soil hydrology: stages of development, current state, and nearest prospects

ISSN 1064�2293, Eurasian Soil Science, 2010, Vol. 43, No. 2, pp. 158–167. © Pleiades Publishing, Ltd., 2010.Original Russian Text © E.V. Shein, 2010, published in Pochvovedenie, 2010, No. 2, pp. 175–185.

158

INTRODUCTION

Soil water is the main driving factor of soil pro�cesses. This is an undisputable truth shared by all theresearchers. The theoretical premises, basic concepts,and experimental approaches of soil hydrology are offundamental importance for the entire soil scienceand neighboring sciences. However, the conceptualbasis of soil hydrology and the theory of soil hydrolo�gical processes have always been a matter of scientificdiscussions. Thus, N.A. Kachinskii and A.A. Rodeargued about the mobility of film water and about thedefinition and determination of the field water capa�city; Richards criticized the latter notion; numerousdiscussion accompanied the development of the the�ory of water retention curves; etc. At present, theseand other discussions are still continued from time totime, though the rapid progress of soil hydrology isquite evident and is related to the appearance of newapproaches, models, and quantitative methods.

The aim of this paper is to review the stages of thedevelopment of soil hydrology, its current state, andpromising directions of future studies. The modernchallenges facing soil hydrology and the developmentof new branches of this science—hydropedology, bio�hydrology, etc.—are also discussed.

STAGES OF THE DEVELOPMENT OF QUANTITATIVE SOIL HYDROLOGY

In this paper, the problems of quantitative hydro�logy of soils are considered. From my point of view,

these problems are related to soil physics. As for therole of the hydrological factor in soil evolution and theclassification of soil water regimes, these problemsshould be considered within the framework of pedo�logy rather than soil physics; soil physics is a quantita�tive science with its own basic concepts and defini�tions. Thus, in this paper, the field of soil hydrologyrelated to various calculations and prognostic mathe�matical modeling is discussed. This field is particularlyimportant for the purposes of soil reclamation andrehabilitation, for which quantitative estimates of thestatics and dynamics of soil water and dissolved sub�stances are necessary.

From my point of view, three major stages can bedistinguished in the development of quantitative soilhydrology.

(I) The stage of hydrological constants.(II) The stage of application of thermodynamic

characteristics of soil water for the description of itsmovement; the development of this approach led tothe progress in application of mathematical hydrolog�ical models.

(III) The modern stage that takes into account thespecificity of the movement of water in a landscape.

Let us analyze these stages separately.Stage I is the stage of classical hydrology. Its ideas

were based on the analysis of different forces acting onthe soil water. Thus, different forms of soil water andthe corresponding soil hydrological constants (soilwater capacities) were distinguished. Some of these

SOIL PHYSICS

Soil Hydrology: Stages of Development, Current State, and Nearest Prospects

In Memory of Aleksandr Mironovich GlobusE. V. Shein

Faculty of Soil Science, Moscow State University, Moscow, 119992 RussiaE�mail: [email protected]

Received April 3, 2009

Abstract—The main stages of the development of soil hydrology are described. These are: (1) the stage ofclassical hydrology based on the concepts of soil water forms and soil hydrological constants; (2) the stage ofthermodynamic approaches toward assessing the statics and dynamics of soil water (soil hydrophysics); and(3) the modern stage of diverse approaches taking into account the specificity of water movement in a heter�ogeneous pore space (the development of preferential water flows), the specificity of the hydrological prop�erties of soils in dependence on the scale of their examination, and the impact of the living soil phase on thesoil hydrological processes. The diversity of modern approaches toward soil hydrology is reflected in thenames of new branches of this science, such as hydropedology, geohydrology, biohydrology, etc. At the mod�ern stage, all the conceptual approaches typical of the earlier stages of the development of soil hydrology arealso applied. At present, soil hydrology is an actively developing field of soil science with clearly understoodlimits of application, advantages, and disadvantages of the methods typical of the first two stages. On thisbasis, an integral quantitative multilevel concept of soil hydrology is being developed.

