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Dep. of Crop and Soil Sciences, 116 ASI Building, The Pennsylvania State University, University Park, PA 16802
* Corresponding author (henrylin{at}psu.edu)
Received 20 June 2002.
ABSTRACT
There is a growing recognition that synergy could be generated by bridging traditional pedology with soil physics and hydrology to enhance integrated studies of soil–water relationships across spatial and temporal scales. Hydropedology is suggested as such a bridge to address: (i) knowledge gaps between pedology, soil physics, and hydrology; (ii) multiscale bridging from microscopic to mesoscopic and macroscopic levels; and (iii) data translations from soil survey databases into soil hydraulic information. Knowledge gaps include flow and transport in the structured unsaturated zone, soil structure quantification, preferential flow modeling, landscape hydrology, soil spatial and temporal variability, quantitative use of field soil morphology for inferring soil hydrology, mechanisms controlling individual and interactive soil–water processes at multiple scales, pedotransfer functions (PTFs), and others. Hydropedology integrates the pedon and landscape paradigms to link phenomena occurring at microscopic (e.g., pores and aggregates), mesoscopic (e.g., pedons and catenas), and macroscopic (e.g., watersheds, regional, and global) scales. Through approaches such as PTFs, hydropedology also facilitates the bridging of data between soil survey databases and soil hydraulic information needed in simulation models. The bridging of disciplines, scales, and data represents potentially unique contributions of hydropedology to integrated soil and water sciences. It is hoped that hydropedology would contribute to our enhanced understanding of a variety of environmental, ecological, agricultural, and natural resource issues of societal importance. These include water quality, soil quality, landscape processes, watershed management, nutrient cycling, contaminant fate, waste disposal, precision agriculture, climate change, and ecosystem functions.
Abbreviations: GIS, geographic information systems • NRC, National Research Council • PTF, pedotransfer function • REA, representative elementary area • REV, representative elementary volume
IT IS WELL RECOGNIZED that the progress of science depends increasingly on an advanced understanding of the interrelationships among different fields and their components (American Association for Advancement of Sciences Council, 2001). In addressing present directions and future research in vadose zone hydrology, Jury (1999) pointed out that the toughest problems require interdisciplinary research. Bouma et al. (1999), discussing the changing paradigm of agricultural research in relation to precision agriculture and pedology, indicated that new interdisciplinary approaches are needed to improve the understanding of complex interactions between multiple factors affecting crop production and farm decision-making. In reviewing three families of statistically based models of soil variation developed since the mid 1960s, Heuvelink and Webster (2001) suggested that a joint effort of scientists with varied backgrounds is required if we are to translate conceptual models of soil formation (such as the State Factor Model of Jenny, 1941) into operational mathematical formulae. A number of recent reports of the National Research Council (NRC) also highlighted the significance of integrated soil and water studies in the context of agriculture (NRC, 1993a), groundwater vulnerability (NRC, 1993b), watershed management (NRC, 1999), earth sciences (NRC, 2001a), water resources (NRC, 2001b), and environmental sciences (NRC, 2001c).
To address diverse soil and water issues at various spatial and temporal scales, it becomes clear that bridging traditional pedology with soil physics, hydrology, and other related disciplines is necessary as well as synergistic. This bridging is justified not only by the interrelationship among the disciplines but also by the complex nature of the problems. This review attempts to examine the differences and connections between pedology, soil physics, and hydrology and suggests that hydropedology is a natural linkage among them. The bridging of disciplines, scales, and data is discussed here as an important component for developing synergistic and integrative hydropedology.
