technology and new instruments are providing us with field-vision of greater acuity about when preferential flow and transport matters, and these data are helping us unravel the complexity of the biophysical interactions that establish and sustain preferential flow and transport processes in soil.

Profile heterogeneities

Figure 2 Measurements of the pattern of rainfall and soil water content recorded at a site growing pasture on a Tokomaru silt loam near Palmerston North, New Zealand. The time domain reflectometer (TDR) rods were either inserted vertically 0–30 cm, or they were inserted horizontally in a face at a depth of 40 cm.

withdrawn for analysis of the flux concentration of solutes. The rims of the cylinders enclosing the undisturbed cores were set flush with the soil surface, so that run-off and run-on processes would not be impeded. During the autumnal period of 30 May–7 June 2006, there was in total some 174 mm of rain. The TDR data in Figure 2 reveal that the soil above 400 mm was, at this stage, still slowly rewetting following the summer drying. Figure 3 shows that despite this, there was substantial drainage at 400 mm, with the frequency distribution of fluxes appearing distinctly lognormal. One fluxmeter obviously

The channel-system concept of Dixon & Peterson (1971) showed how land management, especially that which affects the soil surface, can alter the soil’s macroporous structure and its connectedness down through the soil, and so determine the scale and nature of the preferential flows. The challenge has been to turn these qualitative concepts into quantitative schemes to predict the preferential functioning of the soil’s heterogeneities of layering, which can lead to fluid fingering (Philip, 1975) or funnelling (Kung, 1993), or of aggregates and macropores, which also create and maintain preferential water-flow processes (Van Genuchten & Wierenga, 1976). Heterogeneities and connected pathways also have an impact on gaseous exchange processes, both near the soil surface (Scotter et al., 1967), and deeper down above the phreatic surface of groundwater. Increasing the complexity of mechanistic models to take into account the heterogeneous topology of soil (Gerke & van Genuchten, 1993) has not provided practical means of predicting preferential flows, because of ‘equifinality’ and the difficulty of uniquely specifying the model parameters (Beven, 1993). Better linking of the topological descriptions of macroporous form to preferential flows and transport functioning may well be realised through the application of new mathematical tools and methods such as transfer-function analysis and mixing-tank models, fractal and network analyses, rivulet flows, plus spectral decomposition of flows or inversion using new algorithms (Gwo, 2001). New insights and interpretations are possible.

Instabilities

Figure 3 Histogram of the drainage totals recorded by 24 fluxmeters installed in a Tokomaru silt loam. The drainage was recorded at depth 400 mm, and the data relate to the period 30 May to 7 June 2006, during which some 174 mm of rain fell at the site.

Even in uniform, isotropic media, preferential flow and transport can occur, such as when wetting fronts become unstable. Unlike the surface phenomena discussed above, these instabilities occur under unsaturated conditions. Theory (Raats, 1973), plus observations in Hele–Shaw cells (White et al., 1976), lysimeters (Yao & Hendrickx, 2001), and the field (Glass et al., 1988), have led to a good understanding of the conditions that initiate fingering, and a determination of the conditions that determine the size, spatial separation and persistence of the fingers. New light-transmission technologies have enhanced the utility of Hele–Shaw cell experiments (Roth, 2008), and the application of classical scaling models of capillary, gravity and viscous forces has resulted in new knowledge and understanding about fingering and unstable flows.


6 B. E. Clothier et al.

Plant root functioning Much to the chagrin of some soil physicists, the big movers of water and solute in soil are not the forces of capillarity and gravity acting alone in an inert porous medium. Rather it is the ever-changing distributed pattern of the soil’s pressure head created by plant roots that exerts major control over the hydrology of surface soil (Clothier & Green, 1997). Figures 4 and 5 present the seasonal pattern of soil-water content measured by an array of nine TDR stations in the rootzone around a kiwifruit vine. The three-wire TDR probes had rods of length 150 mm, and at each station probes were inserted to depths of 0–150, 450–600, and 900–1050 mm. These data relate to the summer of 1994, with the DOE (day of experiment) numbering from 1 January 1993. The experimental set-up was similar to that we have described for our 1992 experiment in Green & Clothier (1995). Unlike the automatic and remotely multiplexed TDR data of Figure 2, such wireless technology did not exist in 1994. So the 27 TDR probes of Figures 4 and 5

