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Hydropedology as a powerful tool for environmental policy research
Geoderma 131 (2006) 275 – 286 www.elsevier.com/locate/geoderma
Hydropedology as a powerful tool for environmental policy research Johan Bouma Environmental Sciences Group, Wageningen University and Research Center, The Netherlands Available online 10 May 2005
Abstract Rather than produce clear-cut answers to well-defined problems, research on future environmental policy issues requires a different approach whereby researchers are partners in joint learning processes among stakeholders, policy makers, NGOs (Non-Governmental Organisations) and industry. This relates to the strategic bup-to-globalQ as well as to the operational bdown-to-local Q level. Researchers can play a key role in facilitating the learning process by contributing knowledge and by helping formulate options for action which will imply definition of tradeoffs between contrasting demands and interests. This holds for scientists in general but also for pedologists and hydrologists when dealing with land use policy issues which always include environmental aspects. By combining hydrological and pedological expertise in hydropedology, better contributions can be made to this process than by operating separately as in the past. The considerable empirical knowledge of the pedologist on soil and landscapes can be sharpened by process knowledge of the hydrologist who, in turn, gains by obtaining more realistic model representations. Examples are provided discussing measurement of (i) Bypass flow; (ii) K-sat; (iii) water table levels; (iv) water accessibility; and (v) hydrophobicity. Furthermore, pedotransfer functions and realistic landscape models are discussed which express pedological expertise. The image of a throbbing landscape with characteristic water fluxes in space and time, affected and to be affected by human management, is seen as vital and highly visible input into interdisciplinary discussions in research teams dealing with land use. Suggestions for future action in hydropedology include the need for more field work, exploring the characteristic effects of soil management on given types of soils and, in general, a more pro-active research approach. D 2005 Elsevier B.V. All rights reserved. Keywords: Pedology; Soil physics; Hydrology; Land use planning; Landscape processes; Soil morphology
1. Introduction The general policy focus on sustainable development, as agreed upon by many countries in Johannes-
E-mail address: [email protected]. 0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2005.03.009
burg in 2002, requires an integrated research approach considering economic, social and environmental issues (bProfit, People, PlanetQ). This also applies to prominent land-related elements of sustainable development such as climate change, biodiversity loss, water scarcity and land degradation. All these issues are quite complex, they are international in character,
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their effect on daily live may be rather invisible in the short term but potentially quite disruptive in a longer time frame and they are all associated with much uncertainty. Science worldwide is struggling to come to grips with these problems. Researchers, including soil scientists and hydrologists, are faced with a new research paradigm because there are no single magic solutions to such problems but, rather, action programs have to be negotiated with all stakeholders involved to come to solutions that always represent a compromise between conflicting demands and interests and this is a cumbersome procedure. For example, Sonneveld and Bouma (2003) reviewed the turbulent negotiations in The Netherlands following the implementation of manure regulations imposed by the European Union. Even though this problem was a lot simpler than the ones associated with climate change, it turned out to be quite complicated to develop solutions acceptable to all. Imposing policy measures top–down without interaction with the stakeholders involved has never been successful, and this has become increasingly clear in recent times. Now, rather, policy development involving stakeholders (which has often been ignored in the past) is becoming an important part of the implementation process of environmental regulations (e.g., Bouma, 2001a,b; Sonneveld and Bouma, 2003). What, then, is the new role of research in this changing relationship between the government and its citizens? Increasingly, research obtains a function to facilitate, feed, stimulate and mediate interaction processes between policy makers, stakeholders and NGOs (Non-Governmental Organisations) with the objective to create conditions for joint learning and a range of possible problem solutions, each one including all explicit and implicit tradeoffs between contrasting economic, social and environmental demands. This requires an immense effort by interdisciplinary research teams but also timely input of disciplinary knowledge at the right moment. This paper will focus on the role of disciplinary soil research in such interdisciplinary teams focusing on pedology and hydrology separately and in combination, exploring the expectation that joint action of both disciplines would not only be attractive for both but also, and particularly, for solving the issues at hand.
2. Opportunities for disciplinarity in an interdisciplinary context Globalisation not only relates to industrial production and services but also, increasingly, to science. The global environmental issues of the future, as mentioned in the introduction, are being studied by international panels of scientists that interact with stakeholder groups, governments, NGOs and industry. An example is the International Panel for Climate Change (the IPCC) which consists of an international group of excellent scientists. Identical developments take place for biodiversity where a Subsidiary Body for Scientific, Technical and Technological Advice (SBSTTA) organises cooperation between researchers and policy makers. For water research, the Global Water Partnership is being established in a comparable way. These panels provide input for international meetings where treaties are negotiated between governments which must next be implemented at national level. The Kyoto protocol, for example, requires a certain negotiated reduction in the emission of greenhouse gases for each country. Acceptance of these reductions by national governments is still problematic because of major economic consequences involved, and the Kyoto protocol of 1997 has still not been accepted by a sufficient number of countries to be formally approved. Considering the unfolding research landscape of the future, the role of hydropedology in formulating environmental policies can best be considered from two points of view: First bup to global Q: its role in the international panels where policy issues are discussed and where negotiations take place. The issue is not so much that pedologists and hydrologists should always be present in such panels but, rather, that what they have to offer in terms of expertise is being taken into account when formulating policy options. This requires effective communication and a critical analysis of the own discipline taking a broad view of the issues at hand and attempting to understand the state of mind of colleagues not only in the natural sciences such as climatology and geology but also, and ever more importantly, of economists and political scientists. We are not used to do this. Our disciplines thrive by having congresses and symposia where we essentially talk to each other and by writing papers for journals that are edited by our direct colleagues.
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Second bdown to localQ: once international treaties have been agreed upon, they have to be followed by implementation on national or local levels. This, again, requires input from science but now on a level that recognizes local conditions and is prepared to fine-tune its approach accordingly. Also here, work is ideally done by interdisciplinary teams. What should be done to act more effectively at international and local level? Do we need different approaches or can we be satisfied with the way we work right now? And would it, in this context, be advisable to join forces of pedology and hydrology or can we continue to operate independently as in the past? Before these questions will be addressed, attention is paid to one important and common element of interdisciplinary activity that is often overlooked: working at different levels of detail.
3. Working at different levels of detail Scientific work is usually focused on the frontiers of science using advanced equipment and methodologies, as this provides the best opportunities for publication of results in prestigious international journals. Pedology and hydrology are no exception to this fact of scientific live. Such specialistic input is, however, not always effective when working in international panels or in interdisciplinary teams that have to implement environmental legislation on the national or regional level. Partners in the panels or the teams will not understand the jargon involved and they certainly will not be able to see such highly specialized work in a broader context. This, obviously, is detrimental to mutual communication which, in turn, is crucial for effective interdisciplinary work. What to do? Bouma (1997b, 2001a) has suggested a step-by-step approach which starts from the assumption that after a hundred years of soil and water research there is an answer for any question that can possibly be raised. Most likely, for many modern issues, the instant answer based on experience will be a poor one, but this offers the possibility to suggest new research or a review of literature that will improve the answer. Always, a certain cost is involved and this cost will steadily increase as the information becomes, step-by-step, more sophisticated. But some issues being raised in an interdisci-
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plinary context require only generalized, empirical data from any given discipline. Then, supply of sophisticated data may represent overkill (e.g., Bouma, 1993). Other issues require more sophisticated data but by starting even then with the most simple input and by discussing this with stakeholders and colleagues from other disciplines and by identifying gaps in knowledge, a joint learning trajectory is introduced that is crucial for creating true cooperation and interchange. We may call this the salesman principle: interaction with potential buyers may follow once his foot does not allow the door to be closed! Scientists playing such a role as a salesman should be prepared to invest time, creativity and energy to establish such communication. The Hoosbeek and Bryant diagram has been used by Bouma (1997b, 2001a) to illustrate this process, considering expressions of know-how in two dimensions, ranging from qualitative to quantitative on one dimension and from empirical to deterministic on the other. In addition, the analysis was made for different scale levels ranging from the microscopic to the world at large (Fig. 1). Different knowledge levels were distinguished: K1—user experience; K2—expert knowledge, based mainly on experience; K3—same, but also including measurements and use of simple (statistical) models; K4—knowledge based on measurements and deterministic models for entire systems, and K5—knowledge based on complex, specialized measurements and models for certain aspects only. Bouma (1997b) illustrated the procedure by defining four increasingly detailed and costly procedures to determine the vulnerability of European Soils for acidification. Most likely, the error associated with these procedures will decrease as the procedures become more detailed, even though this may not be the case because of natural variability. This natural variability, to be characterized by (geo)statistical procedures, may be so high that highly detailed experiments at point level produce only pseudoaccuracy. But even if work at higher K levels results in lower error, we still have to balance this lower error against the associated costs that increase strongly as the knowledge level increases. Rather than jump right away to a given type of (often detailed) research at K5 level, it pays to start simple and follow a gradual and systematic (bstep-by-stepQ) buildup. Along these lines, Bouma and Droogers (1999) presented a case study
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Fig. 1. Procedures studying water movement at the landscape level combining different types of knowledge, applied at different spatial levels, and represented for pedology and hydrology separately and for hydropedology as an interdisciplinary activity.
