Transcript
Page 1: Two Indices of Soil Structure Based on Prediction of Soil Water Processes

DIVISION S-6—SOIL & WATER MANAGEMENT& CONSERVATION

Two Indices of Soil Structure Based on Prediction of Soil Water ProcessesG. W. Geeves, H. P. Cresswell,* and B. W. Murphy

ABSTRACTEvaluation of soil structure should reflect the nature and degree

of soil physical limitations to land use for local climatic conditions.This can be achieved through mechanistic simulation of soil-plant-atmosphere processes. We propose two alternative indices of soilstructure, based on predicted infiltration. The potential runoff index(PRI) is the runoff predicted from a l-in-20-yr average recurrenceinterval storm event of 30-min duration using an event-based soilwater infiltration model that utilizes Richards' equation. The runoffrecurrence index (RRI) is the average recurrence interval of a 30-min duration storm of intensity just sufficient to result in runoff. Theseindices were applied to data from 37 sites (mainly Palexeralfs, butincluding Natrixeralfs, Rhodoxeralfs, and Haploxeralfs) in southeast-ern Australia, where rainfall partitioning between infiltration andrunoff can significantly affect agricultural production. Sites with differ-ent land use histories were ranked on the basis of the RRI. Woodlandsites have a significantly greater mean ranking (mean ranking [RRI] =33.5), indicating more favorable soil structure, compared with lessconservative agricultural land uses such as heavily grazed pasture(mean ranking = 11.7) or intensively cultivated cropping where stub-ble was not retained (mean ranking = 12.4). Both indices integratethe effects of soil structure, as represented through soil hydraulicproperties, with local rainfall characteristics. They also account forsoil horizon interactions influencing infiltration. The saturation-excessrunoff generation predicted for 12 of the 37 sites indicates that struc-tural amelioration of B horizon soil should be a high priority atthese sites.

BREWER (1964) defined soil structure as the size,shape, and arrangement of the particles and voids.

Adverse changes to the structure and physical proper-ties of soils under agriculture have been termed soilstructural degradation (e.g., Williams and Chartres,1991). Such degradation is a consequence of tree clear-ing, repeated cultivation, stubble burning, subsequentloss of soil organic matter, surface sealing under sparsegroundcover, and surface compaction caused by stocktrampling or passage of machinery. In the cereal beltof southern New South Wales and northern Victoria,structural degradation can limit the productive potentialof land and is regarded as a significant problem by bothfarmers and soil conservation agencies (e.g., Geeves etal., 1995). Two important limitations to sustainable ce-real production in this region relate to the partitioningof rainfall between infiltration and runoff, a processcontrolled by soil structure. First, the amount of plant-available soil water limits potential cereal production

G.W. Geeves and B.W. Murphy, Dep. of Land and Water Conserva-tion, P.O. Box 445, Cowra, NSW 2794, Australia; and H.P. Cresswell,CSIRO Land and Water, GPO Box 1666, Canberra, ACT 2601, Aus-tralia. Received 15 Nov. 1996. *Corresponding author ([email protected]).

Published in Soil Sci. Soc. Am. J. 62:223-232 (1998).

in most seasons. Useable rainfall lost to surface runoffmay reduce potential cereal production at a rate of 15to 20 kg ha'1 mm-1 (e.g., French and Schultz, 1984;Cornish and Murray, 1989). Second, surface runoff re-sulting from prolonged or intense rainfall is a primaryagent of rill and interrill erosion. In addition, off-siteproblems, caused by runoff from agricultural land underintense rainfall, can include localized flooding and sedi-mentation damage to public utilities.

There is a need for practical measures or indices suit-able for assessing the nature and degree of soil structuraldegradation. Such measures or indices will enable thesetting of priorities for site amelioration, evaluation ofthe efficacy of ameliorative management systems, andevaluation of the degree of degradation at representa-tive sites within the agricultural environment for publicenvironmental auditing. Despite the availability of stan-dard field and laboratory methods for determining soilphysical properties, there is a lack of consensus on ap-propriate methodologies for quantitatively assessingsoil structure.

Three distinct aspects of soil structure, namely struc-tural form, stability, and resiliency have been recog-nized. Kay (1990) defined structural form as the hetero-geneous arrangement of solid and void space that existsin soil at a given time. Structural form can be describedthrough total porosity, pore-size distribution, and conti-nuity of the pore system. Structural stability is definedas the ability of the soil to retain its arrangement ofsolid and void space when exposed to different stresses.Stability characteristics are generally specific for a char-acteristic of structural form and the type of stress beingapplied. Structural resiliency is the ability of a soil torecover its structural form through natural processeswhen the applied stresses are reduced or removed.