were ‘manually’ multiplexed, on just a daily basis. Nonetheless these data do disclose insights into how plant roots can establish preferential flow and transport processes. The surface roots of the vine are seen to be functioning hyperactively, and they preferentially cycle the water content of soil rapidly in response to rain, and the four large irrigations. Deeper down, the temporal pattern of water content change is damped and lagged. The soil at this site is a Manawatu silt loam, and near the surface the roots have dried the soil down. Deurer & Bachmann (2007) have shown that drying of the soil by plant roots to small water contents can induce buried pockets of hydrophobicity. They predicted that rewetting of the soil leads to umbrellalike preferential flows around the dry and water-repellent pockets. We did not observe such a phenomenon here, though. In the fine sand horizon, at 900 mm (Figure 4c), the pattern of water content is much lagged, reaching a minimum in early winter. Closer inspection of the 900 mm data reveals two preferential flow phenomena (Figure 5). Three traces are highlighted in the upper graph of Figure 5(a) to reveal preferential wetting,

Figure 4 The depthwise pattern of soil water content measured by TDR probes around a single kiwifruit vine. The three-wire probes were of length 150 mm, and the depths shown are 0–150 mm (a), 450–600 mm (b), and 900– 1050 mm (c). The experiment commenced on 1 January 1993. Four irrigations by minisprinkler are shown in (a), and the pattern of rainfall is included in (b). # 2007 The Authors Journal compilation # 2007 British Society of Soil Science, European Journal of Soil Science, 59, 2–13


Figure 5 Those traces from TDR probes in Figure 4(c) at depth 0.9 m, which exhibit evidence of preferential wetting are highlighted in (a), and those from Figure 4(c) which reveal evidence of a rapid preferential dry-down due to localised surges in root growth are highlighted in (b).

presumably due to macropore flow, or even fingering for there is a fine-over-coarse interface at around 400 mm. The other preferential phenomenon is reflected in the two bold traces of the bottom graph in Figure 5(b). In mid summer, two of the wettest locations at 900 mm suddenly undergo a pattern of rapid drying to become some of the driest spots by autumn. Local surges in root growth are considered responsible for this, as kiwifruit exhibit such ‘explosions’ of local root growth (Hughes et al., 1995). Other species have been found to exhibit high-density root clustering, which would lead to preferential flows and transport in surface soils (Shane & Lambers, 2005). Plant roots through their functioning can determine the pattern of preferential flows in surface, be this through the creation and maintenance of connected macroporous pathways, through their preferred pattern of near-surface hyperactivity, or through their tendency for local and ephemeral cycles of root growth and decay. Useful models have been developed to predict the threedimensional pattern of root functioning (Vrugt et al., 2001), and this will be an area of fruitful future research.

preferential flows through the transport volume of the soil underneath. The canopies of plants can act as reverse umbrellas and locally focus the flux of rainwater towards the perimeters of the interfaces between the trunks and the soil. Right there, the local flux density can well exceed that prevailing above the canopy, vo(t), and furthermore it can often lead to local ponds of free water, Equation (1), because the local flux exceeds the soil’s hydraulic capacity. It is in this near-trunk zone that the plant itself has, via its roots, created a network of macropores, so that the pond of free-water can easily be preferentially captured by the soil, and transported into the root zone of the plant. Thus we can expect local preferential flow to prevail, such that the plant benefits. At the larger scale of plant communities, the intimacy of this connection between water capture and utilization by an individual plant can lead to organized patterns of vegetation (Esteban & Faireń, 2006). Preferential patterns at a very large scale result from local preferential phenomena. Agriculture can create distributed point sources of water and chemicals across landscapes at coarse spatial and temporal scales. In sum, these point sources conspire, through the cumulative effects of the preferential flows, to cause the vexatious problem of non-point source pollution. Such examples of this are the fixed point-source drippers used for irrigation and fertigation of some crops, and the time-varying pattern of urine spots on pastures that preferentially leak nitrogen (Ball et al., 1979). In Figure 6, we present a photograph of a common land-use change that is happening in parts of the world: pastoral land is being converted to vineyards in which timber posts treated with copper, chromium and arsenic (CCA) are used to support vines. At nearly 600 posts per hectare this represents an areally

Point sources Water flows and solute sources are rarely uniformly distributed in space across the soil’s surface, and their heterogeneity can also create preferential flows and transport in surface soil at several scales. Here we discuss some of the point sources commonly found at the surface in the field that will create conditions of

Figure 6 Conversion of pastoral land into a vineyard in Marlborough, New Zealand. The vineyard support posts, installed at a density of 600 ha1, are treated with copper, chromium and arsenic (CCA) to lengthen their effective lifetime. Over their effective lifetime of 20 years, the CCA leaks locally from the posts across an array of point sources several metres apart.