on the soil moisture supply capacity, distinguishing five K levels. When members in panels or teams, as described above, can present their input for their discipline at different degrees of detail, there is a higher probability for effective communication and output as compared with a condition where every discipline presents data at the K5 level only, or if some disciplines offer data at K3 level and others at K5 level etc. This example applies to the widely used successful DSSAT models that simulate soil water movement and crop growth (e.g., Bouma and Jones, 2001). Here, emphasis has been on aspects of plant growth, leading to ever more specific (K5) representations of crop development. At the same time the original soil water module (of the K3 tipping-bucket type) was not changed and this has resulted in the end in a somewhat unbalanced model where soil and hydraulic aspects are rather poorly represented. To be clear, only soil scientists are to blame here for the fact that soil expertise is not adequately represented in the model. Such examples of unbalanced model representations are particularly relevant for pedology and hydrology. To many blue-blooded hydrologists, activities of pedologists are difficult to judge from a scientific point of view. In their view, pedologists use bfunnyQ names to describe soils and they make too many empirical statements about soil behavior which are not supported by solid measurements. On the other hand, pedologists are taken back by the representation of natural soils in terms of homogeneity and isotropy that hydrologists and soil physicists need when
running model representations of soils: this clearly does not reflect real conditions being experienced in the field. This certainly applies to regional hydrological models where pixel-data often cannot reflect occurrence of various soil horizons and effects of soil patterns. One important task of the hydropedology working group of SSSA is to debunk the above stereotype visions on both sides of the fence. Pedology has its roots in soil survey which considers landscape processes and soil structure descriptions that have been somewhat neglected in the period in which soil classification received most attention. These two aspects are important for soil physics and hydrology to improve their quantitative characterization of flow regimes in the field of watersheds with undisturbed soils. At the same time, pedologists need to benefit from flow theory when transforming their qualitative descriptions into quantitative expressions, which is increasingly necessary to answer modern policy questions. Combining pedological and hydrological expertise can be particularly attractive when presenting soil information to panels and teams as discussed: pedology has much useful descriptive information at the K1, K2 and K3 level (see illustration in Fig. 1 where the characterization of water movement at landscape level is visualized). At the same time, hydrology and soil physics have much process and modeling information at K3, K4 and K5 level (Fig. 1). Combining both implies that the entire K-range is covered whereby hydrology would benefit from pedological characterizations and pedology would
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benefit from hydrological flow theory. The assumption that the joint package would be more powerful than the two parts by themselves will now be further explored.
4. Hydropedology: more than the sum of the two disciplinary parts? Some examples will be described that illustrate the benefits of a combined hydropedological approach. Selection of examples from the work of our own research group, as presented below, does certainly not imply that relevant work has not been done elsewhere, but allows illustrations to be specific and based on experience. McKenzie et al. (1991) and Bruand (1990) also relate soil morphology to soil water properties. 4.1. Bypass flow Sprinkling irrigation of heavy clay soils next to the river Rhine in The Netherlands caused problems in the seventies of the last century, because water applications in summer did not result in calculated soil water contents that agreed with measured values. In fact, differences were very large. Calculations suggested field capacity while measurements indicated wilting point. The reason was bypass flow which is the vertical movement of free water along the walls of macropores (in this case shrinkage cracks) through an otherwise unsaturated soil matrix. Physical flow theory did not allow for this: flow is either saturated or unsaturated and soils are implicitly considered to be homogeneous and isotropic. The most recent Methods of Soil Analysis by the Soil Science Society of America (Dane and Clark, 2002) still reflects this, although bypass flow is considered now for the first time (Booltink and Bouma, 2002). Application of blue-stained water at different rates and quantities and observation of stained flow patterns in the soil along the cracks allowed us to define critical flow rates and quantities that could be adsorbed by the peds on the soil surface, avoiding bypass flow (Bouma and Dekker, 1978). Also, a simple field technique was devised to measure bypass flow as a function of rain intensity and quantity and the soil moisture content (e.g., Bouma, 1997a). Of course, cracking of clay
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soils was already well known before our work, but deriving cracking patterns theoretically from physical soil swell-shrink characteristics turned out to be impossible because of the sensitivities involved: very small pores, with a volume that cannot reliably be measured with physical methods, can conduct large volumes of water. A procedure whereby macropores are morphologically studied in the field, preferably functionally characterized by staining, followed by modeling whereby bypass flow is incorporated as a separate module in existing physical flow models, represents an effective combined procedure of hydropedology that avoids purely qualitative descriptions by soil morphology (K2) and irrelevant model representations by soil physics (K4) (see Hoogmoed and Bouma, 1980). 4.2. Saturated hydraulic conductivity (K-sat) K-sat is an important physical soil characteristic: it defines the flux under saturated conditions at unit hydraulic gradient. Many measurement methods are available (Soil Science Society of America, 2002) but they still ignore the effect of the volume of samples and pore-continuity of interpedal voids. Soil survey information can be used to improve estimation of Ksat (e.g., McKenzie and Jacquier, 1997). Taking a quantitative approach, Anderson and Bouma (1973) showed that any K-sat could be measured in the B2t of a Wisconsin Hapludalf by varying the height of the sample. This was due to the well-developed blocky structure: vertical continuity of the cracks between the peds decreased as the sample became longer, resulting in lower K-sat values even though the sample was completely saturated in all cases and flow occurred at unit hydraulic gradient. Two applications of this insight materialized: (i) morphological measurement of crack patterns could be used to calculate K-sat, using a vertical pore-continuity model, and (ii) representative elementary volumes of samples were defined as a function of ped sizes. Standard volumes of samples are incorrect; sample volumes should be a function of the size of the peds: the larger the peds, the larger the samples. The pore-continuity model was refined later using micromorphological studies and here we showed that very small pore-necks, immeasurable in terms of volumes by soil physical methods, governed K-sat in prismatic clay soils, where big
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cracks occurred when the soil was dry (Bouma et al., 1979). Comparable studies were made for large vertical worm and root channels, showing big differences between measured K-sat values, comparing measured values in large columns that were either attached to the subsoil (K-sat: 0.5 m/day) or detached (50 m/day). Of particular interest were K-sat values measured in soils with macropores with a light crust. The soil matrix was then still saturated, as evidenced by a pressure head of 0 kPa throughout, but macropores were filled with air. Theoretically, the measured flux was still a K-sat value, but much lower than the other ones (at 0.05 m/day) (see Bouma, 1997a and Fig. 2). A study was made in a silty clay soil with glossic features, where samples in the bleached cracks yielded a K-sat of 6.9 m/day, while values were 0.3 m/ day inside the compact peds (Fig. 2) (Bouma et al., 1989a,b). Placing samples at random in this soil leads to a very high variability that cannot be reduced by applying statistics, but only by making a morphological analysis before samples are taken and by systematic subsampling within the two populations. The conclusion can be that studying soil structure with morphological methods is important when choosing the proper measurement method for K-sat, again demonstrating a powerful combination of pedology (K2) and hydrology (K4) expertise.
4.3. Where is the water table? When working in pedal soils in The Netherlands, measurement of water table depth has often presented problems. Levels fluctuate wildly, particularly after rainfall while levels observed in piezometers differed significantly at very short distances. Also, tensiometers can produce different results at short distances. Bouma et al. (1980) studied this phenomenon, showing the effects of water flowing along cracks along the faces of peds, not only in the unsaturated zone of the soil but also in the saturated zone (Fig. 3). After rainfall the water level in the cracks rises very rapidly to subside slowly as water moves slowly into the surrounding, unsaturated peds. When piezometers intercept these cracks through which the free water moves, they show high fluctuations. When piezometers are inside the peds, they hardly move at all, unless they are not well sealed on the outside which is likely to result in vertical flow along the cracks from the soil surface into the piezometer, incorrectly suggesting a bperchedQ water table inside the (unsaturated) peds. The field practice in the seventies was to observe water table levels in unlined boreholes. This resulted in much confusion because a bwater table levelQ was observed at the bottom of each unlined augerhole,
Fig. 2. Illustrations of using soil morphological structure descriptions for improving physical measurements of K-sat, including (from left to right) effects of sampling volume (A), use of glossic features (B), and effects of large-pore continuity (C).
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Fig. 3. Possible effects of pedality on measuring water table levels with boreholes and piezometers and on measuring soil water potentials with tensiometers.
every augerhole producing a different bwater table levelQ as water moved along air-filled cracks into the hole while the surrounding soil was unsaturated. Use of lined piezometers solved this problem. When the water moves out of the cracks, the remaining peds may remain saturated for a while, suggesting overall saturation of the soil as measured by tensiometry, which does, in fact, not occur as the surrounding cracks are already filled with air. Once this mechanism is understood, much bvariabilityQ can be explained. Another problem can occur when tensiometers are placed with a downward angle into a vertical profile wall. This may induce flow along the sides, incorrectly suggesting saturation as the water reaches the cup of the tensiometer. Placement under a slight upward angle avoids this problem. Understanding the flow regime, as influenced by cracking patterns, helps to explain what would otherwise by highly confusing hydrological measurement results and can contribute to better instrument design and measurement protocols. Thus, pedology expertise helps to guide and interpret physical measurements,
again combining K2 expertise of pedology with K4/ K5 expertise of soil physics/hydrology. 4.4. Water accessibility Soils with large compact peds, such as prisms and clods formed by tillage under adverse conditions, often show concentrations of roots at the ped surface, indicating that the roots were unsuccessful in penetrating the peds or clods. Field conditions have been observed where plants were wilting even though water contents of the root zone were well above wilting point. This phenomenon has been attributed to limited accessibility. Modeling studies, implicitly assuming unlimited accessibility, yielded poor results. Droogers et al. (1997) studied these processes in large undisturbed field samples and they defined accessibility as a function of ped sizes. This, in turn, could be incorporated in existing simulation models for plant growth, again demonstrating the positive effects of combining pedological expertise (K2) with hydrological process knowledge (K5 in this case).