PEDOLOGY, SOIL PHYSICS, AND HYDROLOGY
Traditionally, pedologists have focused on field soil profiles (pedons) as observed in the landscape, soil physicists have emphasized theoretical studies and laboratory investigations using small soil samples (only in recent decades has a shift to the field regime begun), and hydrologists have most often been concerned with landscape or watershed-scale processes. There are also distinct differences in the methods of investigations among these disciplines. Pedologic studies traditionally have been observational and descriptive. Recently, more attention has been given to quantitative methods such as pedogenesis modeling (Hoosbeek and Bryant, 1992) and pedometrics (McBratney et al., 2000). Soil physical and hydrologic studies, on the other hand, emphasize instrumentation and mathematical modeling. Nevertheless, pedologists, soil physicists, and hydrologists share many common interests and have mutually benefited from each other's work. For example, through their knowledge of soil–landform relationships and the principles of geology and geography, pedologists have developed soil-forming theories and established soil classification systems that provide an overall framework for our understanding of global soil resources. Wilding (2000) and others noted that pedologists have studied soil moisture and temperature regimes in various soil taxonomic units (e.g., Soil Survey Staff, 1999), identified the occurrence and distribution of water and root restrictive layers such as fragipans (e.g., Calmon et al., 1998), documented cracking and fissuring patterns in soils (e.g., Vertisols) and saprolites that impact bypass flow (e.g., Lin et al., 1996; Li et al., 1997), identified systematic vs. random spatial variability fundamental to sampling design efficiency (e.g., Wilding and Drees, 1983), and utilized soil color patterns (e.g., redoximorphic features) to infer soil aeration and moisture regimes (e.g., Veneman et al., 1998). Soil physicists and hydrologists apply the principles of physics and hydrology to the characterization and quantification of soil physical and hydrologic properties and processes that are relevant to soil morphology, genesis, classification, and mapping. Soil physicists have been leaders in measuring and modeling processes that take place a few meters above and below the earth's surface (Sposito and Reginato, 1992), and hydrologists have been experts in hydrologic cycles (NRC, 1991). Soil physicists and hydrologists have elucidated water flow through soils and over the landscape, and have made significant progress in monitoring and modeling soil moisture, heat, and gas fluxes in soil profiles and through the soil–plant–atmosphere continuum. Their work contributed to the enhanced understanding of landscape hydrology, hillslope dynamics, catena distribution, wetland functions, soil hydromorphology, nutrient transport, onsite waste disposal, and many other issues that are of increasing interests to pedologists (e.g., Rabenhorst et al., 1998; Richardson and Vepraskas, 2001). Soil physicists' and hydrologists' efforts also have encouraged better utilizations of soil survey databases (e.g., Bouma, 1989; Wagenet et al., 1991; Rawls et al., 2001).
Recent literature and professional activities have suggested that synergy could be generated by linking pedology with soil physics and hydrology. For example, Nielsen et al. (1998), in discussing emerging technologies for scaling field soil water behavior, expected that new paradigms for local and regional scales of homogeneity in pedology would emerge, with soil mapping units containing spatial and temporal soil water scale factors. Wilding et al. (1994) also made this point in an earlier publication, although they emphasized that one can utilize the spatial variability in landscape mapping units to advantage in building PTFs. Kutílek and Nielsen (1994) pointed out that models of soil porous systems describing flow and transport phenomena should properly mimic the morphological reality of the soil and the classification used in soil macro- and micromorphology. Through collaborative efforts of soil physicists and pedologists, Quisenberry et al. (1993) suggested a soil classification scheme using soil surface texture, subsurface clay mineralogy, and subsoil structure to characterize water movement and chemical transport through soils in South Carolina. Lin et al. (1996)(1997, 1998), Vervoort et al. (1999), Shaw et al. (2000), and others have illustrated close relationships between soil structure and preferential flow as a result of joint efforts by pedologists, soil physicists, and hydrologists. The compilations of articles by Rabenhorst et al. (1998) and by Richardson and Vepraskas (2001) demonstrated that soil morphology is a valuable field tool for evaluating soil hydrology.
Pedology has much to offer to soil physics and hydrology, and vice versa. For instance, soil mapping provides the classical foundation for our understanding of soil variation across the landscape. Also, soil profile descriptions have been the major source of information on in situ soil structure and various soil hydromorphological features that are signatures of soil hydrology, and soil survey databases provide a wealth of information that soil physics and hydrology could utilize. Soil classification offers a hierarchical system for organizing, modeling, and transferring our knowledge about different soils. Soil genesis provides insights regarding soil evolution with time. On the other hand, soil hydrology is a major driving force behind pedogenesis, morphology, and soil distribution. It controls a variety of soil physical, chemical, and biological processes that lead to the formation of different soils and diverse land uses. Soil moisture and temperature regimes play a critical role in classifying soils, and the spatial and temporal distribution of water provides clues to soil variation and mapping. Consequently, a new paradigm of integrated soil and water sciences would be based on spatial covariation of soil and water that is coevolved with time.