8 B. E. Clothier et al. averaged loading of nearly 18 kg As ha1 in the posts. It does leak out, preferentially at 600 points for every planted hectare (Robinson et al., 2006). The challenge is to understand the preferential flow and transport processes close to the post, and then upscale these to predict the likely impact at the regional scale, both on soil health, and on the quality of the receiving groundwaters underneath.

What are preferential flow and transport processes worth? Traditional economics has viewed capital as simply being cash, investment and monetary instruments. However, sustainable development is now seen to rely on four types of capital: the traditional capital of finance, plus the manufactured capital of infrastructures, and human capital in the form of intelligence, culture and organization, as well as the natural capital of our renewable and non-renewable stocks of natural resources that support life and economic activities (Hawken et al., 1999). Our soil comprises a substantial element of the earth’s natural capital – and it leaks preferentially. The soil’s ecosystem services are the flows of energy and materials from the soil’s natural capital stocks. These are valuable. The Millennium Ecosystem Assessment project (Millennium Ecosystem Assessment, 2005) noted that ecosystem services cover cultural roles, regulating activities, provisioning aspects and supporting functions. The soil’s ecosystem services, such as flows of mass and energy, nonetheless often operate preferentially, and this common aspect of the soil’s functioning might add value to these functions, or degrade them. If so, then what is preferential flow and transport worth? There is a need to develop an integrated and accepted system of valuing or measuring natural capital and ecosystem services, and Fenech et al. (2003) commented that ‘. . . turning the idea of natural capital into a practical means of measuring or modelling both economic and ecological systems requires considerable study and innovation’. Costanza et al. (1997) made a start by estimating the value of the whole world’s ecosystem services and natural capital. They considered 17 ecosystem services across 16 biomes, and concluded that the annual value of the world’s ecosystem services is US$33 trillion (1012). We have used their results to make a crude estimate of the value played by preferential flow and transport across those 17 services (Table 1). We acknowledge that our simple procedure is fraught with difficulties, yet we hope that this opening gambit will stimulate further investigation of how preferential flow can either enhance or diminish the value of the soil’s ecosystem services. Fundamentally, we need to understand better why it is important that we enhance our knowledge of preferential flow and transport, and why this understanding is relevant to the functioning of agricultural, urban and natural ecosystems. Curiosity alone is an insufficient reason to study preferential flow and transport in soil.

Table 1 The ecosystem services selected by Costanza et al. (1997). A tick indicates a positive role for macropores, a cross a negative impact, and a dash implies neutrality Ecosystem service including ecosystem ‘goods’

Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Gas regulation Climate regulation Disturbance regulation Water regulation Water supply Erosion control and sediment retention Soil formation Nutrient cycling Waste treatment Pollination Biological control Refugia Food production Raw materials Genetic resources Recreation Cultural

Preferential flow impact O O O O O O O O O O — O O —

All 17 ecosystem services selected by Costanza et al. (1997) involve soil (Table 1). We deem that only two are not affected by preferential flow processes. Twelve, we consider benefit positively from preferential flow (Table 1). We consider that the service of nutrient cycling benefits from preferential flows, and we suggest, as an example, that when nitrogen mineralizes within the soil’s matrix, it is isolated from by-pass flows that might otherwise preferentially leach it from the root zone. Yet, within the matrix, that mineralized nitrogen is still accessible to plant roots. Conversely, for the service of waste treatment, as Moore (1898) noted, preferential flow can, by far-reaching and rapid transport, carry contaminants to receiving waters. Therefore, for this service, we assign a negative value to preferential flow. Likewise, for the service of water supply, because connected macropores enhance the leakiness of the root zone (Figure 1), preferential flows would also have a negative value. For brevity, we restrict our valuation of preferential flows to simple arithmetic, and hope that this stimulates more detailed calculations in the future. Twelve ecosystem services benefit, we consider, from preferential flow processes, whereas three are affected negatively. Therefore, the net benefit is 9/17 of the total. Let us presume that of the total, 10% of each service derives from the preferential flows and heterogeneous transport processes that are created by the soil’s structural characteristics. From Costanza et al. (1997) we note that of the annual global value of services of US$33 trillion, the terrestrial forests (US$4.7 trillion), grass and rangelands (US$906 billion (109)) and croplands (US$128 billion) provide just some US$5.74 trillion



worth of services. Using our simple multipliers would suggest that preferential flows and transport in these terrestrial ecosystems are globally worth US$304 billion per year. Certainly, the science of preferential flow and transport is an endeavour worth pursuing. Yet we need to understand better the positive benefits and deleterious impacts of preferential flow on the natural capital value of our soil.