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4.5. Hydrophobicity The work of Dekker and Ritsema has demonstrated the importance of hydrophobicity on soil water movement in The Netherlands. When ignored, simulations of water regimes which implicitly assume onedimensional homogeneity, produce poor results (Dekker and Ritsema, 2003). Dekker et al. (1984) showed that assumed lateral flow of water on top of a compact spodic subsurface horizon in a nature area in The Netherlands did not occur but that lateral movement of water was due to surface runoff originating from hydrophobicity of the soil surface. Extensive field studies, covering the entire country, have shown that most soils, as distinguished by the national soil survey, are susceptible to hydrophobicity under dry conditions, but land use history is an important factor as well. A hydrophobicity module has been added to the regular soil–water flow model, allowing realistic simulations for water movement for soils that exhibit this phenomenon (Ritsema et al., 2001). When simulations are to be made of soil–water movement of field soils, first a screening is made as to whether or not soils are hydrophobic. If so, the particular flow routine is included. This is another example where pedological expertise, which here relates to soil surveys covering the entire country and simple field measurements of hydrophobicity (K3), are crucial input for soil–water simulations (K4 and K5), yielding a much better result than either of the two disciplines could have produced on its own. 4.6. Pedotransfer functions Pedotransfer functions relate simple soil characteristics to be found in soil surveys to more complex parameters that are used in modeling and that are relatively difficult to measure (e.g., Bouma, 1989, McBratney et al., 2002). Many publications have appeared on this topic, particularly for hydraulic conductivity and moisture retention in soil physics, illustrating the significance of the combined approach of hydropedology (e.g., Wo¨sten et al., 2001). But soil– chemical pedotransfer functions have been derived as well (Breeuwsma et al., 1986). Nemes (2002) has shown that simulations of water regimes for Hungarian soils could only be made thanks to the availability of pedotransfer functions. Lack of funds would not
have allowed extensive measurement campaigns. Still, there is a clear risk here. Uncritical application of automatically generated pedotransfer functions is likely to produce poor results when made by non-soil scientists with no feeling for what may broadly be expected or when the functions are derived from data obtained for other soils than the ones being characterized. McBratney et al. (2002) broadened the concept to soil inference systems where pedotransfer functions form knowledge rules for inference engines, including uncertainty analysis. This is a promising future approach which extends the concept beyond the derivation of moisture retention and hydraulic conductivity data which is the most common interpretation of pedotransfer functions so far. 4.7. Redox features as wetness indicators In the above examples, soil morphology or other data from soil surveys were used to improve the physical characterization of soil water regimes, usually by deterministic modeling. There are also examples where soil morphological data uniquely characterises flow regimes that would be very difficult to document with soil physical or hydrological techniques. Examples are moisture regimes in soils with peds or with periodic bperchedQ water tables on top of slowly permeable subsurface soil horizons, where occurrence of characteristic grey reduction and iron oxidation patterns can provide important clues as to flow patterns during the year. They can be quite helpful when placing monitoring or measurement equipment in soils in the field (e.g., Bouma et al., 1989a,b). Much has been published on this topic and the reader is referred to literature for more examples. 4.8. Flow processes in landscapes So far, examples have been discussed related to the dynamic behaviour of soils. Pedology has, of course, also a strong landscape dimension which is expressed by soil maps. To the uninitiated user, such maps may suggest occurrence of adjacent homogeneous soil bodies that differ from each other. This, of course, is not the case. Different mapping units have characteristically different internal variabilities (e.g., Marsman and de Gruijter, 1983) which are increasingly being
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reported in survey reports, using statistical and geostatistical techniques that allow transformation of point to area data (Brus and de Gruijter, 1997). Finke (2000) compared the increase of quality when incorporating variability data with cost involved. When considering soil physical data, we often find that pedological differences do not necessarily correspond with differences in soil physical behaviour. For example, Wo¨sten et al. (1985) reported that only 30% of the different land units on a given soil map also behaved differently from a soil physical point of view, allowing combination of mapping units into bfunctional unitsQ. Hydrological landscape models work with pixels, each to be fed with typical, representative data. The surface relief can now be well expressed in great detail using data from Digital Terrain Models, fed by satellite data (e.g., Schoorl et al., 2002). Infiltration characteristics at the surface are often derived from soil survey data providing texture and organic mater contents which allow estimates to be made when measurements are not available or feasible. Some potential pitfalls of that procedure have been mentioned above when discussing K-sat. After infiltration, however, water never follows flow patterns as would occur in a homogeneous and isotropic porous medium. Occurrence of slowly permeable or irregular subsoil horizons or geologic formations usually strongly alters flow patterns. Here, again, pedology can have important input in improving the quality of hydrological models in areas of land, such as watersheds. Several papers are appearing now in the hydrology literature that start to take account of subsurface pedological horizons (e.g., Rosney et al., 2002; Chamram et al., 2002; Wullschleger et al., 2001).
5. The throbbing landscape as a starting point for research The above examples indicate that a combination of pedological and hydrological expertise is particularly valuable for the realistic characterization of soil water regimes in landscapes. Empirical observations by pedologists can significantly improve the quality of parameter measurements by hydrologists, to be used in modelling. In turn, interpretations of soil surveys
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can thereby be improved by adding quantitative expressions, including bwhat. . .if Q explorations, to the usual qualitative, empirical characterizations in pedology in terms of relative limitations for certain types of land use. How can this be effectively presented in the interdisciplinary panels and teams mentioned above? Traditionally, our disciplines are not prepared to make a special effort here as we implicitly assume that our maps and modelling results will find their way if they are good. This, perhaps unfortunately, is not true anymore. Citizens of today and politicians have a short attention span and react to ever more intense and extraordinary impulses. This is not only relevant for commercials but also for reporting the results of scientific studies. Images are important and those images have to be bsoldQ. Fortunately, in hydropedology, pedology and hydrology can together create images that are beyond reach for each individual discipline by itself. This we like to present in terms of a bthrobbingQ landscape. In fact, water fluxes into and through soils in a landscape are the essence of live and resemble, in a way, the manner in which blood circulates in a human body. We could even compare blood pressure with the pressure potential of water in soil: when it is too high or too low soil functioning is clearly hampered. We could therefore speak of a throbbing landscape in which water enters and leaves, on an hourly, weekly, monthly and yearly basis. Of course, not only water governs soil functioning. But once water regimes have been characterized, chemical and biological processes can be added as they strongly depend on the water regime and on interaction processes with the soil. Using modern ICT (Information and Communication Technology) routines allows visualizations that can help to convey messages on limitations or potentials that are associated with natural and man-induced water regimes in landscapes. What are the fluxes in a dry year? A wet year? What does that imply for different forms of land use? How does or can human management affect these fluxes? etc. Such integrated information is needed for the interaction and negotiation in the panels and teams mentioned earlier. This also works well in Dutch planning which considers the blayer modelQ consisting of three blayersQ: the first one represents the natural dynamics
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of land and water; the second one all networks of roads, railways and waterways and the third one: settlements. Ideally, new land use plans should consider the sequence from one to three, considering first the dynamics of land and water which is most difficult to affect or should not be affected in sensitive areas where substantial damage could occur. Next, infrastructure networks have a higher degree of permanence than settlements which readily expand and contract (e.g., Metropolitan Debate, 1998). This approach offers an attractive platform for applied hydropedology, working with geologists as well. The manner in which the natural landscape bthrobsQ offers clues as to bwhatQ can best be done bwhereQ with the lowest risks and the greatest opportunities.
6. Challenges ahead A broad well-planned discussion is needed to formulate the most important challenges for hydropedology research in future. The new SSSA working group on hydropedology provides a useful platform for this. For a start, two procedural approaches to research are mentioned here that we believe are relevant for hydropedology in future. A listing of future environmental issues as such is beyond the scope of this text and can be found in documents of environmental agencies, policy think-tanks and research institutions. 6.1. Incorporating effects of management Pedology is focused on natural processes and does not reflect effects of short-term soil management in soil classification. This was done on purpose to avoid frequently changing classifications of a given soil following different types of soil management, such as tillage and fertilisation. Exceptions are, for example, Plaggen soils in Europe, that were formed for periods of over 900 years by adding each year a thin layer of animal manure mixed with local soil. The manure added fertility in centuries when no chemical fertilizer was available, while the added local soil raised the soil slowly by, on average, 1 mm a year. This resulted in 90-cm thick surface horizons to be observed now that are classified as such, even though they resulted from management. Similar examples can be given for
rice soils in Asia. When dealing with issues of land use in future, however, effects of more short-term soil management which are not expressed by soil classification are very important, certainly when defining different types of land use than the current ones, a common process in land use planning and negotiation. We expect that different soil types will react characteristically different to particular types of management, but we do not really know this as effects of different types of management on different types of soil has never been a general topic in soil research. In The Netherlands two major soil series were investigated to establish effects of different types of short-term soil management on longer term soil properties for cases where this land management did not result in a change of soil classification. Droogers and Bouma (1997) suggested the term genoform for the genetic soil type and different phenoforms of that particular genoform to express the effects of different types of management. Pulleman et al. (2000) worked this out for a Typic Fluvaquent and Sonneveld et al. (2002) for a Typic Haplorthod. In order to find these relationships, extensive field studies were made where existing soil maps were used to determine locations where the particular genoform could be found. Next, farmers were interviewed, land use history was reconstructed and soil properties, including hydraulic soil characteristics, were measured. Next, a limited number of significantly different phenoforms were identified and regression analysis was used to express relationships between land use and soil properties for a given soil series. Sonneveld et al. (2002) added an additional element by also including landscape features. This work showed that much soil information, to be gained by field research, is as yet waiting to be discovered in the field. Obviously, the type of information gained this way is impossible to obtain with classical field experiments: it would take decades of experimental work for which no funding is available. The alternative is, of course, modelling but the error and uncertainties involved here are quite large and the relatively simple procedure of selectively sampling the vast amount of documentation that is out there to be explored, is tempting. This has the important side effect of requiring field work which increasingly is being substituted by laboratory and office activity, breaking the liveline that pedology always had with bthe fieldQ. Back to the field!