The existence of these three aspects of soil structureplus the diversity of processes that they affect haveresulted in the use of a wide range of parameters tomeasure and describe soil structure. Many of these arenonquantitative and do not relate closely to the soil-plant-water processes controlling plant production andenvironmental degradation. The most important charac-teristics of soil structure are those with the greatestimpact on the soil processes causing the most severelimitation to specific land uses (Kay, 1990). Researcheffort should be focused on improving measurement ofthe critical properties that control these soil processes.These measurements should then be used to predictthe nature and degree of impacts on the soil processescausing limitation to land use. Then the soil property

Abbreviations: ARI, average recurrence interval; PRI, potential run-off index; RRI, runoff recurrence index.

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224 SOIL SCI. SOC. AM. J., VOL. 62, JANUARY-FEBRUARY 1998

can be interpreted on the basis of the predicted behavioror response of the soil process that it controls.

We seek to use measurements of soil hydraulic prop-erties to predict the nature and degree of impacts ofland use on important soil-water processes. We usedthis approach in developing indices to quantitativelyassess soil structural form (not stability or resiliency)for the cereal belt of southern New South Wales andnorthern Victoria (Australia). Of the various processescontrolled or influenced by structural form, we havechosen to focus on the infiltration process because ofits importance to crop production and soil degradation.Rainfall characteristics interact with soil hydraulic prop-erties, surface slope, vegetation, surface roughness, de-pressional storage, topography, and antecedent watercontent to partition infiltration and runoff. The soil hy-draulic properties that exert control over infiltration andsoil water movement are the soil moisture characteristicand hydraulic conductivity functions. Measurements ofthese, when combined and utilized with the Richardsequation, can be used to assess soil structural formthrough the hydrologic behavior of the soil (given thelocal climatic characteristics).

We propose two indices of soil structural form. Theyassess current profile condition through mechanisticsimulation of profile response to site-specific inputs andarbitrarily chosen initial and boundary conditions. Theindices relate directly to infiltration and are calculatedusing methods that represent this process mechanisti-cally. They require measurements of hydraulic proper-ties that are sensitive to changes in soil structural form.We demonstrate the indices using data from farms in thecereal belt of southern New South Wales and northernVictoria and we contrast the indices with simple mea-sures of soil structure proposed or used by others.

MATERIALS AND METHODSSampling

Soil data used here were collected from 37 sites withinan area extending from Wellington (New South Wales) to

Table 1. Criteria used for classifying site land use.fLand useclassf Criteria

DDa Cropped using direct drilling, stubble retained until late burn.DDb Cropped using reduced tillage and stubble incorporation or

early burning.TTa Cropped using multiple-pass tillage with fined implements,

stubble incorporated.TTb Continuously cropped with multiple-pass disk tillage, stubble

burnt or heavily grazed.Gl Highly productive pasture, lightly grazed.Gin Low-productivity pasture but not overgrazed, or productive

pasture heavily grazed.Gh Heavily grazed pasture, little surface cover, sometimes surface

compaction evident.W Relatively undisturbed woodland.

t Cropping management histories were classified as traditional tillage (TT)where the following combinations of operations occurred: stubble burntbefore March and at least two tillage operations performed; stubbleincorporated with a two-way disk followed by futher tillage passes; stub-ble incorporated with a tined implement followed by two or more tillagepasses. The direct drill (DD) class included the following combinationsof operations: no cultivation before sowing, stubble retained or burntlate (March or later); single shallow tillage pass with tined implementbefore sowing, stubble incorporated or burnt early.

Charlton (Victoria) in the cereal belt of southern New SouthWales and northern Victoria, Australia. These 37 sites are asubset of 77 sites that were sampled in the post-harvest periodbetween November 1990 and April 1991. Site details, samplingmethodology, and results for the complete set are describedby Geeves et al. (1995). Sites were selected to represent thefull range of management histories and major soil types inthe region. Some relatively undisturbed woodland sites weresampled to represent pre-agricultural conditions. The manage-ment history of each site was established from questionnairescompleted by landowners. Land use was classified accordingto the criteria in Table 1. A similar range of soil types isrepresented within each land use class. The 37 sites encompassA horizon clay content from 7 to 42% and include mainlyPalexeralfs, with Natrixeralfs, Rhodoxeralfs, and Haploxeralfs(Soil Survey Staff, 1996). Three of the 37 sites (all Palexeralfs)were resampled in the post-harvest period between November1992 and April 1993 in order to enable better characterizationof within-site variability in soil properties.