The 1989 Monte Verita` workshop The 1989 workshop was the first scientific meeting ever held on Monte Verita`, and the published report of Think-Tank 4 (Roth et al., 1990) showed what were considered then to be the imperatives for better understanding the benefits and impacts of preferential flow. Seventeen participants evaluated ‘the role of preferential flow on solute transport through unsaturated soil’. They identified the five issues listed below. We now summarize their synopses.

Modes of appearance There was general agreement that preferential flow was widespread.

Methods of observation It was lamented that traditional methods of measuring preferential flow are unreliable.

Understanding of mechanisms Whereas it was agreed that our understanding of the physics of preferential flows was good, linking such flows to observable soil features was not. The roles of soil-surface condition and hydrophobicity were highlighted.

require specific experimental or theoretical approaches . . . [and] many of the issues raised were not resolved in discussion’. So how far have we come in the intervening 17 years?

The 2006 Monte Verita` Workshop The published abstracts of this recent workshop (Roulier & Schulin, 2006) reveal the breadth and depth of research conducted in the first 5 years of the 21st century, and we measure these developments against the five issues identified in 1989.

Modes of appearance We have made considerable progress in understanding the modes of appearance of preferential flow. Greater emphasis is now given to the role of hydrophobicity in creating preferential flows, and consideration is now given to the ‘wettable cross section’ as a dynamic property of flow in water-repellent soils (Wessolek et al., 2008). Advances have been made in linking storm and rain-event characteristics to the thresholds that initiate and maintain preferential flow events (McGrath et al., 2008). Connected pathways, linked to soil ecological functioning, are now being used to predict transport distances in structured soils. Unlike 1989, upscaling featured strongly: preferential flows from pore to core, from plots to hillslopes, right through to catchments (Lin & Zhou, 2008). At the larger scale, spectral analysis of hydrological responses has indicated the fractal nature of travel times, and these have been used to infer the catchment-scale preferential flows of reactive solutes. Whereas in 1989 the focus was on the preferential flow of water, now greater emphasis is on how these flows heterogeneously transport a range of substances, including nitrates, organic compounds (Wehrer & Totsche, 2008), metals (Roulier et al., 2008), biota, pesticides (Kahl et al., 2008), pharmaceuticals, and colloids (Burkhardt et al., 2008), as well as transport in the presence of water uptake by plants (Roulier et al., 2008). Real progress has been made.

Experimental approaches It was rued that our understanding was limited by a dearth of measurements, and it was suggested that more effort be put into field observations.

Modelling issues The group agreed that modelling of preferential phenomena was in its infancy, yet there was a reluctance to propose specific direction for quantitative modelling. There was, nonetheless, the issuing of a caution against over-developing model complexity, because of the difficulty of uniquely parameterizing such predictive schemes. This is what Beven (1993) latterly referred to as the conundrum of ‘equifinality’. Think-Tank 4 concluded by noting that the ‘. . . preferential flow is a generic term referring to a host of processes that may

Methods of observation Befitting the 21st century, new technologies have enabled us to make big strides in our ability to observe the many modes of preferential flow and heterogeneous transport at various spatial and temporal scales. We are in an observation-led phase with new discoveries of the causes and consequences of preferential flow. New and innovative technologies from outside soil science are opening new vistas, and providing us with insight into the internal details of macropore flows. New means of interpreting dye data (Bogner et al., 2008; Burkhardt et al., 2008; Duwig et al., 2008), plus light-transmission, magnetic resonance and X-ray imaging through tomographic analyses, are enabling us to see right inside the soil and detect flows and pathways. These images and analyses are whetting the appetites of modellers. But significant challenges lie ahead for describing and


10 B. E. Clothier et al. parameterizing the complexity we are now seeing with greater clarity and no longer through a glass darkly. Not only have new methods of observation been developed at the pore-and-core scale, but also we are obtaining better data in the field though new technologies. New means of interpreting dye-tracer experiments in the field using extreme-value statistics are providing quantitative measures of preferential flow (Bogner et al., 2008), and field tracer-experiments are revealing the impact of land management systems on preferential flow systems. Fluxmeters, buried multi-port sampling lysimeters, and large undisturbed lysimeters (Durner et al., 2008) are currently providing us with direct measures of preferential flows and transport under natural field conditions. As we have shown in Figures 2 and 3, new devices can capture real-time observations wirelessly, and they can operate remotely and unattended for long periods of time. No longer are our experimental observations bedevilled by boundary and initial conditions that do not mimic nature.