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6.2. Pro-active design Somehow, most work in pedology and hydrology has been rather reactive in character. Either questions raised by others needed to be answered or given conditions were characterized, more often than not representing problems caused by poor land use. This has, of course, allowed excellent research but why not also take an occasional more pro-active approach? Why not take a given soil material, consider climate conditions and design a soil structure that would best satisfy conflicting demands, for instance, a structure that would allow optimal rooting, would supply a relatively high amount of moisture, would be trafficable, would avoid bypass flow of agro-chemicals etc. Bouma et al. (1999) have made an attempt to do this. Such work provides an excellent basis for joint work of pedologists and hydrologists. The focus could also be shifted to the landscape scale, taking into account the blayerQ model mentioned above. As is, we dutifully document errors that engineers and architects have made. Why not design optimal soil and landscape structures based on comparing effects of different flow patterns and deliver these to engineers and architects with an invitation to realize them in practice?
References Anderson, J.L., Bouma, J., 1973. Relationships between hydraulic conductivity and morphometric data of an argillic horizon. Soil Sci. Soc. Am. Proc. 37, 408 – 413. Booltink, H.W.G., Bouma, J., 2002. Bypass flow. In: Dane, J., Clark, T. (Eds.), Methods of Soil Analysis: Part 4. Physical Methods, vol. 5. SSSA, Madison, WI, pp. 930 – 937. Bouma, J., 1989. Using soil survey data for quantitative land evaluation. In: Stewart, B.A. (Ed.), Advances in Soil Science, vol. 9. Springer Verlag, New York, pp. 177 – 213. Bouma, J., 1993. Soil behaviour under field conditions: differences in perception and their effects on research. Geoderma 60, 1 – 15. Bouma, J., 1997a. Long-term characterization: monitoring and modelling. In: Lal, R., Blum, W.H., Valentin, C., Stewart, B.A. (Eds.), Methods for the Assessment of Soil Degradation. Advances in Soil Sci., pp. 337 – 358. Bouma, J., 1997b. Role of quantitative approaches in soil science when interacting with stakeholders. Geoderma 78, 1 – 12. Bouma, J., 2001a. The new role of soil science in a network society. Soil Sci. 166, 874 – 879. Bouma, J., 2001b. The role of soil science in the land negotiation process. Soil Use Manag. 17, 1 – 6.
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Bouma, J., Dekker, L.W., 1978. A case study on infiltration into dry clay soil: I. Morphological observations. Geoderma 20, 27 – 40. Bouma, J., Droogers, P., 1999. Comparing different methods for estimating the soil moisture supply capacity of a soil series subjected to different types of management. Geoderma 92, 185 – 197. Bouma, J., Jones, J.W., 2001. An international collaborative network for agricultural systems applications (ICASA). Agric. Syst. 70, 355 – 368. Bouma, J., Jongerius, A., Schoonderbeek, D., 1979. Calculation of saturated hydraulic conductivity of some pedal clay soils using micromorphological data. Soil Sci. Soc. Am. J. 43, 261 – 264. Bouma, J., Dekker, L.W., Haans, J.C.F.M., 1980. Measurement of depth to water table in a heavy clay soil. Soil Sci. 130, 264 – 270. Bouma, J., Fox, C.A., Miedema, R., 1989. Micromorphology of hydromorphic soils: applications for soil genesis and land evaluation. In: Douglas, L.A. (Ed.), Soil Micromorphology: A Basic and Applied Science. Developments in Soil Science, vol. 19, pp. 257 – 279. Bouma, J., Jongmans, A.G., Stein, A., Peek, G., 1989. Characterizing spatially variable hydraulic properties of a boulder clay deposit. Geoderma 45, 19 – 31. Bouma, J., Droogers, P., Peters, P., 1999. Defining the bidealQ soil structure in surface soil of a Typic Fluvaquent in The Netherlands. Soil Sci. Soc. Am. J. 63, 343 – 348. Breeuwsma, A., Wosten, J.H.M., Vleeshouwer, J.J., van Slobbe, A.M., Bouma, J., 1986. Derivation of land qualities to assess environmental problems from soil survey. Soil Sci. Soc. Am. J. 50, 186 – 190. Bruand, A., 1990. Improved predictions of water retention properties of clayey soils by pedological stratification. J. Soil Sci. 41, 491 – 497. Brus, D.J., de Gruijter, J.J., 1997. Random sampling or geostatistical modelling? Choosing between design-based and model-based sampling strategies for soil (with discussion). Geoderma 80, 1 – 44. Chamram, F., Gessler, P.E., Chadwick, O.A., 2002. Spatially explicit treatment of soil-water dynamics along a semi arid catena. Soil Sci. Soc. Am. J. 66, 1571 – 1583. Dane, J., Clark, T. (Eds.), 2002. Methods of soil analysis: Part 4. Physical methods. SSSA Book Series, vol. 5. Soil Sci. Soc. Am. Inc., Madison, WI. Dekker, L.W., Ritsema, C.J., 2003. Wetting patterns in water repellent Dutch soils. In: Ritsema, C.J., Dekker, L.W. (Eds.), Soil Water Repellency. Elsevier, Amsterdam, pp. 151 – 166. Dekker, L.W., Wosten, J.H.M., Bouma, J., 1984. Characterizing the soil moisture regime of a Typic Haplohumod. Geoderma 34, 27 – 42. Droogers, P., Bouma, J., 1997. Soil survey input in exploratory modelling of sustainable land management practices. Soil Sci. Soc. Am. J. 61, 1704 – 1710. Droogers, P., van der Meer, F.B.W., Bouma, J., 1997. Water accessibility to plant roots in different soil structures occurring in the same soil type. Plant Soil 188, 83 – 91. Finke, P.A., 2000. Updating groundwater table class maps 1:50000 by statistical methods: an analysis of quality versus cost. Geoderma 97, 329 – 350.
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Hoogmoed, W.B., Bouma, J., 1980. A simulation model for predicting infiltration into cracked clay soil. Soil Sci. Soc. Am. J. 44, 458 – 461. Marsman, B.A., de Gruijter, J.J., 1983. Quality of soil maps: a comparison of survey methods. Soil Survey Papers, vol. 15. Netherlands Soil Survey Institute, Wageningen, The Netherlands. McBratney, A.B., Minasny, B., Cattle, S.R., Vervoort, R.W., 2002. From pedotransfer functions to soil inference systems. Geoderma 109, 41 – 73. McKenzie, N.J., Jacquier, D.W., 1997. Improving the field estimation of saturated hydraulic conductivity in soil survey. Aust. J. Soil Res. 35, 803 – 825. McKenzie, N.J., Smetten, K.R.J., Ringrose Voase, A.J., 1991. Evaluation of methods for inferring air and water properties of soils from field morphology. Aust. J.Soil Res. 29, 587 – 602. Metropolitan Debate, 1998. In: de Hoog, Maurits (Ed.), Laag Land. dRO, Amsterdam (Dienst Ruimtelijke Ordening), Desing team Stad. Amsterdam (in Dutch). Nemes, A., 2002. Unsaturated soil hydraulic database of Hungary. HUNSODA. Agrokem. Talajtan. 51, 17 – 26. Pulleman, M.M., Bouma, J., van Essen, E.A., Meijles, E.W., 2000. Soil organic matter content as a function of different land use history. Soil Sci. Soc. Am. J. 64, 689 – 694. Ritsema, C.J., van Dam, J.C., Dekker, L.W., Oostindie, K., 2001. Principles and modelling of flow and transport in water repellent surface layers and consequences for management. Intern. Turfgrass Soc. Res. 9, 615 – 623.
Rosney, P., de Polcher, J., Bruen, M., Laval, K., 2002. Impact of a physically based soil water flow and soil–plant interaction representation for modeling large scale land surface processes. J. Geophys. Res., [Atmos.], 107 – 112. Schoorl, J.M., Veldkamp, A., Bouma, J., 2002. Modeling water and soil redistribution in a dynamic landscape context. Soil Sci. Soc. Am. J. 66, 1610 – 1619. Soil Science Society of America, 2002. Methods of soil analysis: Part 4. Physical methods. In: Dane, J.H., Topp, G.C. (Eds.), Soil Sci. Soc. Am., Madison, WI. Sonneveld, M.P.W., Bouma, J., 2003. Methodological considerations for nitrogen policies in the Netherlands including a new role for research. Environ. Sci. Policy 6 (6), 501 – 511. Sonneveld, M.P.W., Bouma, J., Veldkamp, A., 2002. Refining soil survey information for a Dutch soil series using land use history. Soil Use Manag. 18, 157 – 163. Wo¨sten, J.H.M., Bouma, J., Stoffelsen, G.H., 1985. The use of soil survey data for regional soil water simulation models. Soil Sci. Soc. Am. J. 49, 1238 – 1245. Wo¨sten, J.H.M., Pachepsky, Ya.A., Rawls, W.J., 2001. Pedotransfer functions: bridging the gap between available basic soil data and missing soil hydraulic characteristics. J. Hydrol. 251, 123 – 150. Wullschleger, S.D., Jackson, R.B., Currie, W.S., Friend, A.D., Luo, Y., Mouillot, F., Pan, Y., Shao, G.F., 2001. Below-ground processes in gap models for simulating forest response to global change. Climate Change 51, 449 – 473.