Field and Laboratory Measurement of Soil PropertiesHydraulic conductivity and sorptivity were determined us-

ing disk permeameters of 200-mm diameter (Perroux andWhite, 1988; White et al., 1992). Measurements at the soilsurface were made at -0.10 J kg"1 matric potential. Hydraulicproperties of any existing surface seals were not determinedseparately from the underlying A horizon. Subsurface mea-surements at —0.10 J kg"1 matric potential were made onledges cut at depths corresponding to the base of the tilledlayer (between 0.05- and 0.10-m depths), and a further mea-surement at -0.10 J kg"1 matric potential was made in thetop of the B horizon. Measurement depths were chosen afteridentifying soil horizons and soil features in a shallow pit.Hydraulic conductivity was calculated using the method ofWhite et al. (1992).

Soil core samples 0.098 m in diameter and 0.075 m in depthwere collected directly beneath the surfaces used for the diskpermeameter measurements using a modified Tanner sampler(Mclntyre, 1974a). The soil moisture characteristic was deter-mined on these core samples using ceramic tension tablesand a pressure plate apparatus following the procedure ofMclntyre (1974b). The cores were successively equilibratedon ceramic tension plates at matric potentials of 0, -1.0, -3.0,-5.0, -10.0, and —34.0 J kg"1. Undisturbed subsamples werethen placed in a pressure plate apparatus and equilibratedsuccessively at -66, -100, -300, -500, and -1500 J kg"1

matric potential. Bulk density was determined using theknown sample volume and the combined oven dry weight ofall sample material. Soil property measurements were unrepli-cated at the 37 sites in the 1990-1991 sampling. At the threesites (Sites 19, 116, and 125) resampled in 1992-1993, surfacesoil hydraulic conductivity and sorptivity and A horizon soilmoisture characteristic measurements were completed at sixlocations per site. The B horizon hydraulic properties werenot measured at this second sampling.

Two Indices of Soil Structural FormSoil structural form was assessed by analyzing runoff re-

sponse to site-specific soil hydraulic property and rainfall in-puts. Richards' equation was utilized for mechanistic runoffsimulation.

Two indices of soil structural form are proposed:The Potential Runoff Index (PRI) is defined as the total

depth of surface runoff predicted under specified conditionswhen a one-dimensional representation of a soil profile is

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GEEVES ET AL.: TWO SOIL STRUCTURE INDICES 225

subject to the expected rainfall event of 30-min duration withan average recurrence interval of 20 yr. The conditions speci-fied are as follows: antecedent soil matric potential throughoutthe profile is -10.0 J kg"1; surface depressional storage re-mains constant in time at 2 mm; surface detention is negligible;bypass flow is negligible; and unit hydraulic gradient is main-tained at the base of the profile.

The Runoff Recurrence Index (RRI) is defined as the aver-age recurrence interval of the expected rainfall event of 30-minutes duration that is just sufficient in magnitude to resultin the prediction of runoff under conditions as specified abovefor the PRI.

We emphasize that the concept of these indices is not tospecifically incorporate all properties and processes that mightcontribute to infiltration and runoff. Producing specific runoffpredictions to match runoff observations at each site beingassessed is not the aim. Instead runoff simulation is used toindicate relative differences between sites on the basis of asubset of soil properties with full knowledge of exactly whatthat subset of properties includes and excludes. Some pro-cesses influencing runoff are specifically excluded from theanalysis (effects of surface vegetative cover and aggregatebreakdown, for example) because the aim is to isolate andlink runoff response to soil structural form. The indices aredesigned to respond only to hydraulic properties of the Aand B horizons of the soil profile as they interact with localrainfall characteristics.

Simulation of Soil-Water ProcessesThe two indices are defined independently of any particular

method for predicting infiltration. However, there would belittle value in using empirical methods that fail to account forthe main processes controlling infiltration. In this study, thetwo indices were calculated using the SWIM simulation model(Soil Water Infiltration and Movement; Ross, 1990a,b). SWIMis a one-dimensional model that simulates infiltration, runoff,evaporation, redistribution, and deep drainage in a verticalprofile of soil divided into horizontal layers. It is based onan efficient numerical solution of Richards' equation (Ross,1990a). Input variables required for SWIM that reflect soilstructural form include the soil moisture characteristic, therelation between hydraulic conductivity and soil water content(or soil matric potential), surface depressional storage, andhydraulic conductance of a surface seal (if present). Parame-ters from the Campbell (1985) soil moisture characteristicfunction were determined from the measured data and usedas model inputs. These, together with hydraulic conductivityat —0.10 J kg"1, were used to predict the unsaturated hydraulicconductivity for each soil layer following Campbell (1985).Climatic inputs required are rainfall and potential evapotrans-piration. Further description of SWIM is given by Ross(1990b).