HYDROPEDOLOGY
Hydropedology is suggested as an intertwined branch of soil science and hydrology that embraces interdisciplinary and multiscale approaches for the study of interactive pedologic and hydrologic processes and properties in the earth's critical zone (Fig. 1). The critical zone, as defined by the NRC (2001a), extends through the root zone, deep vadose zone, and groundwater zone, and includes the land surface and its canopy of vegetation, rivers, lakes, and shallow seas. Interactions at this interface between the solid earth and its fluid envelopes determine the availability of nearly every life-sustaining resource (NRC, 2001a). Hence, the NRC has identified integrated studies of the critical zone as one of the compelling research areas in the 21st century. Hydropedology, in combination with hydrogeology, provides a systematic approach to the study of the earth's surface and subsurface environments (Fig. 1). It should be pointed out that soil science traditionally has limited its investigations to the upper few meters beneath the earth's surface (emphasis on the root zone), whereas hydropedology extends all the way from the land surface to the groundwater table, encompassing both the root and deep vadose zones.
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As a bridge connecting pedology, soil physics, and hydrology, hydropedology integrates the pedon and landscape paradigms to link phenomena occurring at microscopic (e.g., pores and aggregates) to mesoscopic (e.g., pedons and catenas) and macroscopic (e.g., watersheds, regional, and global) scales (Fig. 1 and 2). Hydropedology is also linked to other related biogeosciences such as geomorphology, hydroecology, and other branches of soil science (Fig. 3). Besides the bridging of disciplines and scales, hydropedology also facilitates the transfer of data between soil survey databases and soil hydraulic information for simulation models, especially through approaches such as PTFs (Fig. 4). The bridging of disciplines, scales, and data represents potentially unique contributions of hydropedology to integrated soil and water sciences. For example, hydropedology could address the knowledge gaps among soil structure, preferential flow, and water quality (Fig. 5); help resolve differences in observational and/or modeling scales (Fig. 6); and encourage enhanced utilizations of the wealth of soil survey databases and soil classification systems (Fig. 7).
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Knowledge Gaps Needing an Integration of Classical Pedology, Soil Physics, and Hydrology
Increasing concerns over chemical pollution in the environment have generated renewed interests in flow and transport through soils and over the landscape. Despite the significant progress made in the past decades, transport processes that occur in structured soils and fractured geological materials, which are of vital concern for protecting water resources and safeguarding nuclear-waste and toxic-chemical disposal sites, are far from well understood (NRC, 2001d). Mechanisms controlling individual and interactive soil–water processes at multiple scales remain unclear. Besides scaling and data issues to be discussed below, other examples of knowledge gaps that would benefit from hydropedology include the following (the list is meant to be illustrative rather than exhaustive):
Soil Structure. Quantification of soil structure and its impacts on flow and transport in field soils remain unresolved. We need ways of representing soil's natural "architecture" in a manner that can be coupled into models of flow, scaling, and rate processes. A new and versatile geometric foundation for representing porous media (e.g., fractal geometry, percolation theory, and geometric modeling) is emerging as one of the possible ways that could yield improvements in media scaling, flow modeling, and soil hydraulic function characterization (e.g., Jury, 1999; Gerke and van Genuchten, 1996). These would require joint efforts of pedologists, soil physicists, hydrologists, and others.
Preferential Flow. Our ability to determine and predict preferential flow dynamics, velocity, pathway, its significance in different soils, and its interface with the soil matrix is unsatisfactory. Since most field soils are structured to varying degrees and since soil layering is the rule in the field, quantitative relationships between preferential flow and soil texture, structure, and layering could provide a means of estimating a priori how important preferential flow is in a given soil (especially when linked to soil map units). Other soil features such as cutans and slickensides are also good indicators of preferential flow and the macropore–matrix interfaces (e.g., Wilding and Hallmark, 1984; Lin et al., 1996; van Genuchten et al., 1999b). A soil classification scheme may be developed to differentiate various soils in terms of their flow patterns and transport mechanisms.