Hydropedology as a powerful tool for environmental policy research Johan Bouma Environmental Sciences Group, Wageningen University and Research Center, The Netherlands Available online 10 May 2005
Abstract Rather than produce clear-cut answers to well-defined problems, research on future environmental policy issues requires a different approach whereby researchers are partners in joint learning processes among stakeholders, policy makers, NGOs (Non-Governmental Organisations) and industry. This relates to the strategic bup-to-globalQ as well as to the operational bdown-to-local Q level. Researchers can play a key role in facilitating the learning process by contributing knowledge and by helping formulate options for action which will imply definition of tradeoffs between contrasting demands and interests. This holds for scientists in general but also for pedologists and hydrologists when dealing with land use policy issues which always include environmental aspects. By combining hydrological and pedological expertise in hydropedology, better contributions can be made to this process than by operating separately as in the past. The considerable empirical knowledge of the pedologist on soil and landscapes can be sharpened by process knowledge of the hydrologist who, in turn, gains by obtaining more realistic model representations. Examples are provided discussing measurement of (i) Bypass flow; (ii) K-sat; (iii) water table levels; (iv) water accessibility; and (v) hydrophobicity. Furthermore, pedotransfer functions and realistic landscape models are discussed which express pedological expertise. The image of a throbbing landscape with characteristic water fluxes in space and time, affected and to be affected by human management, is seen as vital and highly visible input into interdisciplinary discussions in research teams dealing with land use. Suggestions for future action in hydropedology include the need for more field work, exploring the characteristic effects of soil management on given types of soils and, in general, a more pro-active research approach. D 2005 Elsevier B.V. All rights reserved. Keywords: Pedology; Soil physics; Hydrology; Land use planning; Landscape processes; Soil morphology
1. Introduction The general policy focus on sustainable development, as agreed upon by many countries in Johannes-
E-mail address: [email protected]. 0016-7061/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2005.03.009
burg in 2002, requires an integrated research approach considering economic, social and environmental issues (bProfit, People, PlanetQ). This also applies to prominent land-related elements of sustainable development such as climate change, biodiversity loss, water scarcity and land degradation. All these issues are quite complex, they are international in character,
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their effect on daily live may be rather invisible in the short term but potentially quite disruptive in a longer time frame and they are all associated with much uncertainty. Science worldwide is struggling to come to grips with these problems. Researchers, including soil scientists and hydrologists, are faced with a new research paradigm because there are no single magic solutions to such problems but, rather, action programs have to be negotiated with all stakeholders involved to come to solutions that always represent a compromise between conflicting demands and interests and this is a cumbersome procedure. For example, Sonneveld and Bouma (2003) reviewed the turbulent negotiations in The Netherlands following the implementation of manure regulations imposed by the European Union. Even though this problem was a lot simpler than the ones associated with climate change, it turned out to be quite complicated to develop solutions acceptable to all. Imposing policy measures top–down without interaction with the stakeholders involved has never been successful, and this has become increasingly clear in recent times. Now, rather, policy development involving stakeholders (which has often been ignored in the past) is becoming an important part of the implementation process of environmental regulations (e.g., Bouma, 2001a,b; Sonneveld and Bouma, 2003). What, then, is the new role of research in this changing relationship between the government and its citizens? Increasingly, research obtains a function to facilitate, feed, stimulate and mediate interaction processes between policy makers, stakeholders and NGOs (Non-Governmental Organisations) with the objective to create conditions for joint learning and a range of possible problem solutions, each one including all explicit and implicit tradeoffs between contrasting economic, social and environmental demands. This requires an immense effort by interdisciplinary research teams but also timely input of disciplinary knowledge at the right moment. This paper will focus on the role of disciplinary soil research in such interdisciplinary teams focusing on pedology and hydrology separately and in combination, exploring the expectation that joint action of both disciplines would not only be attractive for both but also, and particularly, for solving the issues at hand.
2. Opportunities for disciplinarity in an interdisciplinary context Globalisation not only relates to industrial production and services but also, increasingly, to science. The global environmental issues of the future, as mentioned in the introduction, are being studied by international panels of scientists that interact with stakeholder groups, governments, NGOs and industry. An example is the International Panel for Climate Change (the IPCC) which consists of an international group of excellent scientists. Identical developments take place for biodiversity where a Subsidiary Body for Scientific, Technical and Technological Advice (SBSTTA) organises cooperation between researchers and policy makers. For water research, the Global Water Partnership is being established in a comparable way. These panels provide input for international meetings where treaties are negotiated between governments which must next be implemented at national level. The Kyoto protocol, for example, requires a certain negotiated reduction in the emission of greenhouse gases for each country. Acceptance of these reductions by national governments is still problematic because of major economic consequences involved, and the Kyoto protocol of 1997 has still not been accepted by a sufficient number of countries to be formally approved. Considering the unfolding research landscape of the future, the role of hydropedology in formulating environmental policies can best be considered from two points of view: First bup to global Q: its role in the international panels where policy issues are discussed and where negotiations take place. The issue is not so much that pedologists and hydrologists should always be present in such panels but, rather, that what they have to offer in terms of expertise is being taken into account when formulating policy options. This requires effective communication and a critical analysis of the own discipline taking a broad view of the issues at hand and attempting to understand the state of mind of colleagues not only in the natural sciences such as climatology and geology but also, and ever more importantly, of economists and political scientists. We are not used to do this. Our disciplines thrive by having congresses and symposia where we essentially talk to each other and by writing papers for journals that are edited by our direct colleagues.
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Second bdown to localQ: once international treaties have been agreed upon, they have to be followed by implementation on national or local levels. This, again, requires input from science but now on a level that recognizes local conditions and is prepared to fine-tune its approach accordingly. Also here, work is ideally done by interdisciplinary teams. What should be done to act more effectively at international and local level? Do we need different approaches or can we be satisfied with the way we work right now? And would it, in this context, be advisable to join forces of pedology and hydrology or can we continue to operate independently as in the past? Before these questions will be addressed, attention is paid to one important and common element of interdisciplinary activity that is often overlooked: working at different levels of detail.
3. Working at different levels of detail Scientific work is usually focused on the frontiers of science using advanced equipment and methodologies, as this provides the best opportunities for publication of results in prestigious international journals. Pedology and hydrology are no exception to this fact of scientific live. Such specialistic input is, however, not always effective when working in international panels or in interdisciplinary teams that have to implement environmental legislation on the national or regional level. Partners in the panels or the teams will not understand the jargon involved and they certainly will not be able to see such highly specialized work in a broader context. This, obviously, is detrimental to mutual communication which, in turn, is crucial for effective interdisciplinary work. What to do? Bouma (1997b, 2001a) has suggested a step-by-step approach which starts from the assumption that after a hundred years of soil and water research there is an answer for any question that can possibly be raised. Most likely, for many modern issues, the instant answer based on experience will be a poor one, but this offers the possibility to suggest new research or a review of literature that will improve the answer. Always, a certain cost is involved and this cost will steadily increase as the information becomes, step-by-step, more sophisticated. But some issues being raised in an interdisci-
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plinary context require only generalized, empirical data from any given discipline. Then, supply of sophisticated data may represent overkill (e.g., Bouma, 1993). Other issues require more sophisticated data but by starting even then with the most simple input and by discussing this with stakeholders and colleagues from other disciplines and by identifying gaps in knowledge, a joint learning trajectory is introduced that is crucial for creating true cooperation and interchange. We may call this the salesman principle: interaction with potential buyers may follow once his foot does not allow the door to be closed! Scientists playing such a role as a salesman should be prepared to invest time, creativity and energy to establish such communication. The Hoosbeek and Bryant diagram has been used by Bouma (1997b, 2001a) to illustrate this process, considering expressions of know-how in two dimensions, ranging from qualitative to quantitative on one dimension and from empirical to deterministic on the other. In addition, the analysis was made for different scale levels ranging from the microscopic to the world at large (Fig. 1). Different knowledge levels were distinguished: K1—user experience; K2—expert knowledge, based mainly on experience; K3—same, but also including measurements and use of simple (statistical) models; K4—knowledge based on measurements and deterministic models for entire systems, and K5—knowledge based on complex, specialized measurements and models for certain aspects only. Bouma (1997b) illustrated the procedure by defining four increasingly detailed and costly procedures to determine the vulnerability of European Soils for acidification. Most likely, the error associated with these procedures will decrease as the procedures become more detailed, even though this may not be the case because of natural variability. This natural variability, to be characterized by (geo)statistical procedures, may be so high that highly detailed experiments at point level produce only pseudoaccuracy. But even if work at higher K levels results in lower error, we still have to balance this lower error against the associated costs that increase strongly as the knowledge level increases. Rather than jump right away to a given type of (often detailed) research at K5 level, it pays to start simple and follow a gradual and systematic (bstep-by-stepQ) buildup. Along these lines, Bouma and Droogers (1999) presented a case study
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Fig. 1. Procedures studying water movement at the landscape level combining different types of knowledge, applied at different spatial levels, and represented for pedology and hydrology separately and for hydropedology as an interdisciplinary activity.