The SWIM model was parameterized with the hydraulicproperty data collected as detailed above for each of the sites.A 0.8-m-deep soil profile was used for the simulations, withat least two layers differentiated on the basis of in situ profiledescriptions. The arbitrarily chosen initial and boundary con-ditions specified in the definitions of the indices, given above,were used for all simulations. Surface seal conductance wasarbitrarily set at a value consistent with no seal being present.Depressional storage was set at the specified constant value of2 mm. Surface detention was arbitrarily reduced to negligiblelevels at all sites. Gravitational drainage was assumed at thebottom boundary of the profile (0.8-m depth) throughout allsimulations. At the beginning of each simulation, the profile

was assumed to have a uniform matric potential of —10.0J kg"1.

In calculating the PRI, the water balance of each site wassimulated using a single l-in-20-yr average recurrence interval(ARI) design storm rainfall event of 30-min duration and fora further 210 min post rainfall (i.e., 240 min total). In calculat-ing the RRI, a range of 30-min-duration rainfall events weredesigned with recurrence intervals ranging from 1 to 100 yr.The water balance was again simulated for 240 min for eachdesign event. The characteristics of the design storm eventfor each individual site were derived using the method ofPilgrim (1987). This method uses log-Pearson Type III statisti-cal distributions fitted to historical rainfall records to estimaterainfall intensity for various frequency-duration combina-tions. Spatial interpolation of these intensities to locationslying between pluviometer sites is based on a combinationof geographical location, topographic factors, and regressionestimates of rainfall. Temporal distribution of rainfall intensityduring the 30-min event is based on temporal patterns ofintense rainfall bursts that have been recorded by pluviome-ters located within the same broad climatological zone. Thetotal amount of rainfall for the single 30-min-duration, l-in-20-yr ARI storm events ranged from 25.9 to 34.4 mm at thedifferent sites (Table 2). Design rainfall data were suppliedto the SWIM model as cumulative values at 5-min intervals.It was assumed that the soil surfaces were not vegetated andthat evaporation was negligible during the short simulation.Each of 37 different sites in the cereal belt were parameterizedin this way and values of the indices determined from therunoff predicted. At the three sites where replicate soil prop-erty measurements were available, both indices were deter-mined for each of the six sampling locations. Results from theinitial (1990-1991) sampling of the 37 sites are presented anddiscussed first, while results from the three sites resampled in1992-1993 are discussed separately.

RESULTS AND DISCUSSIONConsideration of the results requires an appreciation

of the assumptions, simplifications, and limitations in-herent in the proposed indices and the likely impactson the ability of the indices to quantitatively assess soilstructural form. The results follow a brief discussion ofthese issues.

Assumptions and SimplificationsDefinition of the proposed indices necessitates the

prescription of arbitrary antecedent soil matric poten-tials, surface depressional storage, surface detention,and lower boundary conditions for all soil profiles.These sets of conditions can strongly influence predic-tion of soil water infiltration and movement throughsoils. While these initial and boundary conditions areuniform across all profiles, their arbitrarily chosen val-ues may influence the relative site rankings that the twoindices produce. The following discussion indicates thelikely influence of assumptions and simplifications.

Antecedent Soil Matric Potential

Antecedent soil matric potential was specified as-10.0 J kg^1 throughout the profile. In many areas ofsoutheastern Australia, soils will often remain aroundthis matric potential for periods during the winter

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226 SOIL SCI. SOC. AM. J., VOL. 62, JANUARY-FEBRUARY 1998

Table 2. Calculated values of the Potential Runoff Index (PRI) and the Runoff Recurrence Index (RRI) for the 37 sites.

Site

13568910111419222324272830313233101102104106108111112116117124125132134135143144146147

Location

CowraCowraWellingtonWellingtonWellingtonWellingtonWellingtonCowraParkesGrenfellGrenfellQuandiallaYoungWest WyalongWest WyalongAlburyHardenHardenHardenCowraCowraTemoraReeftonWallendbeenAriah ParkTemoraWagga WaggaCoolamonThe RockYerong CreekCharltonCharltonCharltonBurcherOothaCondobolinCondobolin