Experimental approaches These new methods of observation have stimulated perceptive experimental approaches that have greatly advanced our knowledge and understanding of preferential flow processes over the last two decades. Soil ecological considerations are now being addressed in experimental approaches, and from tomographic data the path lengths observed in the pore network are being translated into effective conductivities. Experimental data are now being used in inverse modelling schemes to back-calculate the soil’s hydraulic properties, even in free form (Durner et al., 2008) to measure the impact of preferential flow paths on the soil’s hydraulic characteristics. From the experimental data that are becoming more available, model-independent classification schemes of macropores are being developed, so that existing modelling schemes can predict macropore flows at non-investigated sites.

Understanding of mechanisms Better knowledge of the modes of appearance of preferential flows, enhanced by new methods of observations and innovative experimental approaches, have combined to result in a huge increase in our understanding of preferential flow mechanisms – across a wide range of spatial and temporal scales (Roth, 2008). The last 20 years have seen a spectacular growth in our scientific understanding of preferential flow and transport processes. Classification of mechanisms and behaviours has enabled us to understand the influences of capillarity, viscosity and gravity at the pore-and-core scale (Roth, 2008). It has been possible, using classical physics, to separate fluid controls from medium characteristics in the initiation of instabilities. Upscaling, connectedness and thresholds have been considered, so that our understanding at the small scale can be applied at the slope

and basin scale. At the catchment scale, there are two main processes by which solutes are transported rapidly to receiving waters – surface runoff and preferential flow. It can be argued that the impacts of these are virtually indistinguishable, and that a lumped system could be considered with a single fast-flow component. More, and better, data would be useful to test this possible simplification.

Modelling In the 1990s, better mechanistic models were developed to incorporate preferential flow processes (Gerke & van Genuchten, 1993). Yet the complexity of such schemes can limit their utility through an inability to parameterize the models uniquely, despite new methods for measuring, inter alia, the mobile fraction of the soil’s water (Clothier et al., 1992), and for obtaining explicit representation of preferential flow paths. Nonetheless, the advent of software codes such as HYDRUS-2D has placed modelling power in the hands of users. These schemes, when used wisely, enable practical predictions to be made of preferential flows and transport (Ga¨rdena¨s et al., 2006). Practical models, such as MACRO (Jarvis, 1994), have also been developed, and their utility is enhanced through the code being freely available, and because their parameterization requirements are not quite as onerous. A current emphasis is on how we apply the models we do have, with the critical issue being model parameterization. Increasingly ‘soft data’ are being used, so that blind predictions at non-investigated sites can be produced. Comprehensive databases, such as UNSODA, can be used to parameterize models, and through the use of pedotransfer functions models can more easily be implemented, albeit with appropriate caveats. At present then, we have useable modelling schemes with which we can, with caution, predict the features and impacts of preferential flow and transport in soils. The uncertainty in such predictions is, as yet, unknown. Non-mechanistic modelling approaches are also being considered, such as the use of the Kitanidis dilution index for predicting solute arrival times and concentrations at depth. At the coarser scale, that of the expanse of the whole country of Canada, useable indicators of preferential flows are being used in risk assessments of water contamination, so that sustainable landmanagement policies can be developed. That is the real challenge: from macropores of 103 m to countries of 106 m, and from events of 101 s to secular trends of 109 s! There are many rungs in the ladder of upscaling, and as yet we have limited modelling tools to assist the ascent from one level to the next. A significant modelling exigency that needs to be addressed is the linking of the soil’s macroporous information and physical understanding of preferential-flow processes to the larger hydrological scales – both temporally and spatially. This linking problem was highlighted by Van der Velde et al. (2006), who worked on the raised coral atoll of Tongatapu. Although they