DOI: 10.1134/S1064229310020055

EURASIAN SOIL SCIENCE Vol. 43 No. 2 2010

SOIL HYDROLOGY: STAGES OF DEVELOPMENT, CURRENT STATE 159

notions were of great practical significance, includingthe major notion of the field water capacity (knownunder different terms in Russian literature). The theo�retical definition of this notion included the condi�tions to be observed for its experimental determina�tion. In particular, in theory, this notion implied thefree outflow of water from the soil thickness, theabsence of layering in the soil profiles, the moisteningof a given soil layer, etc.). In practice, it was impossibleto meet all these requirements. Some of them requiredfurther specification. For example, the free outflow ofwater from the lower soil boundary may be achieved bydifferent methods: free drainage, deep drainage, seep�age face, horizontal drains of different kinds, etc. Eachof these methods affects the conditions of water out�flow from the soil profile and, hence, the value of thefield water capacity. For a number of cases, additionalnotions were introduced, such as the dynamic watercapacity [9]. At present, the conventional character ofthe field water capacity is well understood [28, 30].Indeed, the equilibrium state of soil water in the soilafter the free outflow of excessive water is dictated bythe initial conditions (the initial distribution of waterin the soil profile), the conditions at the boundaries ofthe considered soil thickness, and the hydrophysicalproperties of the soil layers, including their hydrody�namic characteristics. It is impossible to ensure thesame initial and boundary conditions for the entirediversity of soils. This leads to biased determination ofthe field water capacity. At present, soil hydrophysicsconsiders this notion to be a qualitative rather than aquantitative soil characteristic. It is a useful notioncreating a “common platform” for soil hydrologists.However, the calculations based on the value of thefield water capacity are rather risky because of the dis�cussed limits in the experimental determination of thisvalue.

The development of thermodynamic notions aboutthe pressure (potential) of water in the soil led to theidea about the interdependence between the particularsoil water capacity and the soil water potential. Forexample, the water contents corresponding to thewater pressure of –330 and –15000 cm (of water col�umn) were considered to be close to the field watercapacity and to the wilting point, respectively. Thisapproach made it possible to perform laboratorydeterminations of these important hydrological con�stants in the soil samples. In turn, the determined val�ues could be used to calculate the range of availablemoisture, etc. A.D. Voronin developed an approachbased on the application of various linear equations tothe water retention curve (WRC) for calculating differ�ent water capacities [1]. This was a well�groundedapproach tested on numerous objects. It is still appliedas a theoretically substantiated approach towards thedetermination of soil water capacities. However, atpresent, it is evident that the soil water content at agiven soil water pressure is not necessarily equal to thegiven soil water capacity [30]. Indeed, the WRC is a

curve without clearly pronounced bends pointing tothe qualitative changes in the mechanisms of waterretention. Even if this curve is expressed in a differen�tial form, one cannot identify stable extrema pointingto qualitative changes in the forms of soil water on thiscurve. Thus, this approach for determining soil hydro�logical constants cannot be considered a physicallysubstantiated approach. It yields little promise for thefuture development of soil hydrology.

It seems that the stage of the active development ofthe theory of soil hydrological constants has come tothe end. Though these constants are still used for thetheoretical and practical purposes as qualitative com�parative soil characteristics, they do not enable us toperform physically grounded calculations and quanti�tative predictions. The theory of soil hydrological con�stants is being replaced by the thermodynamics of soilwater describing its energy status.

Stage II is the stage of application of thermody�namic approaches to soil water studies. The first worksin this field date back to the beginning of the 20th cen�tury. The thermodynamic approach in soil hydrologywas actively developed in the United States. Owing toW. Gardner and L. Richards, new methods, devices(tensiometers), and calculation procedures foundtheir application. It was suggested to modify the Darcyequation describing water movement in saturated soilsto the conditions of unsaturated soil. The latter equa�tion is known as the Richards–Klute equation,because these scientists managed to combine themajor equation of water transport with the continuityequation. As a result, they obtained a differentialequation that has no analytical solutions, but can besolved by numerical methods. This equation can beused for calculating water transport in porous mediaon the basis of data on water potential.

Thus, a physically substantiated quantitativeapproach to soil water studies was shaped. A.M. Glo�bus suggested that it could be referred to as soil hydro�physics [2–4]. He made a significant contribution tothe development and practical application of thisapproach in Russian soil physics. In the 1960s–1980s,soil hydrophysics became the central concept of thequantitative soil hydrology.