Landuset

DDaWW/GmTTaDDaDDbDDbWTTaTTaGmGmTTbGmWDDbDDbTTaWTTbTTbTTaTTbTTbDDbGmDDaTTaDDbDDbDDb/GmTTbDDaTTaGhGhGh

l-in-20-yr ARI30-min rainfall

————————— mm ———31.831.834.434.434.434.434.431.833.931.231.130.730.730.130.127.229.729.729.731.831.829.830.329.629.929.829.129.128.928.825.925.925.931.733.132.832.8

PRI

00

15.516.77.15.77.102.9

18.40

10.617.99.003.12.13.005.6

11.92.98.7

10.56.30.76.21.83.709.505.97.42.2

15.39.8

RRI

51>100

1.2<1

6.56.85.3

279

<1472.7

<13.9

>1007.7

137.8

>1008.1172.41.33.4

165.1

145.8

861.3

303.93

10<1

2.3

t DDa, cropped using direct drilling, stubble retained until late burn; DDb, cropped using reduced tillage and stubble incorporation or early burning;TTa, cropped using .multiple-pass tillage with tined implements, stubble incorporated; TTb, continuously cropped with multiple-pass disk tillage, stubbleburnt or heavily grazed; Gl, highly productive pasture, lightly grazed; Gm, low-productivity pasture but not overgrazed, or productive pasture heavilygrazed; Gh, heavily grazed pasture, little surface cover, sometimes surface compaction evident; W, relatively undisturbed woodland.

months. Less negative antecedent soil matric potentialwould tend to increase PRI values and decrease RRIvalues. Runoff resulting from saturation excess wouldbe predicted earlier, in greater volume, and possibly atmore sites. The saturation-excess mechanism is wherewater perched on subsurface layers causes saturation ofthe surface soil and subsequent runoff. This may alterrelative site rankings and result in greater emphasis ondifferences in structural form of subsurface layers andB horizons. The use of a large antecedent water contentmeans that the indices are responsive to the sections ofthe soil moisture characteristic and unsaturated hydrau-lic conductivity function between saturation and —10.0 Jkg"1 matric potential. This is the section of the predictedhydraulic conductivity function in which we have themost confidence due to the use of a measured saturatedor near-saturated hydraulic conductivity match point.

Surface Depressional StorageStorage was set temporally constant at 2 mm in order

to preclude the influence of site-specific surface geome-try. Specifying greater surface depressional storagewould have the effect of reducing PRI values and in-creasing RRI values. Relative site ranking for both indi-ces would be unaffected by an increase in depressional

storage providing that it is not sufficient to precludepredicted runoff. In reality, surface depressional storagemay change throughout a rainfall event in response toraindrop impacts, erosive runoff, and site-specific sur-face geometry. Given the relatively low initial surfacedepressional storage specified, ignoring this dynamicprocess is unlikely to affect rankings produced by ei-ther index.

Surface DetentionSpecified as being minimal and temporally constant

for both indices. Surface detention of greater magnitudewould not affect calculated values of the RRI but wouldresult in lower PRI values. Relative site ranking pro-duced should be insensitive to changes in surface deten-tion providing that detention is not sufficient to reducepredicted runoff to negligible levels. Surface detentionis strongly affected by slope. Standardizing surface de-tention removes slope effects, allowing comparison ofdifferent sites on the basis of structural form of the soilprofile and local rainfall characteristics alone.

Temporally Constant Surface Hydraulic ConductivityRapid soil wetting and compaction caused by ener-

getic raindrop impacts on unprotected soil can result in

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GEEVES ET AL.: TWO SOIL STRUCTURE INDICES 227

surface sealing with rapid reductions in the permeabilityof the soil surface during rainfall events. Surface sealinghas been avoided in the definition of the two proposedindices for two reasons. First, the proposed indices as-sess soil structural form, whereas rates of surface sealingare influenced primarily by soil structural stability. Sec-ond, rates of surface sealing can be strongly influencedby two highly temporally variable parameters, namelysurface soil water potential and surface protective cover,which can vary independently of soil structural form.For the sites assessed in this study, measurements ofsurface hydraulic conductivity were made on settled soilsurfaces during the post-harvest period after the siteshad experienced significant amounts of cumulative rain-fall. The measured values of surface hydraulic conduc-tivity probably reflect a degree of surface sealing. Simu-lation of any further surface sealing would be unlikelyto have any significant effect on relative site rankings.

Lower Boundary ConditionFor both indices, gravitational drainage at the base

of the soil profile is specified. In reality, drainage at theprofile base may be other than gravitational. However,the short duration of the rainfall event and the 0.8-mdepth of profile specified for the simulation precludesany significant effects of the bottom drainage conditionon predicted runoff.

Negligible Bypass FlowFor both indices bypass flow is specified as negligible.