measured high saturated conductivities in the surface soil of order 105 m s1, and they observed preferential flows there during the tropical downpours that can sometimes exceed 300 mm per event, a transfer-function analysis between the pattern of rainfall and the electrical conductivity of the underlying freshwater lens suggested a time lag of around 4 months. This strongly points to a need for greater understanding of the ‘convergence and divergence’ processes in the vadose zone which links the overlying macroporous soils to the receiving groundwaters underneath. Van der Velde et al. (2006) then temporally extended their hydrological analysis to consider the inter-annual rainfall variability consequent upon the El Ninõ– Southern Oscillation (ENSO). They statistically unravelled the link between the salinity of the freshwater lens under the atoll, and the ENSO index. Non-mechanistically, they made a fairly good prediction of the salinity of the groundwater, some 10 months in advance, simply by using the ENSO index. They could provide, without considering preferential flow mechanisms, a decision-support tool for better managing Tonga’s natural capital, and for ensuring sustainable maintenance of the islands’ ecosystem services which are, in part, supported by preferential flow in the surface soil. The prime intellectual challenge is to model better the links between our observations of macroporous structures and our understanding of preferential flow mechanisms in soil, with our measurements of the hydrological behaviours we see in the quantity and quality of our receiving waters. Where to then with modelling? Brederhoeft (2005) noted that the foundation of modelling is the conceptual model. ‘Surprise’ he defined to be when new data render the prevailing conceptual model invalid. The solution to this problem of ‘surprise’, Brederhoeft (2005) suggested is twofold: to collect as much data as is feasible, and for the model developer to remain open to the possibility of having to change the conceptual model. Sound advice, we reckon.

Prognosis Of all the issues identified on Monte Verita` in 1989, modelling has advanced the least over the intervening years. However, our understanding of the mechanisms of preferential flow that we are gathering from new experimental approaches and better observations of the modes of appearance of preferential flows and transport will, we are sure, lead to better concepts of modelling, and better parameterization of our models. We can begin to assess perceptively the impact of land management on the provision of ecosystem services by macropores and paths of preferential flow. Nonetheless, economic valuation of the impact of preferential flow on ecosystem services remains a challenge, despite our rudimentary attempts here. It is, we consider, a challenge worthy of improved responses, for these should lead to better policies for encouraging sustainable developments based on the preferential attributes of our soils’ natural capital.

The scientific study of preferential flows and transport in soil is in good heart. Hannes Flu¨hler set us on the right path.

References Andrews, S.S., Karlen, D.L. & Cambardella, C.A. 2004. The soil management assessment framework: a quantitative soil quality evaluation method. Soil Science Society of America Journal, 68, 1945–1962. Ball, R., Keeney, D.R., Theobald, P.W. & Nes, P. 1979. Nitrogen balance in urine-affected areas of a New Zealand pasture. Agronomy Journal, 71, 309–314. Barlow, M. & Clarke, T. 2002. Blue Gold: The Fight to Stop the Corporate Theft of the World’s Water. The New Press, New York. Beven, K.J. 1993. Prophecy, reality and uncertainty in distributed hydrological modelling. Advances in Water Resources, 16, 41–51. Blo¨schl, G. & Sivapalan, M. 1995. Scale issues in hydrological modeling: a review. Hydrological Processes, 9, 251–290. Bogner, C., Wolf, B., Schlather, M. & Huwe, B. 2008. Analysing flow patterns from dye tracer experiments in a forest soil using extreme value statistics. European Journal of Soil Science, 59, 103–113. Brederhoeft, J. 2005. The conceptualization model problem – surprise. Hydrogeology Journal, 13, 37–46. Burkhardt, M., Kasteel, R., Vanderborght, J. & Vereecken, H. 2008. Field study on colloid transport using fluorescent microspheres. European Journal of Soil Science, 59, 82–93. Clothier, B.E. & Green, S.R. 1997. Roots: the big movers of water and chemical in soil. Soil Science 162, 534–543. Clothier, B.E. & Heiler, T.D. 1983. Infiltration during sprinkler irrigation: theory and field results. In: Advances in Infiltration (ed. D. C. Slack), pp. 275–284. American Society of Agricultural Engineers, St Joseph, MI. Clothier, B.E., Kirkham, M.B. & McLean, J.E. 1992. In situ measurement of the effective transport volume for solute moving through soil. Soil Science Society of America Journal, 56, 733–736. Clothier, B.E., Vogeler, I. & Magesan, G.N. 2000. The breakdown of water repellency and solute transport through a hydrophobic soil. Journal of Hydrology, 231–232, 255–264. Costanza, R., d’Arge, R., de Groot, R., Farber, S., Grasso, M., Hannon, B. et al. 1997. The value of the world’s ecosystem services and natural capital. Nature, 387, 253–260. Dekker, L.W., Oostinidie, K. & Ritsema, C.J. 2005. Exponential increase of publications related to soil water repellency. Australian Journal of Soil Research, 43, 403–441. Deurer, M. & Bachmann, J. 2007. Modelling water movement in heterogeneous water-repellent soil: 2. A conceptual numerical simulation. Vadose Zone Journal, 6, 446–457. Dixon, R.M. & Peterson, A.E. 1971. Water infiltration control: a channel concept. Soil Science Society of America Journal, 35, 968–973. Durner, W., Jansen, U. & Iden, S. 2008. Effective hydraulic properties of layered soils at the lysimeter scale determined by inverse modelling. European Journal of Soil Science, 59, 114–124. Duwig, C., Delmas, P., Mu¨ller, K., Prado, B., Ren, K., Morin, H. et al. 2008. Quantifying flourescent tracer distribution in allophanic soils to image solute transport. European Journal of Soil Science, 59, 94–102.