In any case, the corresponding calculations, theanalysis of the state of soil water, and the analysis ofinterphase interactions (soil solution–solid phase–gaseous phase) were based on the water retention curveshowing the dependence of the volumetric water con�tent on the capillary�sorption (matric) pressure of soilwater. Globus suggested that this dependence could bereferred to as main hydrophysical characteristic of soil.This concept proved to be very useful in different fieldsof soil hydrology [2–4, 13]. The works of A.D. Voro�nin, A.M. Globus, I.I. Sudnitsyn, and other scientistsdemonstrated that the WRC depends on the soil struc�ture, the properties of soil solid phase, and the compo�sition of soil solution. The analysis of this curve in dif�ferent ranges of the matric pressure made it possible to

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obtain quantitative characteristics of the specific soilsurface, the pore�size distribution, the rheologicalstate of the soil, and its physicomechanical properties.This field of soil physics was named by Voronin “thestructural�functional hydrophysics of soils” [1].

The development of thermodynamic approaches tosoil water studies by different schools of thought pro�ceeded differently. In Russia, the analysis of the WRCfor obtaining information about the state and structureof the solid, liquid, and gaseous soil phases predomi�nated. The WRC (the main hydrophysical characteris�tic) was considered as the dependence of the waterpressure on the gravimetric water content, whichoffered additional possibilities for the analysis of theequilibrium system containing solid, liquid, andgaseous soil phases. In foreign countries (particularly,in the United States), the practical use of the WRC forcalculating the transport of water and dissolved sub�stances in soils predominated. The rapid progress incomputation techniques related to the use of comput�ers together with the further development of the theoryof soil water transport led to the appearance of mathe�matical models of water transport based on two majorfunctions: the WRC and the unsaturated water con�ductivity function (the dependence of hydraulic con�ductivity on the water pressure). At present, these arewell�developed models with adequate mathematicalsupport. The WRC is approximated by the Van Genu�chten equation, the parameters of which for the entirerange of mobile water characterize the specific shapeof the WRC. Together with the Van Genuchten–Mualem equation, they make it possible to determinethe water conductivity function on the basis of data onsaturated hydraulic conductivity [14, 17, 35]. In modernsoil hydrology, the use of predictive mathematical modelsof water transport is based on the WRC parametersincluded in the Van Genuchten equation and the satu�rated hydraulic conductivity values. There are manymathematical models of that kind applicable for a widerange of soil–climatic and hydrological conditions. Inparticular, one of the first Russian models describingwater transport was developed by Ya.A. Pachepskii,E.V. Mironenko, and R.A. Shcherbakov (the MOISTmodel) [10, 11]. The HYDRUS model developed byM.Th. van Genuchten and J. Simunek is applied topredict water, salt, and heat transport. The SWAP(Soil–Water–Atmosphere–Plant) model simulatestransport of water, solutes, and heat in saturated andunsaturated soils [34]. There are models combiningsoil hydrological characteristics with crop productivity[12] and many other physically substantiated modelsin the form convenient for their practical use.

This situation seemed to be quite satisfactory and sta�ble. Indeed, the physical grounds of the models, theexperimental methods, and the mathematical support ofthe models reflected the current state of soil hydrology.

However, new experimental data on the movementof solutes in soils could not be described adequatelywithin the framework of the hydrophysical models. In

particular, these models could not predict the very fasttransport of contaminants, nitrates, and pesticidesinto the groundwater [27, 29, 32]. In relation to this,special experiments on the adaptation and verificationof the models were conducted. In these experiments,boundary conditions and the dynamics of moistureand water pressure were controlled for some time.Then, the results of these experiments were comparedwith model predictions for the specified time. In mostcases, the agreement between the experimental dataand the model predictions was quite satisfactory interms of the statistical assessment of the differencebetween the calculated and measured data. This dif�ference was explained by the spatial variability in thesoil water content and pressure in the experiments, thenonequilibrium conditions in the experiments, andthe insufficient accuracy of determination of bound�ary conditions [14, 19, 29, 30]. However, the mainsource of possible errors of modeling is believed to berelated to the scale effect. Usually, the determinationof necessary WRC parameters is performed in thesamples of small sizes (up to 50 cm3). These experi�mental data are used to calculate water exchangebetween soil horizons and soil profiles, the objects ofprincipally different sizes. In this case, the scale effectrelated to the application of data obtained on a givenscale to the processes manifesting themselves on a dif�ferent scale. These scale�related errors are important.At present, we do not know the quantitative regulari�ties of the transition from a given scale to anotherscale. These scale�related errors can be reduced via theprocedure of model adjustment (the purposeful alter�ation of model parameters to achieve the best fit of thecalculated and real values). After this, the model canbe used for a wide range of soil conditions and differ�ent boundary and initial conditions, i.e., to apply it forpolyvariant predictions.