In profiles where large continuous macropores transmitinfiltrating water, such as in forest soils, the soil matrixmay be effectively bypassed. For such profiles, hydraulicconductivity measured at negative water pressures maylead to underestimation of the capacity to accept infil-trating water. This could lead to unrealistically highvalues for the PRI and low values for the RRI.

Bypass flow is an important process related to struc-tural form that would be best included in the indicesproposed here. To do this requires means to adequatelysimulate bypass flow and to measure the hydraulic char-acteristics controlling the bypass flow process. Suchmethods are not adequate at present and this is the onlylimitation to incorporation of bypass flow into theindices.

The arbitrary assumptions explained above representa degree of simplification in representation of the infil-tration process. They enable the two indices to respondjust to hydraulic properties of the A and B horizons ofthe soil profile and their interaction with local rainfallcharacteristics. The influence of factors that relate togeometry of the soil surface (i.e., depressional storageand surface detention) have been removed from theassessment. Their removal is not a suggestion that soilsurface geometry is insignificant in influencing infiltra-tion or that it has no potential as a management toolin preventing or reducing runoff. The processes isolatedfrom the analysis were chosen to suit the purpose forwhich the indices were devised. The proposed runoffindices were not devised to incorporate all processes

influencing runoff or to predict the runoff that could beobserved at any specific site. Their purpose is to enableevaluation of the structural form of a particular soilprofile independent of factors such as the geometryof the soil surface at that site, surface vegetation, andstructural changes during rainfall.

The set of processes on which the indices are basedcan be expanded or reduced depending on the aim ofthe assessment, providing that appropriate soil data areavailable and that the modeling framework used is capa-ble of representing them.

The accuracy of infiltration and runoff predictionswill always be constrained by the quality of the soilhydraulic property input data and by the accuracy ofthe method chosen to simulate field soil-water processesand the effects of structural form on these processes.The indices proposed here have been defined indepen-dently of the particular method used in this study topredict infiltration. However, they should be evaluatedusing an infiltration prediction method that adequatelydescribes the infiltration process.

Indices Differentiate between Sites that Differin Land Use or Rainfall Regime

Application of the PRI using the l-in-20-yr ARI rain-fall across the 37 sites (1990-1991 sampling) resulted ina range of values from 0 to 18.4 mm, while the RRIresulted in a range of values from <1 yr to >100 yr(Table 2).

Both proposed indices differentiated between adja-cent sites where alternative land management has af-fected structural form as measured by hydraulic proper-ties. For example, comparison of sites co-located atCowra in New South Wales (Sites 1, 3, and 101) showsa larger PRI value and a smaller RRI value for Site 101,which was subject to continuous cropping with multiple-pass disk tillage and stubble burning, than for less dis-turbed woodland at Site 3 and direct drill cropping atSite 1 (Tables 1 and 2). Although Site 1 has also beenunder continuous cropping, the soil physical conditionhas been less adversely affected by the direct drill tillageregime and no runoff is predicted under the conditionssimulated for the PRI. Site pairs 27-28 and 32-33 arealso examples of adjacent woodland and agriculturalsites. Both the PRI and the RRI differentiate these sitesand predict that the woodland sites (28 and 33) have lesspotential to produce runoff or require a larger returninterval storm before runoff is predicted.

The effect of land use is less clear in the full set of37 sites studied. Table 3 lists mean site rankings basedon RRI values for each of the differing land uses (wherea larger mean site ranking equates to larger RRI valuesand more desirable soil structural form). While meanranking is significantly larger for woodland sites thanfor the least conservative agricultural land use classes(i.e., TTb and Gh), differences between mean rankingsfor agricultural land use classes were statistically insig-nificant. This result is not surprising given that the sam-ple of sites covers a considerable range of soil materialsand rainfall regimes and is not statistically balanced

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228 SOIL SCI. SOC. AM. J., VOL. 62, JANUARY-FEBRUARY 1998

Table 3. Effect of land use on mean site ranking based on theRunoff Recurrence Index (RRI).

Land useclasst

DDaDDbTTaTTbGinGhW

Numbersampled

4876435

Mean site rankingbased on RRIi

20.6ab19.8ab16.4b13.1b21.6ab12.7b34.5a

t DDa, cropped using direct drilling, stubble retained until late burn; DDb,cropped using reduced tillage and stubble incorporation or early burning;TTa, cropped using multiple-pass tillage with tined implements, stubbleincorporated; TTb, continuously cropped with multiple-pass disk tillage,stubble burnt or heavily grazed; Gl, highly productive pasture, lightlygrazed; Gm, low-productivity pasture but not overgrazed, or productivepasture heavily grazed; Gh, heavily grazed pasture, little surface cover,sometimes surface compaction evident; W, relatively undisturbed wood-land.