Soil Hydromorphology. Soil macro- and micro-morphology have long been used to infer soil moisture, hydraulic properties, and to provide a basis for soil genesis and classification. Morphological features offer clues regarding flow processes in field soils, and provide information that cannot be easily obtained through other methods (such as redoximorphic features, aggregate consistency, and ped strength). However, quantitative use and modeling of soil morphological data have been lacking. The continued acquisition of soil water table and soil moisture data through field monitoring and remote sensing could give rise to a more quantitative use of field soil morphology for inferring water table behavior, drainage classes, and soil hydraulic properties. As demonstrated by Bouma (1990), Lin et al. (1999a)(b), and others, morphometric data could be used to quantitatively derive flow parameters.
Water Movement over the Landscape. "Where, when, and how" water moves through various landscapes and how water flow impacts soil processes and subsequently soil spatial patterns need to be better understood. Conceptual models of water movement over the landscape are key aspects of contaminant transport, water quality, watershed management, wetland delineation, and terrestrial ecosystem functions. Joint efforts among soil physicists, hydrologists, pedologists, geomorphologists, and others could help develop such conceptual models that would likely go beyond the classical Darcy–Buckingham's laws for saturated and unsaturated flow. For example, hydrological models generally do a poor job in accurately predicting lateral flow and baseflow vs. runoff in total streamflow (Wood, 1999). However, sloping topography, stratification, and soil layering all favor lateral flow (Richardson et al., 2001). The convergence of lateral flow within a landscape results in the formation and distribution of streams and rivers and contributes to the spatial heterogeneity of soil and vegetation across the landscape (Wood, 1999).
Soil Variation. One especially frustrating issue facing soil scientists, hydrologists, and others in dealing with the variably unsaturated zone, both in terms of experimentation and modeling, is the overwhelming heterogeneity of the subsurface (van Genuchten et al., 1999b). Knowledge of soil spatial and temporal variation is essential to effective modeling, reliable prediction, accurate mapping, and appropriate scaling. Although pedologists, soil physicists, and hydrologists have made tremendous efforts in understanding and modeling soil variation, their efforts do not seem to have converged well in the past. Recently, the merger of geostatistics with soil classification for handling synergistically continuous and discrete soil spatial variation has appeared (Heuvelink and Webster, 2001). Particularly promising is the development of so-called environmental correlation modeling (McKenzie and Ryan, 1999; Ryan et al., 2000) or landscape-guided soil mapping (Heuvelink and Webster, 2001), where landform and environmental attributes such as digital elevation models, land use and land cover, parent materials, and others serve as additional information in mapping, kriging, and modeling.
Bridging Scales from Laboratory to Field to Landscape
Soil in nature is spatially heterogeneous and temporally dynamic. A motivating challenge is to transfer results from laboratory studies using soil cores to soil mapping units in the field, then to watershed, regional, and global scales. Similarly, downscaling is needed when dynamic processes or static properties in a larger area are observed (e.g., through remote sensing), but require translation to smaller but inherently heterogeneous subareas if they are to be made useful for site-specific applications. A major recurring problem in soil and hydrologic sciences is the representation of soil properties or processes in the presence of large spatial and temporal variability at a scale different from the one in which observations are made. This inevitable but challenging scale transfer or multiscale bridging issue remains at the heart of many hydrologic and pedologic studies.