In the recent years, it has become evident that thescale factor is not the only source of errors. Anothersource of potential errors is related to the specificity ofsoil as a complex structured natural body, the move�ment of water in which cannot be described preciselywithin the framework of classical hydrophysicalapproaches. The following groups of processes andphenomena may serve as the potential sources oferrors in the hydrophysical modeling.

(1) The presence of preferential flows of water,heat, and solutes in the soil.

(2) The scale�related changes in the hydrologicalproperties and functions of soils.

(3) The anisotropy of soil properties, i.e., the dif�ference in the soil hydrophysical characteristicsdepending on the direction of their study.

(4) The complexity of achieving equilibrium con�ditions in the soil and the dependence of hydrophysi�cal functions on the nonequilibrium conditions oftheir determination and use.



(5) The hysteresis phenomenon, i.e., the depen�dence of current properties on the prehistory of theprocess.

(6) The hydrophilicity / hydrophobicity of the solidphase surface.

(7) The shrink–swell phenomena, i.e., the changesin soil pore space in dependence on the soil water con�tent, the chemical composition of the liquid phase,and other factors.

Let us analyze these processes and phenomena andtheir effect on the soil water movement.

Preferential water flows. Preferential flows of waterand solutes in soils are conditioned by the heterogene�ity of soil pore space; they are characterized by rapidtransport of substances through a part of soil porespace (the transport zone) upon the high (exceedingthe rate of water absorption) input of considerableamounts of water onto the upper soil boundary, theabsence of frontal water movement. and the low watersorption capacity [15]. As follows from this definitionand several similar definitions [14, 16], neither theRichards–Klute equation, nor chemical equations ofequilibrium exchange and sorption can describe thisphenomenon. It should be specially described by somemechanism. There may be several mechanismsresponsible for the development of preferential flows:

(a) The formation of separate “water channels”(preferential flow pathways) that are called “fingerflows” and are conditioned by the spatial heterogene�ity of the soil properties (its hydrophobicity, density,etc.);

(b) The rapid transport along conductive zones ofthe soil pore space (macropores, fissures) followed byexchange with stagnant zones; and

(c) The unevenness of water transport related to thespatial variability in the soil properties (its bulk densityand initial water content).

In the case of preferential water flows, we deal witha mechanism ensuring virtually instantaneous (incomparison with a slow movement of the wettingfront) transport of water along macropores, fissures,and other preferential pathways with the subsequentredistribution of water between the transport zone andthe stagnant zone. This phenomenon is typical of var�ious soils; according to the factual data, it is most pro�nounced in separate periods with the maximumhydrological loads. However, its role cannot be overes�timated, particularly in the case of the transport of dis�solved toxicants. It should be adequately describedand taken into account in soil hydrological models[14–17, 25, 31, 32].

Scale�related changes in the main hydrological prop�erties and functions. It should be noted that none of theexisting methods for determining the hydrologicalproperties of soils is supplied with special recommen�dations on the scale of necessary experiments. At thesame time, the understanding of the fact that the val�ues of particular properties depend on the scale (area,

volume) of their determination has been gained by soilhydrology long ago. In particular, the scale effect insoil hydrology was described by Dmitriev [6, 20].However, the proper understanding and description ofthe physical basis of scale�related phenomena has yetto be achieved.

Let us consider one of the examples. Figure 1 pre�sents the results of determination of saturated hydrau�lic conductivity by different methods: the method oftubes of 17 to 25 cm2 in size and the method of framesof different areas (600–625, 1930–1985, and 2300–2500 cm2). In all the experiments conducted on agro�gray soils of the Vladimir opolie region, the water headof 5–6 cm was maintained and the experiments lasteduntil the establishment of steady�state infiltration. Theresults indicate that the saturated hydraulic conduc�tivity directly depends on the test area: the larger thearea of ponded infiltration, the higher the saturatehydraulic conductivity (Ksat). Which value should beused in further calculations? What problems should besolved with the use of particular values? Can weextrapolate the obtained dependence of the saturatedhydraulic conductivity on the test area over largerareas? It seems that the answers to these questions maybe found within the framework of some new conceptsdiffering from the classical concepts of soil hydrology.