I Different letters denote significant differences among means at P = 0.05(analysis of variance).

in relation to land use and rainfall. It indicates thatadditional factors such as surface soil depth, which maybe poorly related to current land use, can influencepredicted runoff.

A particular soil structural form that results in runoffoccurring very infrequently under one rainfall regimemay result in unacceptably frequent runoff under a dif-ferent rainfall regime. Both proposed runoff indices canaccount for such interaction between soil structural formand the variation in expected rainfall characteristics thatoccurs in the cereal belt of southern New South Walesand northern Victoria. For example, Sites 5, 6, 8, and9 lie near Wellington in central New South Wales andhave similar predicted l-in-20 ARI rainfall intensity(34.4-mm rainfall in 30 min; Table 2). They have PRIvalues of 15.5, 16.7, 7.1, and 5.7 mm, respectively, thatrank them 4, 3,14.5, and 19 out of the 37 example sites.Additional simulations were run for these sites usingdesign rainfall events of a lesser intensity that may beexpected with an ARI of l-in-20 in the Charlton areaof Victoria (25.9 mm in 30 min). The recalculated PRIvalues represent predictions of 7.6-, 8.6-, 0-, and 0-mmrunoff, respectively, which correspond to significantlylarger (more favorable) site rankings of 12,10,32.5, and32.5 out of 37 sites. This demonstrated capability torepresent predicted interaction between soil structuralform and regional climate distinguishes the proposedindices from other simpler structural measures that arebased purely on soil properties or features.

Accounting for Alternative RunoffGeneration Processes

Indices based on simulated soil behavior allow somescrutiny of the physical processes that underlie the pre-dicted behavior at each site. In the case of the indicesproposed here, examination of the detailed simulationoutput produced in calculating index values indicatesthe relative significance of alternative mechanisms of run-off generation. Subsurface (below the surface but withinthe A horizon) or B horizon soil hydraulic propertieslimited simulated infiltration, and increased predictedrunoff, for 12 out of the 37 sites (sampled 1990-1991).

This simulated runoff resulted from the saturation-excess mechanism where water perched on subsurfacelayers causes saturation of the surface soil and subse-quent runoff. In such instances of saturation-excess run-off, improvement of surface soil structure alone is un-likely to result in greatly reduced runoff. Table 4 listssurface, subsurface, and B horizon steady-state infiltra-tion rates and identifies those sites where low subsurfaceor B horizon permeability may result in saturation-excess runoff. The saturation-excess mechanism mayexert a significant influence on runoff generation inmany agricultural catchments in southeastern Australia(e.g., Burch et al., 1987). The capability to represent thispotentially important mechanism significantly enhancesthe applicability of the proposed indices.

Comparison of Site Rankings Producedwith the Two Indices

Comparison of relative site rankings produced by cal-culated values of the two indices (see Fig. 1) indicatesa high degree of similarity (Spearman's coefficient ofrank correlation = 0.98). Similarity in rankings is ex-pected since the values of both indices have been calcu-lated using the same infiltration model and identical soilhydraulic property data (1990-1991 sampling). Aberra-tions in the ranking occur for eight sites where PRIvalues are 0 mm, for four sites where RRI values are<1 yr, and for three sites where RRI values are >100yr. In these cases, the respective indices are "off scale"and do not provide the capability to differentiate be-tween sites. For the sites evaluated in this study, thereare fewer values that are "off scale" for the RRI thanthe PRI.

Indices Contrasted with Other Less ComplexFlow-Related Parameters

The proposed indices incorporate information on soilhydraulic properties with expected local rainfall charac-teristics to allow evaluation of a potential soil structurallimitation to land use. We now contrast the runoff indi-ces with three less complex parameters used to measureand describe soil structure. Each represents a measureof a physical property related to the movement andstorage of water in soil. Figures 2, 3, and 4 compareRRI site rankings with site ranking based either onsorptivity (Sw) measured at a matric potential of —0.10J kg"1 (Perroux and White, 1988), hydraulic conductivity(Kio) measured at a matric potential of —0.10 J kg"1

(Perroux and White, 1988), or volume of channels orvoids >0.1 mm in equivalent diameter (£100 index;Lance, 1987). These properties were measured at thesoil surface or in the A horizon (1990-1991 sampling).Site rankings clearly differ. Only rankings based on sur-face Kw show meaningful correlation with rankingsbased on the RRI (Spearman's coefficient of rankcorrelation = 0.76).