12 B. E. Clothier et al. Esteban, J. & Faireń, V. 2006. Self-organised formation of banded vegetation patterns in semi-arid regions: a model. Ecological Complexity, 3, 109–118. Fenech, A., Foster, J., Hamilton, K. & Hansell, R. 2003. Natural capital in ecology and economics: an overview. Environmental Monitoring and Assessment, 86, 3–17. Flury, M., Flu¨hler, H., Jury, W.A. & Leuenberger, J. 1994. Susceptibility of soils to preferential flow of water – a field study. Water Resources Research, 30, 1945–1954. Ga¨rdena¨s, A.I., Sˇimu˚nek, J., Jarvis, N. & van Genuchten, M.T. 2006. Two-dimensional modelling of preferential water flow and pesticide transport from a tile-drained field. Journal of Hydrology, 329, 647–660. Gee, G.W., Zhang, Z.F. & Ward, A.L. 2003. A modified vadose zone fluxmeter with solution collection capability. Vadose Zone Journal, 2, 627–632. Gerke, H.H. & van Genuchten, M.T. 1993. A dual-porosity model for simulating preferential movement of water and solutes in structured porous media. Water Resources Research, 29, 305–319. Gish, T.J., Walthall, C.L., Daughtry, C.S.T. & Kung, K.-J.S. 2005. Using soil moisture and spatial yield patterns to identify subsurface flow pathways. Journal of Environmental Quality, 34, 274–286. Glass, R.J., Steenhuis, T.S. & Parlange, J.-Y. 1988. Wetting front instability as a rapid and far-reaching process in the vadose zone. Journal of Contaminant Hydrology, 3, 207–226. Green, S.R. & Clothier, B.E. 1995. Root water uptake by kiwifruit vines following partial wetting the root zone. Plant and Soil, 173, 317–328. Gwo, J.-P. 2001. In search of preferential flow paths in structured porous media using a simple genetic algorithm. Water Resources Research, 37, 1589–1601. Hawken, P., Lovins, A. & Lovins, L.H. 1999. Natural Capitalism. Back Bay Books, New York. Hendrickx, J.M.H. & Flury, M. 2001. Uniform and preferential flow mechanisms in the vadose zone. In: Conceptual Models of Flow and Transport in the Fractured Vadose Zone, pp. 149–187. National Research Council, National Academy Press, Washington, DC. Hughes, K.A., Gandar, P.W. & de Silva, H.N. 1995. Exploration and exploitation of soil by apple, kiwifruit, peach, Asian pear and grape roots. Plant and Soil, 175, 301–309. Jarvis, N.J. 1994. The MACRO Model – Technical Description and Sample Simulations. Reports and Dissertations No 19, Swedish University of Agricultural Sciences, Department of Soil Sciences, Uppsala. Jarvis, N.J. 2007. A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factors, and consequences for water quality. European Journal of Soil Science, 58, 523–546. Kahl, G., Ingwersen, J., Nutniyom, P., Totrakool, S., Pansobat, K., Thavornyutikarn, P. et al. 2008. Loss of pesticides to a stream in a mountainous area of Northern Thailand: results from a fieldscale experiment. European Journal of Soil Science, 59, 71–81. Klute, A. 1986. Water retention: laboratory methods. In: Methods of Soil Analysis. Part 1 (ed. A. Klute), pp. 635–662. American Society of Agronomy, Madison, WI. Kung, K.-J.S. 1993. Soil processes and chemical transport: laboratory observation of funnel flow mechanism and its influence on solute transport. Journal of Environmental Quality, 22, 91–102.