Anisotropy of soil properties. This phenomenon maybe considered using the WRC obtained for the secondhumus horizon (Ah) of gray forest soils in the Vladimiropolie region [15, 16]. The measurements were per�formed for the samples of 0.03 l in volume sampled inthe horizontal (lateral) and vertical directions.

280

240

200

160

120

80

40

02300–25001930–1985600–62517–25

2

3

S, сm2

1

Ksa

t, cm

/day

Fig. 1. Statistical characteristics of saturated hydraulicconductivity of the plow layer of an agrogray soil (Vladimiroblast) upon different test areas: (1) median, (2) quartiles,and (3) minimum and maximum values.

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SHEIN

As seen from Fig. 2, the anisotropy of the soil prop�erties is clearly pronounced in this horizon. The soilmonoliths sampled in the horizontal direction (alongthe slope of soil horizons) are characterized by thelower values of the water content upon the given waterpressure (pF) in comparison with the monoliths sam�pled in the vertical direction. Thus, the WRC for thehorizontally oriented samples is shifted to the left incomparison with the WRC for the vertically orientedsamples.

This phenomenon is even more pronounced in thelower part of the Ah horizon, where a general decreasein the water retention capacity takes place. This phe�nomenon is related to the specificity of the soil struc�tural arrangement. The inner relief of the secondhumus horizon is important. Within the framework ofthis article, we should note that the anisotropy of soilproperties (including water retention characteristics)is virtually ignored in the hydrological calculations.The effect of this phenomenon on the hydrologicalcalculations and quantitative descriptions of watertransport has yet to be clarified.

The complexity of achieving equilibrium conditionsin the soil is also important, because the soil hydrolog�ical functions depend on the equilibrium/nonequilib�rium conditions of their determination. Water reten�tion curves obtained by different methods upon differ�ent rates of drying or drainage of the soil mass can befound in literature [5, 18]. It is stressed that the soilwater content at the given water pressure upon the highrate of evaporation (nonequilibrium conditions) ishigher than that upon equilibrium conditions. Thisfact is explained by the discontinuity of the soil waterbody under nonequilibrium conditions: some parts ofthe pore space remain filled with water that has notreached the evaporation (or drainage) front. Thus, thesoil water content remains higher than that underequilibrium conditions. In this context, several meth�odological problems have to be solved. Which of thecurves (obtained upon equilibrium, quasiequilibrium,or nonequilibrium conditions) should be used in cal�culations? It should be noted that the soil water isoften far beyond the equilibrium state under real con�ditions. What is the effect of the nonequilibrium con�ditions on the hysteresis phenomena for most of thehydrophysical soil characteristics? May be, the hyster�esis phenomena are related to the nonequilibriumconditions rather than to the specificity of soil porespace. The effect of nonequilibrium conditions on theWRC in the ranges of the high and low water contentsshould be determined and explained. It can be sup�posed that under the high flow velocities (under con�ditions close to saturated conditions) the appearanceof the zones with the low�mobile water far beyond theequilibrium increases. However, the achievement ofthe equilibrium for the soil with the low water contentis retarded by the low flow velocities. In fact, we dealwith nonequilibrium conditions. The role of this phe�

nomenon in the hydrological models has yet to beclarified.

Hysteresis. Hysteresis—the dependence of hydro�physical properties on the prehistory of the process(system memory)—should also be taken into account.This phenomenon is often considered in dependenceon the direction of the process (whether the soil is sub�jected to wetting or drying). Its physical nature is notfully understood. The theory of stationary anddynamic hysteresis as applied to soil hydrology has yetto be developed.