According to a report by the Soil Science Society of America, Opportunities in Basic Soil Science Research (Sposito and Reginato, 1992), pedologists are foremost among basic soil scientists who help develop integrated-system models to scale up information from small samples to the global pedosphere. Pedologists study both the mechanisms and the magnitudes of spatial and temporal variability (e.g., Wilding and Drees, 1983; Wilding et al., 1994; Mausbach and Wilding, 1991) as a basis for broad generalizations about soil genesis, classification, and mapping (particularly from the perspective of soil-forming factors). Indeed, the purpose of soil surveys is to partition the spatial variability of landforms into stratified subsets that are less variable (Wilding and Drees, 1983; Soil Survey Staff, 1993). When correlated with their classification, information gained from soil surveys on the properties and distributions of soils provides a powerful vehicle for knowledge transfer (Wilding, 2000). Soil physicists and hydrologists also have long been concerned with scaling and spatial and temporal variability. They have studied scaling theories such as similitudes (e.g., Miller and Miller, 1956; Tillotson and Nielsen, 1984) and fractals (e.g., Tyler and Wheatcraft, 1990; Baveye et al., 1999), and have attempted to quantify spatial variability using methods such as geostatistics (e.g., Warrick, 1998) and temporal variability using time series analysis (e.g., Wu et al., 1997). The variability of the field regime has also prompted the development of stochastic methods (e.g., Jury and Kabala, 1993). While significant understanding of scale-dependent soil properties and processes have been obtained in pedology, soil physics, and hydrology, multiscale bridging or scale-transfer remains a significant challenge (Sposito, 1998; Baveye and Boast, 1999).
The development of hydropedology could facilitate multiscale bridging by linking microscopic to mesoscopic and macroscopic levels (Fig. 1 and 2). Hierarchical complexity has been studied in pedology, which has long recognized self-organized complexity in the processes of soil formation and has constructed taxonomic frameworks to summarize that ordering (Buol et al., 1997; Wagenet, 1998). As hierarchy is common to the subjects typically encountered in pedology, soil physics, and hydrology, hierarchical frameworks offer a potential solution in scale bridging. As illustrated in Fig. 6, the hierarchies of soil mapping (for soil distribution) and soil modeling (for soil processes) may serve as useful conceptual frameworks for multiscale bridging in hydropedology (Lin and Rathbun, 2003). The hierarchy of soil mapping relates to the spatial distribution of soil types or specific soil properties across landscapes of varying sizes through different orders of soil surveys, spatial interpolation, and/or spatial aggregation. The hierarchy of soil modeling relates to the representation of soil processes at different scales and the upscaling or downscaling of model input parameters. In an analog to the leaf–tree–forest relationship (Fig. 2), it is apparent that when the sample size is changed from small soil cores to field plots, we need to incorporate soil structural information and the concept of the representative elementary volume (REV) (see Bear, 1972). When the sample size is further enlarged from field plots to watersheds, we need to consider the variation in topography, land use, and the concept of the representative elementary area (REA) (see Wood et al., 1988). Such hierarchy helps explain the observed differences between field-measured hydraulic properties and laboratory-determined values (e.g., Sharma and Uehara, 1968; Field et al., 1984; Bouma, 1992) and the large spatial variation in soil hydraulic properties over the landscape (e.g., Sharma et al., 1980; Duffy et al., 1981).
Bridging Data through Pedotransfer Functions
"Data rich, information poor" (information here connotes interpretation, synthesis, and utilization of data) has been a common syndrome in many disciplines. This problem is largely due to data fragmentation, incompleteness, incompatibility, inaccessibility, or lack of interpretation and synthesis in spite of past extensive and costly data collections. In soil science and hydrology, it is recognized that gaps exist between what we have (e.g., the National Cooperative Soil Survey [NCSS] Program databases) and what we need (e.g., soil hydraulic parameters and PTFs needed for simulation models). Improved procedures to extract useful information from the available databases and to improve and interpret soil survey data for flow and transport characteristics in different soils are needed.