on the soil moisture supply capacity, distinguishing five K levels. When members in panels or teams, as described above, can present their input for their discipline at different degrees of detail, there is a higher probability for effective communication and output as compared with a condition where every discipline presents data at the K5 level only, or if some disciplines offer data at K3 level and others at K5 level etc. This example applies to the widely used successful DSSAT models that simulate soil water movement and crop growth (e.g., Bouma and Jones, 2001). Here, emphasis has been on aspects of plant growth, leading to ever more specific (K5) representations of crop development. At the same time the original soil water module (of the K3 tipping-bucket type) was not changed and this has resulted in the end in a somewhat unbalanced model where soil and hydraulic aspects are rather poorly represented. To be clear, only soil scientists are to blame here for the fact that soil expertise is not adequately represented in the model. Such examples of unbalanced model representations are particularly relevant for pedology and hydrology. To many blue-blooded hydrologists, activities of pedologists are difficult to judge from a scientific point of view. In their view, pedologists use bfunnyQ names to describe soils and they make too many empirical statements about soil behavior which are not supported by solid measurements. On the other hand, pedologists are taken back by the representation of natural soils in terms of homogeneity and isotropy that hydrologists and soil physicists need when
running model representations of soils: this clearly does not reflect real conditions being experienced in the field. This certainly applies to regional hydrological models where pixel-data often cannot reflect occurrence of various soil horizons and effects of soil patterns. One important task of the hydropedology working group of SSSA is to debunk the above stereotype visions on both sides of the fence. Pedology has its roots in soil survey which considers landscape processes and soil structure descriptions that have been somewhat neglected in the period in which soil classification received most attention. These two aspects are important for soil physics and hydrology to improve their quantitative characterization of flow regimes in the field of watersheds with undisturbed soils. At the same time, pedologists need to benefit from flow theory when transforming their qualitative descriptions into quantitative expressions, which is increasingly necessary to answer modern policy questions. Combining pedological and hydrological expertise can be particularly attractive when presenting soil information to panels and teams as discussed: pedology has much useful descriptive information at the K1, K2 and K3 level (see illustration in Fig. 1 where the characterization of water movement at landscape level is visualized). At the same time, hydrology and soil physics have much process and modeling information at K3, K4 and K5 level (Fig. 1). Combining both implies that the entire K-range is covered whereby hydrology would benefit from pedological characterizations and pedology would
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benefit from hydrological flow theory. The assumption that the joint package would be more powerful than the two parts by themselves will now be further explored.
4. Hydropedology: more than the sum of the two disciplinary parts? Some examples will be described that illustrate the benefits of a combined hydropedological approach. Selection of examples from the work of our own research group, as presented below, does certainly not imply that relevant work has not been done elsewhere, but allows illustrations to be specific and based on experience. McKenzie et al. (1991) and Bruand (1990) also relate soil morphology to soil water properties. 4.1. Bypass flow Sprinkling irrigation of heavy clay soils next to the river Rhine in The Netherlands caused problems in the seventies of the last century, because water applications in summer did not result in calculated soil water contents that agreed with measured values. In fact, differences were very large. Calculations suggested field capacity while measurements indicated wilting point. The reason was bypass flow which is the vertical movement of free water along the walls of macropores (in this case shrinkage cracks) through an otherwise unsaturated soil matrix. Physical flow theory did not allow for this: flow is either saturated or unsaturated and soils are implicitly considered to be homogeneous and isotropic. The most recent Methods of Soil Analysis by the Soil Science Society of America (Dane and Clark, 2002) still reflects this, although bypass flow is considered now for the first time (Booltink and Bouma, 2002). Application of blue-stained water at different rates and quantities and observation of stained flow patterns in the soil along the cracks allowed us to define critical flow rates and quantities that could be adsorbed by the peds on the soil surface, avoiding bypass flow (Bouma and Dekker, 1978). Also, a simple field technique was devised to measure bypass flow as a function of rain intensity and quantity and the soil moisture content (e.g., Bouma, 1997a). Of course, cracking of clay
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soils was already well known before our work, but deriving cracking patterns theoretically from physical soil swell-shrink characteristics turned out to be impossible because of the sensitivities involved: very small pores, with a volume that cannot reliably be measured with physical methods, can conduct large volumes of water. A procedure whereby macropores are morphologically studied in the field, preferably functionally characterized by staining, followed by modeling whereby bypass flow is incorporated as a separate module in existing physical flow models, represents an effective combined procedure of hydropedology that avoids purely qualitative descriptions by soil morphology (K2) and irrelevant model representations by soil physics (K4) (see Hoogmoed and Bouma, 1980). 4.2. Saturated hydraulic conductivity (K-sat) K-sat is an important physical soil characteristic: it defines the flux under saturated conditions at unit hydraulic gradient. Many measurement methods are available (Soil Science Society of America, 2002) but they still ignore the effect of the volume of samples and pore-continuity of interpedal voids. Soil survey information can be used to improve estimation of Ksat (e.g., McKenzie and Jacquier, 1997). Taking a quantitative approach, Anderson and Bouma (1973) showed that any K-sat could be measured in the B2t of a Wisconsin Hapludalf by varying the height of the sample. This was due to the well-developed blocky structure: vertical continuity of the cracks between the peds decreased as the sample became longer, resulting in lower K-sat values even though the sample was completely saturated in all cases and flow occurred at unit hydraulic gradient. Two applications of this insight materialized: (i) morphological measurement of crack patterns could be used to calculate K-sat, using a vertical pore-continuity model, and (ii) representative elementary volumes of samples were defined as a function of ped sizes. Standard volumes of samples are incorrect; sample volumes should be a function of the size of the peds: the larger the peds, the larger the samples. The pore-continuity model was refined later using micromorphological studies and here we showed that very small pore-necks, immeasurable in terms of volumes by soil physical methods, governed K-sat in prismatic clay soils, where big
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cracks occurred when the soil was dry (Bouma et al., 1979). Comparable studies were made for large vertical worm and root channels, showing big differences between measured K-sat values, comparing measured values in large columns that were either attached to the subsoil (K-sat: 0.5 m/day) or detached (50 m/day). Of particular interest were K-sat values measured in soils with macropores with a light crust. The soil matrix was then still saturated, as evidenced by a pressure head of 0 kPa throughout, but macropores were filled with air. Theoretically, the measured flux was still a K-sat value, but much lower than the other ones (at 0.05 m/day) (see Bouma, 1997a and Fig. 2). A study was made in a silty clay soil with glossic features, where samples in the bleached cracks yielded a K-sat of 6.9 m/day, while values were 0.3 m/ day inside the compact peds (Fig. 2) (Bouma et al., 1989a,b). Placing samples at random in this soil leads to a very high variability that cannot be reduced by applying statistics, but only by making a morphological analysis before samples are taken and by systematic subsampling within the two populations. The conclusion can be that studying soil structure with morphological methods is important when choosing the proper measurement method for K-sat, again demonstrating a powerful combination of pedology (K2) and hydrology (K4) expertise.
4.3. Where is the water table? When working in pedal soils in The Netherlands, measurement of water table depth has often presented problems. Levels fluctuate wildly, particularly after rainfall while levels observed in piezometers differed significantly at very short distances. Also, tensiometers can produce different results at short distances. Bouma et al. (1980) studied this phenomenon, showing the effects of water flowing along cracks along the faces of peds, not only in the unsaturated zone of the soil but also in the saturated zone (Fig. 3). After rainfall the water level in the cracks rises very rapidly to subside slowly as water moves slowly into the surrounding, unsaturated peds. When piezometers intercept these cracks through which the free water moves, they show high fluctuations. When piezometers are inside the peds, they hardly move at all, unless they are not well sealed on the outside which is likely to result in vertical flow along the cracks from the soil surface into the piezometer, incorrectly suggesting a bperchedQ water table inside the (unsaturated) peds. The field practice in the seventies was to observe water table levels in unlined boreholes. This resulted in much confusion because a bwater table levelQ was observed at the bottom of each unlined augerhole,
Fig. 2. Illustrations of using soil morphological structure descriptions for improving physical measurements of K-sat, including (from left to right) effects of sampling volume (A), use of glossic features (B), and effects of large-pore continuity (C).
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Fig. 3. Possible effects of pedality on measuring water table levels with boreholes and piezometers and on measuring soil water potentials with tensiometers.
every augerhole producing a different bwater table levelQ as water moved along air-filled cracks into the hole while the surrounding soil was unsaturated. Use of lined piezometers solved this problem. When the water moves out of the cracks, the remaining peds may remain saturated for a while, suggesting overall saturation of the soil as measured by tensiometry, which does, in fact, not occur as the surrounding cracks are already filled with air. Once this mechanism is understood, much bvariabilityQ can be explained. Another problem can occur when tensiometers are placed with a downward angle into a vertical profile wall. This may induce flow along the sides, incorrectly suggesting saturation as the water reaches the cup of the tensiometer. Placement under a slight upward angle avoids this problem. Understanding the flow regime, as influenced by cracking patterns, helps to explain what would otherwise by highly confusing hydrological measurement results and can contribute to better instrument design and measurement protocols. Thus, pedology expertise helps to guide and interpret physical measurements,
again combining K2 expertise of pedology with K4/ K5 expertise of soil physics/hydrology. 4.4. Water accessibility Soils with large compact peds, such as prisms and clods formed by tillage under adverse conditions, often show concentrations of roots at the ped surface, indicating that the roots were unsuccessful in penetrating the peds or clods. Field conditions have been observed where plants were wilting even though water contents of the root zone were well above wilting point. This phenomenon has been attributed to limited accessibility. Modeling studies, implicitly assuming unlimited accessibility, yielded poor results. Droogers et al. (1997) studied these processes in large undisturbed field samples and they defined accessibility as a function of ped sizes. This, in turn, could be incorporated in existing simulation models for plant growth, again demonstrating the positive effects of combining pedological expertise (K2) with hydrological process knowledge (K5 in this case).