Lin, H. & Zhou, X. 2008. Subsurface preferential flow as revealed by real-time soil hydrologic monitoring at the Shale Hills catchment. European Journal of Soil Science, 59, 34–49. McGrath, G.S., Hinz, C. & Sivapalan, M. 2008. Modelling the impact of within-storm variability of rainfall on the loading of solutes to preferential flow pathways. European Journal of Soil Science, 59, 24–33. Millennium Ecosystem Assessment 2005. Living Beyond Our Means: Natural Assets and Human Well-Being. A Statement from the Board, p. 28. [WWW document]. URL http://www.maweb.org/proxy/ document.429.aspx. accessed March 2005. Moore, E.C.S. 1898. Sanitary Engineering. B.T. Batsford, London. Organisation for Economic Co-operation and Development. Environmental Country Reviews. OECD, Paris [WWW document]. URL http://www.oecd.org/department/0,2688,en_2649_34307_1_1_1_1_1, 00.html. Perroux, K.M., Smiles, D.E. & White, I. 1981. Water movement in uniform soils during constant-flux infiltration. Soil Science Society of America Journal, 45, 237–240. Philip, J.R. 1957. The theory of infiltration: 4. Sorptivity and algebraic infiltration equations. Soil Science, 84, 257–264. Philip, J.R. 1975. Growth of disturbances in unstable infiltration flows. Soil Science Society America Journal, 39, 1049–1053. Raats, P.A.C. 1973. Unstable wetting fronts in uniform and nonuniform soils. Soil Science Society of America Journal, 37, 681–685. Robinson, B.H., Greven, M.M., Green, S.R., Sivakumaran, S., Davidson, P. & Clothier, B.E. 2006. Leaching of copper, chromium and arsenic from treated timber vineyard posts in Marlborough, New Zealand. Science of the Total Environment, 364, 113–123. Roth, K. 2008. Scaling of water flow through porous media and soils. European Journal of Soil Science, 59, 125–130. Roth, K., Flu¨hler, H., Jury, W.A. & Parker, J.C. 1990. Field-scale Water and Solute Flux in Soils. Birkhaüser Verlag, Basel. Roulier, S. & Schulin, R. (eds) 2006. Preferential Flow and Transport Processes in Soil. Swiss Federal Institute of Technology (ETHZ), Zu¨rich (Abstract). Roulier, S., Robinson, B., Kuster, E. & Schulin, R. 2008. Analysing the preferential transport of lead in a vegetated road-side soil using lysimeter experiments and a dual porosity model. European Journal of Soil Science, 59, 61–70. Scotter, D.R., Thurtell, G.W. & Raats, P.A.C. 1967. Dispersion resulting from sinusoidal gas flow in porous media. Soil Science, 104, 306–308. Shane, M.W. & Lambers, H. 2005. Cluster roots: a curiosity in context. Plant and Soil, 274, 101–125. Sparling, G.P. & Schipper, L.A. 2002. Soil quality at a national scale in New Zealand. Journal of Environmental Quality, 38, 1848– 1857. Van der Velde, M., Green, S.R., Gee, G.W., Vanclooster, M. & Clothier, B.E. 2005. Evaluation of drainage from passive suction and nonsuction flux meters in a volcanic clay soil under tropical conditions. Vadose Zone Journal, 4, 1201–1209. Van der Velde, M., Javaux, M., Vanclooster, M. & Clothier, B.E. 2006. El Ninõ–Southern Oscillation determines the salinity of the freshwater lens under a coral atoll in the Pacific Ocean. Geophysical Research Letters, 33, L21403, doi: 10.1029/2006GLO27748.



Van Genuchten, M.T. & Wierenga, P.J. 1976. Mass transfer studies in sorbing porous media. 1. Analytical solutions. Soil Science Society of America Journal, 40, 473–480. Vidal, J. 2002. Blue gold: Earth’s liquid asset. The Guardian, Thursday 22 August 2002 [WWW document]. URL http://www.guardian. co.uk/worldsummit2002/earth/story/0,,777661,00.html Vrugt, J.A., van Wijk, M.T., Hopmans, J.W. & Sˇimu˚nek, J. 2001. One-, two-, and three-dimensional root water uptake functions for transient modeling. Water Resources Research, 37, 2457–2470. Wehrer, M. & Totsche, K.U. 2008. PAH release from tar-oil contaminated soil material in response to forced environmental gradients:

implications for contaminant transport. European Journal of Soil Science, 59, 50–60. Wessolek, G., Schwa¨rzel, K., Greiffenhagen, A. & Stoffregen, H. 2008. Percolation characteristics of a water-repellent sandy forest soil. European Journal of Soil Science, 59, 14–23. White, I., Colombera, P.M. & Philip, J.R. 1976. Experimental study of wetting front instability induced by a sudden change of pressure gradient. Soil Science Society of America Journal, 40, 824–829. Yao, T. & Hendrickx, J.H.M. 2001. Stability analysis of the unsaturated water flow equation 2. Experimental verification. Water Resources Research, 37, 1875–1881.