With the increasing popularity of coupling geographic information systems (GIS) with vadose zone models and soil survey databases for diverse natural resource applications, the demand for soil hydraulic information has increased significantly in recent years. However, the lack of sufficient field data on soil hydraulic properties often limits the application of contaminant transport and hydrological modeling. Existing methods for direct field measurement of soil hydraulic properties remain complex, time-consuming, and costly (Mualem, 1986; Bouma, 1989), despite decades of work by soil physicists, hydrologists, and others representing different disciplines. Another limitation of direct field measurement is significant spatial and temporal variability, hence demanding a large number of measurements that are often prohibitive in terms of time and money (van Genuchten et al., 1999b). This has prompted efforts to indirectly estimate soil hydraulic properties using more readily available data often found in soil surveys (such as particle-size distribution, bulk density, organic matter content, and others). Such indirect methods, now often referred to as pedotransfer functions (PTFs) as suggested by Bouma and van Lanen (1987), have been attempted for estimating water retention curve parameters, saturated hydraulic conductivity, the unsaturated hydraulic conductivity function, and other soil hydraulic parameters (e.g., Vereecken et al., 1990; Tietje and Hennings, 1996; Batjes, 1996; van Genuchten et al., 1999a; Lin et al., 1999b; Wösten et al., 2001). Compared with other methods of estimating soil hydraulic parameters (e.g., pore-size distribution models and inverse methods), PTFs are inexpensive, easy to derive and use, and, in many practical cases, they provide good estimators for missing hydraulic parameters (e.g., Verhagen and Bouma, 1998; van Genuchten et al., 1999b; Wösten et al., 2001). Besides conventional regression or functional analyses, new techniques such as neural networks (e.g., Schaap et al., 2001), group methods of data handling (e.g., Pachepsky and Rawls, 1999), and classification and regression trees (CART) (e.g., McKenzie and Jacquier, 1997) are increasingly being explored for developing PTFs using a growing number of large soils databases such as the NCSS databases, UNSODA (Leij et al., 1996), HYPRES (Lilly, 1997), WISE (Batjes, 1996), SoilVision (SoilVision Systems Ltd., 2002), and many others.
While various degrees of success have been achieved with different PTFs (Pachepsky et al., 1999; Wösten et al., 2001), limitations of existing PTFs remain. For example, the vast majority of existing PTFs are completely empirical, and limited efforts have been put into systematic probing of underlying mechanisms for the existence of such functions. There is a tendency among soil scientists and others to estimate soil hydraulic functions by regression analysis in a given region, but efforts to apply PTFs derived from one area to soil hydrologic studies in various soil mapping units or soil regions are futile (Kutílek and Nielsen, 1994). Only a few quasi-physical PTFs exist, such as those by Arya and Paris (1981), Haverkamp and Parlange (1986), and Arya et al. (1999), in which particle-size distribution is first translated into an equivalent pore-size distribution model and then further related to water retention curve. However, the flow system in the bundle of capillary tubes differs considerably from the water flow network in real-world soils. Thus, the practical application to field soils of these quasi-physical PTFs, based essentially on the Hagan-Poiseuille's law, is very limited. In addition, existing PTFs have not fully incorporated soil structure and land use information, and have lacked scale and temporal considerations. As such, the accuracy, reliability, and utility of existing PTFs are constrained. In the mean time, the NCSS databases developed in the last century have been underused in addressing environmental issues. Interpretations and applications of the NCSS databases are challenges facing soil scientists in general and pedologists in particular. There are pressures on both pedologists and soil physicists and hydrologists to disseminate soil survey information and utilize it in a variety of applications (van Genuchten, 2002, personal communication).
The combined efforts of pedologists, soil physicists, and hydrologists could open up new opportunities for the next generation of PTFs. For instance, five general categories of PTFs may be identified for potential improvement in estimating dynamic soil properties (Fig. 7). PTF Type I relates use-dependent soil properties to soil hydraulic information, both of dynamic nature requiring regular sampling. PTF Type II includes relatively static soil properties that could be sampled only once into the prediction of dynamic soil properties. PTF Type III further considers soil mapping and classification related information to improve the prediction. Landscape features such as digital elevation models, land use and land cover, and others could serve as additional inputs in PTF Type IV and V, hence connecting the pedon and landscape scales. It is likely that PTFs in combination with routine spatial information from soil survey, topography, and land use could improve the regional estimates of soil hydraulic parameters. In terms of land use and use-dependent soil properties, Droogers and Bouma (1997) suggested the terms genoform, for genetically defined soil series, and phenoform, for soil types resulting from a particular form of management in a given genoform. Such distinction between major soil management types within the same soil series could potentially enhance PTFs that involve soil series and land use as carriers of soil hydraulic information. Realizing the importance of dynamic and use-dependent soil properties, the NCSS program is now considering the possible development of a dynamic soil properties database, which, once developed, would significantly facilitate the enhancement of PTFs.