The amount of systematized data on changes in thesoil hydrophysical functions in dependence on thesample saturation conditions is also insufficient. It isknown that the hydrological properties of soils dependon the degree of drying of the samples before theexperimental tests. In terms of physics, this is theinfluence of initial conditions of the experiment on thedynamics and equilibrium state of the determinedhydrological property. In soil physics, it is generallyagreed that all the tests should be conducted in thecourse of soil drying after its saturation to the neces�sary degree (for the given experiment). For example,during the WRC determination with capillarimeters(tensiometers), the soil is preliminarily saturated withwater (the initial conditions of the experiment). How�

2

1

056463626

(b)

θ, %

2

1

0

(a)pF

1

2

3

Fig. 2. Water retention curves for soil monoliths sampledoriented in vertical and lateral directions from the (a) Apand (b) AhE horizons of agrogray soil (Vladimir oblast):(1) vertical monoliths, (2) lateral monoliths oriented alongthe slope of soil horizons, and (3) lateral monoliths ori�ented across the slope of soil horizons (inner soil relief)(according to [15]).



ever, it is also important, what was the degree of thesoil drying before its saturation. Was it an air�driedsoil? Or an absolutely dried soil? Or a soil with fieldmoistening? The results of the experiment may be dif�ferent. An example can be found in [15]. Samples froma soddy�podzolic soils were air�dried and absolutelydried. After this, they were completely saturated withwater and left in desiccator above pure water table forseveral weeks (for reaching equilibrium conditions).After this, when the initial conditions were strictly lev�eled for the samples, they were placed above saturatedsalt solutions to determine the equilibrium waterdesorption curve (a part of the WRC in the range ofpF 4.4–6.3, or relative water vapor pressure of 0.98–0.17). As follows from Fig. 3, the equilibrium desorp�tion curves obtained during this experiment differedsignificantly in dependence on the initial pretreatmentof the samples: the water content of the samples pre�liminarily dried to the absolutely dry state was lowerthan the water content of the samples preliminarilystored in the air�dry state. What are the reasons for thisdifference? There is no generally accepted hypothesis.It is probable that significant changes in the structureof soil solid phase take place upon the soil drying: thestronger the drying, the more significant thesechanges. According to one of the hypotheses [14],these changes are explained by the organization of soilcolloids into gel structures that include considerableamounts of soil moisture. If these structures aredestroyed upon the soil desiccation, their restorationupon the soil rewetting may require considerable time;it is also possible that they are irreversibly destroyedupon the soil drying to the absolutely dry state. If thesoil is dried incompletely (to the air�dry state), gelstructures with their capacity for water retention maybe partly preserved. In the context of this article, it isimportant that the physical nature of some hydrologi�cal soil properties is still unclear; some of them dependon the prehistory of soil processes. It is a challenge forsoil hydrology to understand when this phenomenonshould be taken into account, and when it can beignored.

Thus, at present, soil hydrology faces many disput�able and unresolved problems that cannot be properlyassessed within the framework of traditional concepts.This casts doubt on the universal applicability of thethermodynamic concept in its traditional form forsolving various applied and theoretical problems. Thisidea is being discussed at various congresses and work�shops [26, 28, 30, 33]. What are the potential solu�tions?

First, we should admit that the key problems facingsoil hydrology are well understood. This concerns thenecessity to put soil hydrology on the landscape levelof studies. This is an urgent need, because the modelsand technologies applied for soil conservation pur�poses, soil rehabilitation, and the environmental pro�tection from pollution require the landscape�basedapproach to find optimum solutions to these prob�

lems. It is also necessary to take into account the entirediversity of soil conditions, including the layered cha�racter of many soils, the soil stoniness, the presence offissures, and other phenomena. It seems unfeasible todevelop specific approaches for each particular case.Some general approach taking into account the diver�sity of soil conditions should be developed. It shouldbe based on our understanding of diverse physical phe�nomena in soils and provide adequate predictions (theerrors of the new models should be smaller than thoseof the existing models). Finally, it is necessary to takeinto account the living phase of soils as one of the pow�erful agents of soil transformation affecting the soilsolid phase, structure, pore space, and interparticleinteractions that are of major importance for hydro�logical models. The living phase of the soils is con�trolled by some biophysical laws that have yet to beunderstood, at least in relation to soil hydrology. Itseems that these problems specify the modern (third)stage of the development of soil hydrology.