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4.5. Hydrophobicity The work of Dekker and Ritsema has demonstrated the importance of hydrophobicity on soil water movement in The Netherlands. When ignored, simulations of water regimes which implicitly assume onedimensional homogeneity, produce poor results (Dekker and Ritsema, 2003). Dekker et al. (1984) showed that assumed lateral flow of water on top of a compact spodic subsurface horizon in a nature area in The Netherlands did not occur but that lateral movement of water was due to surface runoff originating from hydrophobicity of the soil surface. Extensive field studies, covering the entire country, have shown that most soils, as distinguished by the national soil survey, are susceptible to hydrophobicity under dry conditions, but land use history is an important factor as well. A hydrophobicity module has been added to the regular soil–water flow model, allowing realistic simulations for water movement for soils that exhibit this phenomenon (Ritsema et al., 2001). When simulations are to be made of soil–water movement of field soils, first a screening is made as to whether or not soils are hydrophobic. If so, the particular flow routine is included. This is another example where pedological expertise, which here relates to soil surveys covering the entire country and simple field measurements of hydrophobicity (K3), are crucial input for soil–water simulations (K4 and K5), yielding a much better result than either of the two disciplines could have produced on its own. 4.6. Pedotransfer functions Pedotransfer functions relate simple soil characteristics to be found in soil surveys to more complex parameters that are used in modeling and that are relatively difficult to measure (e.g., Bouma, 1989, McBratney et al., 2002). Many publications have appeared on this topic, particularly for hydraulic conductivity and moisture retention in soil physics, illustrating the significance of the combined approach of hydropedology (e.g., Wo¨sten et al., 2001). But soil– chemical pedotransfer functions have been derived as well (Breeuwsma et al., 1986). Nemes (2002) has shown that simulations of water regimes for Hungarian soils could only be made thanks to the availability of pedotransfer functions. Lack of funds would not
have allowed extensive measurement campaigns. Still, there is a clear risk here. Uncritical application of automatically generated pedotransfer functions is likely to produce poor results when made by non-soil scientists with no feeling for what may broadly be expected or when the functions are derived from data obtained for other soils than the ones being characterized. McBratney et al. (2002) broadened the concept to soil inference systems where pedotransfer functions form knowledge rules for inference engines, including uncertainty analysis. This is a promising future approach which extends the concept beyond the derivation of moisture retention and hydraulic conductivity data which is the most common interpretation of pedotransfer functions so far. 4.7. Redox features as wetness indicators In the above examples, soil morphology or other data from soil surveys were used to improve the physical characterization of soil water regimes, usually by deterministic modeling. There are also examples where soil morphological data uniquely characterises flow regimes that would be very difficult to document with soil physical or hydrological techniques. Examples are moisture regimes in soils with peds or with periodic bperchedQ water tables on top of slowly permeable subsurface soil horizons, where occurrence of characteristic grey reduction and iron oxidation patterns can provide important clues as to flow patterns during the year. They can be quite helpful when placing monitoring or measurement equipment in soils in the field (e.g., Bouma et al., 1989a,b). Much has been published on this topic and the reader is referred to literature for more examples. 4.8. Flow processes in landscapes So far, examples have been discussed related to the dynamic behaviour of soils. Pedology has, of course, also a strong landscape dimension which is expressed by soil maps. To the uninitiated user, such maps may suggest occurrence of adjacent homogeneous soil bodies that differ from each other. This, of course, is not the case. Different mapping units have characteristically different internal variabilities (e.g., Marsman and de Gruijter, 1983) which are increasingly being
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reported in survey reports, using statistical and geostatistical techniques that allow transformation of point to area data (Brus and de Gruijter, 1997). Finke (2000) compared the increase of quality when incorporating variability data with cost involved. When considering soil physical data, we often find that pedological differences do not necessarily correspond with differences in soil physical behaviour. For example, Wo¨sten et al. (1985) reported that only 30% of the different land units on a given soil map also behaved differently from a soil physical point of view, allowing combination of mapping units into bfunctional unitsQ. Hydrological landscape models work with pixels, each to be fed with typical, representative data. The surface relief can now be well expressed in great detail using data from Digital Terrain Models, fed by satellite data (e.g., Schoorl et al., 2002). Infiltration characteristics at the surface are often derived from soil survey data providing texture and organic mater contents which allow estimates to be made when measurements are not available or feasible. Some potential pitfalls of that procedure have been mentioned above when discussing K-sat. After infiltration, however, water never follows flow patterns as would occur in a homogeneous and isotropic porous medium. Occurrence of slowly permeable or irregular subsoil horizons or geologic formations usually strongly alters flow patterns. Here, again, pedology can have important input in improving the quality of hydrological models in areas of land, such as watersheds. Several papers are appearing now in the hydrology literature that start to take account of subsurface pedological horizons (e.g., Rosney et al., 2002; Chamram et al., 2002; Wullschleger et al., 2001).
5. The throbbing landscape as a starting point for research The above examples indicate that a combination of pedological and hydrological expertise is particularly valuable for the realistic characterization of soil water regimes in landscapes. Empirical observations by pedologists can significantly improve the quality of parameter measurements by hydrologists, to be used in modelling. In turn, interpretations of soil surveys
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can thereby be improved by adding quantitative expressions, including bwhat. . .if Q explorations, to the usual qualitative, empirical characterizations in pedology in terms of relative limitations for certain types of land use. How can this be effectively presented in the interdisciplinary panels and teams mentioned above? Traditionally, our disciplines are not prepared to make a special effort here as we implicitly assume that our maps and modelling results will find their way if they are good. This, perhaps unfortunately, is not true anymore. Citizens of today and politicians have a short attention span and react to ever more intense and extraordinary impulses. This is not only relevant for commercials but also for reporting the results of scientific studies. Images are important and those images have to be bsoldQ. Fortunately, in hydropedology, pedology and hydrology can together create images that are beyond reach for each individual discipline by itself. This we like to present in terms of a bthrobbingQ landscape. In fact, water fluxes into and through soils in a landscape are the essence of live and resemble, in a way, the manner in which blood circulates in a human body. We could even compare blood pressure with the pressure potential of water in soil: when it is too high or too low soil functioning is clearly hampered. We could therefore speak of a throbbing landscape in which water enters and leaves, on an hourly, weekly, monthly and yearly basis. Of course, not only water governs soil functioning. But once water regimes have been characterized, chemical and biological processes can be added as they strongly depend on the water regime and on interaction processes with the soil. Using modern ICT (Information and Communication Technology) routines allows visualizations that can help to convey messages on limitations or potentials that are associated with natural and man-induced water regimes in landscapes. What are the fluxes in a dry year? A wet year? What does that imply for different forms of land use? How does or can human management affect these fluxes? etc. Such integrated information is needed for the interaction and negotiation in the panels and teams mentioned earlier. This also works well in Dutch planning which considers the blayer modelQ consisting of three blayersQ: the first one represents the natural dynamics
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of land and water; the second one all networks of roads, railways and waterways and the third one: settlements. Ideally, new land use plans should consider the sequence from one to three, considering first the dynamics of land and water which is most difficult to affect or should not be affected in sensitive areas where substantial damage could occur. Next, infrastructure networks have a higher degree of permanence than settlements which readily expand and contract (e.g., Metropolitan Debate, 1998). This approach offers an attractive platform for applied hydropedology, working with geologists as well. The manner in which the natural landscape bthrobsQ offers clues as to bwhatQ can best be done bwhereQ with the lowest risks and the greatest opportunities.
6. Challenges ahead A broad well-planned discussion is needed to formulate the most important challenges for hydropedology research in future. The new SSSA working group on hydropedology provides a useful platform for this. For a start, two procedural approaches to research are mentioned here that we believe are relevant for hydropedology in future. A listing of future environmental issues as such is beyond the scope of this text and can be found in documents of environmental agencies, policy think-tanks and research institutions. 6.1. Incorporating effects of management Pedology is focused on natural processes and does not reflect effects of short-term soil management in soil classification. This was done on purpose to avoid frequently changing classifications of a given soil following different types of soil management, such as tillage and fertilisation. Exceptions are, for example, Plaggen soils in Europe, that were formed for periods of over 900 years by adding each year a thin layer of animal manure mixed with local soil. The manure added fertility in centuries when no chemical fertilizer was available, while the added local soil raised the soil slowly by, on average, 1 mm a year. This resulted in 90-cm thick surface horizons to be observed now that are classified as such, even though they resulted from management. Similar examples can be given for
rice soils in Asia. When dealing with issues of land use in future, however, effects of more short-term soil management which are not expressed by soil classification are very important, certainly when defining different types of land use than the current ones, a common process in land use planning and negotiation. We expect that different soil types will react characteristically different to particular types of management, but we do not really know this as effects of different types of management on different types of soil has never been a general topic in soil research. In The Netherlands two major soil series were investigated to establish effects of different types of short-term soil management on longer term soil properties for cases where this land management did not result in a change of soil classification. Droogers and Bouma (1997) suggested the term genoform for the genetic soil type and different phenoforms of that particular genoform to express the effects of different types of management. Pulleman et al. (2000) worked this out for a Typic Fluvaquent and Sonneveld et al. (2002) for a Typic Haplorthod. In order to find these relationships, extensive field studies were made where existing soil maps were used to determine locations where the particular genoform could be found. Next, farmers were interviewed, land use history was reconstructed and soil properties, including hydraulic soil characteristics, were measured. Next, a limited number of significantly different phenoforms were identified and regression analysis was used to express relationships between land use and soil properties for a given soil series. Sonneveld et al. (2002) added an additional element by also including landscape features. This work showed that much soil information, to be gained by field research, is as yet waiting to be discovered in the field. Obviously, the type of information gained this way is impossible to obtain with classical field experiments: it would take decades of experimental work for which no funding is available. The alternative is, of course, modelling but the error and uncertainties involved here are quite large and the relatively simple procedure of selectively sampling the vast amount of documentation that is out there to be explored, is tempting. This has the important side effect of requiring field work which increasingly is being substituted by laboratory and office activity, breaking the liveline that pedology always had with bthe fieldQ. Back to the field!