HYDROPEDOLOGY APPLICATIONS
Soil and water are integral parts of the earth's critical zone, contributing to the origin and development of life on the planet, the rise and decline of human civilizations, and the sustainability or deterioration of global ecosystems. In these fundamental areas, hydropedology contributes to our enhanced understanding of a variety of issues related to the critical zone, such as water quality, soil quality, landscape processes, watershed management, nutrient cycling, ecosystem health, climate change, onsite waste disposal, land use planning, precision agriculture, and many others of societal importance. For instance, with the development of contaminant hydrogeology (Fig. 8), hydrogeologists have become much more interested in the "mysteries" of the unsaturated zone, as many releases of contaminants to the subsurface occur within or above the vadose zone, including materials applied deliberately (e.g., agricultural chemicals, landfill leachate, or toxic waste dumps) and those released accidentally (e.g., leaking septic tanks, chemical spills, or leaking petroleum tanks). Along with hydrogeologists, hydropedologists play a critical role in the integrated study of contaminant fate in the environment. As another example, nonpoint source pollution has become a focal point of attention by the general public because of its ubiquitous nature and potential chronic health effects (Corwin and Wagenet, 1996). A renewed wave of "watershed thinking" is thus spreading across the USA and around the globe (NRC, 1999). In watershed-based approaches to address water pollution, hydropedology is expected to contribute in a variety of ways, such as the enhanced use of soil survey databases and the development of improved PTFs for watershed water quality modeling. The bridging of pedon-scale observations and landscape-scale phenomena, with the appropriate incorporation of soil structural data into preferential flow modeling, may expedite solutions to many nonpoint source pollution problems.
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Hydropedology is suggested as an intertwined branch of soil science and hydrology that embraces interdisciplinary and multiscale approaches for the study of interactive pedologic and hydrologic processes and properties in the earth's critical zone. Emphasized here are potential "bridges" to address (i) knowledge gaps between traditional pedology, soil physics, and hydrology; (ii) scale differences in microscopic, mesoscopic, and macroscopic studies of soil–water interfaces; and (iii) data translations from soil survey databases into soil hydraulic properties. Such bridges signify the potential unique contributions hydropedology can make to integrated soil and water sciences. While the scope and niche areas of hydropedology are yet to be further defined and accepted by the soil science and hydrology communities, the promotion of hydropedology offers a renewed perspective and a more integrated approach to the study of soil–water interactions across spatial and temporal scales. Finally, the interdisciplinary emphasis of education in the 21st century will make hydropedology a timely addition to the training of the next generation of soil scientists and hydrologists.
ACKNOWLEDGMENTS
The author is grateful to Dr. Rien van Genuchten for his interest in this manuscript and his encouragement to submit it as a possible publication in the Vadose Zone Journal. The author appreciates the valuable comments on an earlier draft of this manuscript from Drs. Randy Brown, Mary Ann Bruns, Dan Fritton, Rien van Genuchten, and Larry Wilding. Valuable comments from anonymous reviewers and the Associate Editor, Dr. Yakov Pachepsky, are also greatly appreciated.
REFERENCES
This article has been cited by other articles:
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  E. R. Landa From agricultural geology to hydropedology: forging links within the twenty-first-century geoscience community Geological Society, London, Special Publications, January 1, 2006; 266(1): 133 - 140. [Abstract] [PDF]
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 A. J. Tugel, J. E. Herrick, J. R. Brown, M. J. Mausbach, W. Puckett, and K. Hipple Soil Change, Soil Survey, and Natural Resources Decision Making: A Blueprint for Action Soil Sci. Soc. Am. J., May 6, 2005; 69(3): 738 - 747. [Abstract] [Full Text] [PDF]
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