The third stage of the development of soil hydro�logy takes into account specific features of the dyna�mics and statics of water in the soilscape; this field ofknowledge is referred to as hydropedology, biohydro�logy, ecohydrology, etc. From my point of view, it isspecified by a more close quantitative integration ofsoil hydrology with other sciences on the basis of deepphysical understanding of the processes of water trans�port in soilscapes. We face the birth of hydropedology,a science that integrates pedology, hydrology, and geo�morphology with the aim to study soil, hydrological,and landscape interactions in time and space [22].This science has three specific features: (a) the assess�ment of regularities governing the distribution of par�ticular soils in soilscapes, (b) the assessment of thespecificity of water transport in soils with due accountfor the presence of transport zones (preferential path�ways) and stagnant zones, and (c) the assessment ofinteraction between the processes of landscape evolu�

10

8

6

4

2

0 p/p01.00.80.60.40.2

1

2

W, %

Fig. 3. The dependence of soil water content on the relativewater vapor pressure in the samples dried to the (1) air�drystate and (2) absolutely dry state before the beginning ofthe experiment (according to [15]).

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tion and the processes of water migration in land�scapes.

These problems dictate the essence of modernapproaches in soil hydrology. Most of the approaches,methods, and models that are currently being devel�oped by scientists are directly related to these threeproblems. Let us briefly consider them.

(1) The spatial distribution of soils (soil properties)in the soilscape (soil cover). The fact that the soil coveris a heterogeneous body and that the areas with differ�ent soils can be found even within plain territories iswell known to soil scientists. The challenge is tounderstand how we can characterize this diversity andhow it can be taken into account in hydrologicalmodels. The classical statistical methods give us thequalitative characterization of soil variability, and geo�statistical methods are aimed at delineation of theareas with similar variability. The problem is to set uphydrophysical functions for different points in a land�scape for 3D calculations of water transport. Atpresent, the idea of pedotransfer functions linking thetraditional and easily measurable (in field or in labora�tory) soil properties with the soil hydrophysical andhydrochemical characteristics is being developed[14, 17]. In general, this direction in the developmentof soil hydrology is in line with the development ofother branches of soil science and related sciencesdealing with the “critical zone” of the Earth [24, 36].

(2) The specificity of water migration in soils withdue account for the existence of the zones of active

water transport and the zones of water stagnation. Thisproblem is directly related to the problem of preferen�tial flows (the zones of active water transport). In thiscontext, the zones of water stagnation are the zones ofintraped space that are slowly moistened by water fromthe zones of preferential flows and then remain in thestate close to saturation for a long time, whereas thezones of water transport are already emptied (Fig. 4b).As noted by many researchers, the movement of waterthrough the zones of preferential flows down to thegroundwater level proceeds rather quickly [7, 8, 23].The mechanism of this phenomenon is well known.Various mathematical models taking into account theexistence of preferential flows have been suggested. Ingeneral, they give more adequate predictions of thetransport of water and solutes (including dissolvedtoxicants, pesticides, etc.) in comparison with tradi�tional models. It is only necessary to have data on thevolumes and conductivity of transport zones (interpedfissures, macropores, etc) and the volumes, waterretention capacity, and conductivity of the intrapedpore space. Unfortunately, there are no well�deve�loped and substantiated methods to determine thesecharacteristics. In some of the approaches, the bulkdensity of peds and the bulk density of soils are appliedto calculate interped pore space; it is also possible totake into account changes in the bulk density of pedswith changes in the water content (shrink/swell pro�cesses). However, it is still unclear how we shoulddetermine the bulk soil density for the soils with coarse

(a)

Front of

Water layer

Preferential flows(Ksat ~ 700 сm/day)

Unsaturatedintrapedzone

capillarymoistening

Peds

CF

GWT

1

(b)

2 3

(Ksat ~ 20–40 сm/day)

Fig. 4. The scheme of water movement in a structured soil upon (a) rainfall of low intensity and (b) ponded soil surface:(1) groundwater table (GWT, saturated zone), (2) capillary fringe zone (CF), and (3) zone of capillary moistening.



peds, how we should determine hydrophysical andhydrochemical parameters separately for peds andinterped spaces, and how we should take into accountmany other important parameters of the geometry ofsoil pore space (e.g., pore sinuosity). It is suggestedthat the methods of 3D tomography and microtomog�raphy can be applied for the description of soilmacropores [26]. However, even if we know approxi�mately the geometry of macropores, we still do notknow which of these pores participate in water trans�port and what conductivity and retention characteris�tics (with respect for water and dissolved substances)do they have?