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6.2. Pro-active design Somehow, most work in pedology and hydrology has been rather reactive in character. Either questions raised by others needed to be answered or given conditions were characterized, more often than not representing problems caused by poor land use. This has, of course, allowed excellent research but why not also take an occasional more pro-active approach? Why not take a given soil material, consider climate conditions and design a soil structure that would best satisfy conflicting demands, for instance, a structure that would allow optimal rooting, would supply a relatively high amount of moisture, would be trafficable, would avoid bypass flow of agro-chemicals etc. Bouma et al. (1999) have made an attempt to do this. Such work provides an excellent basis for joint work of pedologists and hydrologists. The focus could also be shifted to the landscape scale, taking into account the blayerQ model mentioned above. As is, we dutifully document errors that engineers and architects have made. Why not design optimal soil and landscape structures based on comparing effects of different flow patterns and deliver these to engineers and architects with an invitation to realize them in practice?
References Anderson, J.L., Bouma, J., 1973. Relationships between hydraulic conductivity and morphometric data of an argillic horizon. Soil Sci. Soc. Am. Proc. 37, 408 – 413. Booltink, H.W.G., Bouma, J., 2002. Bypass flow. In: Dane, J., Clark, T. (Eds.), Methods of Soil Analysis: Part 4. Physical Methods, vol. 5. SSSA, Madison, WI, pp. 930 – 937. Bouma, J., 1989. Using soil survey data for quantitative land evaluation. In: Stewart, B.A. (Ed.), Advances in Soil Science, vol. 9. Springer Verlag, New York, pp. 177 – 213. Bouma, J., 1993. Soil behaviour under field conditions: differences in perception and their effects on research. Geoderma 60, 1 – 15. Bouma, J., 1997a. Long-term characterization: monitoring and modelling. In: Lal, R., Blum, W.H., Valentin, C., Stewart, B.A. (Eds.), Methods for the Assessment of Soil Degradation. Advances in Soil Sci., pp. 337 – 358. Bouma, J., 1997b. Role of quantitative approaches in soil science when interacting with stakeholders. Geoderma 78, 1 – 12. Bouma, J., 2001a. The new role of soil science in a network society. Soil Sci. 166, 874 – 879. Bouma, J., 2001b. The role of soil science in the land negotiation process. Soil Use Manag. 17, 1 – 6.
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Bouma, J., Dekker, L.W., 1978. A case study on infiltration into dry clay soil: I. Morphological observations. Geoderma 20, 27 – 40. Bouma, J., Droogers, P., 1999. Comparing different methods for estimating the soil moisture supply capacity of a soil series subjected to different types of management. Geoderma 92, 185 – 197. Bouma, J., Jones, J.W., 2001. An international collaborative network for agricultural systems applications (ICASA). Agric. Syst. 70, 355 – 368. Bouma, J., Jongerius, A., Schoonderbeek, D., 1979. Calculation of saturated hydraulic conductivity of some pedal clay soils using micromorphological data. Soil Sci. Soc. Am. J. 43, 261 – 264. Bouma, J., Dekker, L.W., Haans, J.C.F.M., 1980. Measurement of depth to water table in a heavy clay soil. Soil Sci. 130, 264 – 270. Bouma, J., Fox, C.A., Miedema, R., 1989. Micromorphology of hydromorphic soils: applications for soil genesis and land evaluation. In: Douglas, L.A. (Ed.), Soil Micromorphology: A Basic and Applied Science. Developments in Soil Science, vol. 19, pp. 257 – 279. Bouma, J., Jongmans, A.G., Stein, A., Peek, G., 1989. Characterizing spatially variable hydraulic properties of a boulder clay deposit. Geoderma 45, 19 – 31. Bouma, J., Droogers, P., Peters, P., 1999. Defining the bidealQ soil structure in surface soil of a Typic Fluvaquent in The Netherlands. Soil Sci. Soc. Am. J. 63, 343 – 348. Breeuwsma, A., Wosten, J.H.M., Vleeshouwer, J.J., van Slobbe, A.M., Bouma, J., 1986. Derivation of land qualities to assess environmental problems from soil survey. Soil Sci. Soc. Am. J. 50, 186 – 190. Bruand, A., 1990. Improved predictions of water retention properties of clayey soils by pedological stratification. J. Soil Sci. 41, 491 – 497. Brus, D.J., de Gruijter, J.J., 1997. Random sampling or geostatistical modelling? Choosing between design-based and model-based sampling strategies for soil (with discussion). Geoderma 80, 1 – 44. Chamram, F., Gessler, P.E., Chadwick, O.A., 2002. Spatially explicit treatment of soil-water dynamics along a semi arid catena. Soil Sci. Soc. Am. J. 66, 1571 – 1583. Dane, J., Clark, T. (Eds.), 2002. Methods of soil analysis: Part 4. Physical methods. SSSA Book Series, vol. 5. Soil Sci. Soc. Am. Inc., Madison, WI. Dekker, L.W., Ritsema, C.J., 2003. Wetting patterns in water repellent Dutch soils. In: Ritsema, C.J., Dekker, L.W. (Eds.), Soil Water Repellency. Elsevier, Amsterdam, pp. 151 – 166. Dekker, L.W., Wosten, J.H.M., Bouma, J., 1984. Characterizing the soil moisture regime of a Typic Haplohumod. Geoderma 34, 27 – 42. Droogers, P., Bouma, J., 1997. Soil survey input in exploratory modelling of sustainable land management practices. Soil Sci. Soc. Am. J. 61, 1704 – 1710. Droogers, P., van der Meer, F.B.W., Bouma, J., 1997. Water accessibility to plant roots in different soil structures occurring in the same soil type. Plant Soil 188, 83 – 91. Finke, P.A., 2000. Updating groundwater table class maps 1:50000 by statistical methods: an analysis of quality versus cost. Geoderma 97, 329 – 350.
286
J. Bouma / Geoderma 131 (2006) 275–286
Hoogmoed, W.B., Bouma, J., 1980. A simulation model for predicting infiltration into cracked clay soil. Soil Sci. Soc. Am. J. 44, 458 – 461. Marsman, B.A., de Gruijter, J.J., 1983. Quality of soil maps: a comparison of survey methods. Soil Survey Papers, vol. 15. Netherlands Soil Survey Institute, Wageningen, The Netherlands. McBratney, A.B., Minasny, B., Cattle, S.R., Vervoort, R.W., 2002. From pedotransfer functions to soil inference systems. Geoderma 109, 41 – 73. McKenzie, N.J., Jacquier, D.W., 1997. Improving the field estimation of saturated hydraulic conductivity in soil survey. Aust. J. Soil Res. 35, 803 – 825. McKenzie, N.J., Smetten, K.R.J., Ringrose Voase, A.J., 1991. Evaluation of methods for inferring air and water properties of soils from field morphology. Aust. J.Soil Res. 29, 587 – 602. Metropolitan Debate, 1998. In: de Hoog, Maurits (Ed.), Laag Land. dRO, Amsterdam (Dienst Ruimtelijke Ordening), Desing team Stad. Amsterdam (in Dutch). Nemes, A., 2002. Unsaturated soil hydraulic database of Hungary. HUNSODA. Agrokem. Talajtan. 51, 17 – 26. Pulleman, M.M., Bouma, J., van Essen, E.A., Meijles, E.W., 2000. Soil organic matter content as a function of different land use history. Soil Sci. Soc. Am. J. 64, 689 – 694. Ritsema, C.J., van Dam, J.C., Dekker, L.W., Oostindie, K., 2001. Principles and modelling of flow and transport in water repellent surface layers and consequences for management. Intern. Turfgrass Soc. Res. 9, 615 – 623.
Rosney, P., de Polcher, J., Bruen, M., Laval, K., 2002. Impact of a physically based soil water flow and soil–plant interaction representation for modeling large scale land surface processes. J. Geophys. Res., [Atmos.], 107 – 112. Schoorl, J.M., Veldkamp, A., Bouma, J., 2002. Modeling water and soil redistribution in a dynamic landscape context. Soil Sci. Soc. Am. J. 66, 1610 – 1619. Soil Science Society of America, 2002. Methods of soil analysis: Part 4. Physical methods. In: Dane, J.H., Topp, G.C. (Eds.), Soil Sci. Soc. Am., Madison, WI. Sonneveld, M.P.W., Bouma, J., 2003. Methodological considerations for nitrogen policies in the Netherlands including a new role for research. Environ. Sci. Policy 6 (6), 501 – 511. Sonneveld, M.P.W., Bouma, J., Veldkamp, A., 2002. Refining soil survey information for a Dutch soil series using land use history. Soil Use Manag. 18, 157 – 163. Wo¨sten, J.H.M., Bouma, J., Stoffelsen, G.H., 1985. The use of soil survey data for regional soil water simulation models. Soil Sci. Soc. Am. J. 49, 1238 – 1245. Wo¨sten, J.H.M., Pachepsky, Ya.A., Rawls, W.J., 2001. Pedotransfer functions: bridging the gap between available basic soil data and missing soil hydraulic characteristics. J. Hydrol. 251, 123 – 150. Wullschleger, S.D., Jackson, R.B., Currie, W.S., Friend, A.D., Luo, Y., Mouillot, F., Pan, Y., Shao, G.F., 2001. Below-ground processes in gap models for simulating forest response to global change. Climate Change 51, 449 – 473.