In this approach, one more point should be dis�cussed. The fact that a soil can transfer water flowsthrough macropores and fissures was known long ago[7, 8]. However, the same soil can transfer water via agradual, layer�by�layer moistening with a relativelysmooth wetting front. The main factors that dictate thetype of water transport through the given soil are theinitial conditions and the upper boundary conditions.If the latter are relatively “mild” (precipitation of lowintensity), the movement of water through the soil hasa gradual character without finger flows and preferen�tial flows [6, 15, 17]. This type of water movement isschematically displayed in Fig. 4a.

If the same soil is dried considerably and dissectedby fissures (the initial conditions are characterized bythe low pressure of soil water in the profile), and theprecipitation is very intense (showers), so that theinput of water onto the surface exceeds the rate ofwater imbibition into the soil, and the hydraulic headis formed, then the transport of water through the soilwill be specified by preferential flows (Fig. 4b). Underthese conditions of water transport, very specific con�ditions are formed in peds. After the discharge of themain part water from preferential pathways, the pedsare subjected to gradual moistening. Then, water maybe evaporated from ped faces, whereas anaerobic con�ditions may be established inside the peds and exist fora relatively long time. These conditions favor the spe�cific functioning of soil biota; microorganisms mayparticipate in the transport of various ions (Ca, Fe,etc.), and new interparticle bonds of crystallizationtype are formed; in turn, these bonds strengthen thepeds and contribute to the development of specificcolumnar structure [33].

As we can see, the development of this field of soilhydrology is now at the initial semiquantitative level; itis probable that the transition to the quantitative levelwill take place in the nearest future.

Finally, let us consider the third problem, i.e., therelationship between landscape evolution and watertransport in landscapes. This is the traditional field ofhydrology of surface waters, the study of energyreserves in a landscape, and soil erosion. Powerfulmodels with adequate support from satellite imageryhave been suggested to solve these problems. Their dis�cussion is beyond the scope of this paper. However, it

should be noted that the current stage of hydrologicalstudies at the landscape level is characterized by agreater attention to quantitative characteristics ofwater flows, including soil water flows. Instead of tra�ditional hydrological characteristics, such as the soilwater content or soil water pressure, characteristics ofwater flows and their distribution in a landscape areapplied. This approach makes it possible to judge thedirection of water transport, to delineate the majortransport zones in a landscape, and to understand thehydrological functioning of the landscape in general.

In recent years, new directions of soil hydrology havebeen shaped: biohydrology, ecohydrology, geohydrology,and some others [21, 36]. From my point of view, theyare tightly related to the new aspects and challenges ofsoil hydrology discussed in this paper. Thus, biohy�drology points special attention to the spatial distribu�tion of different zones of the functioning of soil biota(stagnant zones, zones of aeration, anaerobic zones,etc.) on different scales of the study. This branch of soilhydrology is interconnected with the problems of bio�spheric gases and climate changes. The study of spe�cific features of the water regime in different zones,including the zones of preferential flows, and of theeffect of hydrophobic properties on water migrationand on the functioning of soil biota is a promisingdirection of biohydrology. Geohydrology is concen�trated on the study of interactions between surface,soil, and ground waters and on the development ofvarious models. The development of new directions insoil hydrology points to qualitative changes in the con�ceptual basis of this science.

CONCLUSIONS

Classical soil hydrology based on soil�hydrologicalconstants and modern soil hydrophysics based on thethermodynamic approach and physical laws governingthe behavior of a continuous capillary–porous bodyare incapable to give quantitative descriptions of theentire diversity of hydrological phenomena. The grow�ing attention of researchers to these phenomena is dic�tated by our understanding of some drawback in theclassical and thermodynamic approaches and by thepractical needs (the growing demand for waterresources, the development of alternative farming sys�tems, the study of the transport of pollutants in land�scapes, etc). This situation has resulted in the appear�ance and rapid development of new branches of soilhydrology—hydropoedology, ecohydrology, and bio�hydrology—that take into account the specificity ofthe statics and dynamics of soil water on different scalelevels, the specificity of soil pore space arrangement,the influence of living phase on the hydrological pro�cesses, etc. It seems that the current stage of the parti�tioning of soil hydrology into separate directionsshould be replaced in the future by the new stage ofintegration of the classical and thermodynamic princi�ples with the modern approaches.

166


SHEIN

ACKNOWLEDGMENTS

This study was supported by the Russian Founda�tion for Basic Research, project nos. 07�04�00131, 07�04�00866, and 08�04�00656.

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soil hydrology: stages of development, current state, and nearest prospects

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