There are several reasons for the lack of similarity:(i) none of the less complex measures, on their own,describe infiltration and soil water movement under avariety of conditions (we note that combinations of soil

Page 7: Two Indices of Soil Structure Based on Prediction of Soil Water Processes

GEEVES ET AL.: TWO SOIL STRUCTURE INDICES 229

Table 4. Land use and selected soil physical properties for the 37 sites.

Sitet Location

Bulk densitySteady-state infiltration rate

(-0.10 J kg ' matric potential)

use! A horizon B horizon Surface Subsurface B horizon

135s.e.68 s.e.910 s.e.111419 s.e.2223 s.e.2427 s.e.2830 s.e.31 s.e.3233101 s.e.102104106108111112116 s.e.117 s.e.124 s.e.125132134135143144146147

CowraCowraWellingtonWellingtonWellingtonWellingtonWellingtonCowraParkesGrenfellGrenfellQuandiallaYoungWest WyalongWest WyalongAlburyHardenHardenHardenCowraCowraTemoraReeftonWallendbeenAriah ParkTemoraWagga WaggaCoolamonThe RockYerong CreekCharltonCharltonCharltonBurcherOothaCondobolinCondobolin

DDaWW/GmTTaDDaDDbDDbWTTaTTaGmGmTTbGmWDDbDDbTTaWTTbTTbTTaTTbTTbDDbGmDDaTTaDDbDDbDDb/GmTTbDDaTTaGhGhGh

—————— Mg1.601.411.401.591.631.561.561.451.461.541.491.561.581.621.461.571.631.601.571.681.061.581.441.461.521.541.571.661.581.551.791.201.581.611.471.381.57

m -*m ——————1.611.601.421.601.611.621.541.641.461.561.621.551.491.591.501.621.521.551.531.701.641.671.571.481.701.731.501.521.491.591.601.441.571.631.391.481.48

1051354510693883702013

106346

58233348223

1604624361313152463

138315530

1542019462119

——— mm h ' ——37234384

3942538

43798

54128

21288

121720-§__1163

18--7

___-

1973

1047141615243

3513209

202311412612663552

101716504

801259656967

t s.e. indicates sites where saturation-excess runoff was predicted for the PRI.:•: DDa, cropped using direct drilling, stubble retained until late burn; DDb, cropped using reduced tillage and stubble incorporation or early burning;

TTa, cropped using multiple-pass tillage with lined implements, stubble incorporated; TTb, continuously cropped with multiple-pass disk tillage, stubbleburnt or heavily grazed; Gl, highly productive pasture, lightly grazed; Gm, low-productivity pasture but not overgrazed, or productive pasture heavilygrazed; Gh, heavily grazed pasture, little surface cover, sometimes surface compaction evident; W, relatively undisturbed woodland.

§ Property was not measured.

hydraulic properties, such as sorptivity, hydraulic con-ductivity, macroscopic capillary length, water content,and soil surface storage, can be used together with rain-fall rate to determine the influence of soil structure oninfiltration and runoff; White, 1988); (ii) they do not,on their own, account for regional variations in expectedrainfall; (iii) they do not account for differences in the

30 4020RRI ranking

Fig. 1. Comparison of site ranking based on the Potential RunoffIndex (PRI) and the Runoff Recurrence Index (RRI).

depth of surface horizons that they characterize or forinteractions between subsurface soil layers or horizons.

Greater similarity in rankings produced by surfaceKw and RRI results from the direct influence of thiswater transmission property on partitioning of infiltra-tion and runoff. Surface Kw influences this partitioning

o 30 4020RRI ranking

Fig. 2. Comparison of site ranking based on the Runoff RecurrenceIndex (RRI) and sorptivity measured at -0.10 J kg"1 matric poten-tial (S,,).

Page 8: Two Indices of Soil Structure Based on Prediction of Soil Water Processes

230 SOIL SCI. SOC. AM. J., VOL. 62, JANUARY-FEBRUARY 1998

40

30

20 -

10 -

1:1

I0 10 30 4020

RRI rankingFig. 3. Comparison of site ranking based on the Runoff Recurrence

Index (RRI) and hydraulic conductivity measured at -0.10 J kg"1

matric potential (A',,,). Sites with saturation-excess runoff indicatedby open circles.

where runoff occurs under intense rainfall as a resultof surface soil hydraulic conductivity being less than therainfall rate (Hortonian flow). Where runoff occurs dueto saturation excess, the influence of surface Kw oninfiltration is much reduced. This is why similarity be-tween site rankings based on RRI and surface Kw in-creases further (Spearman's coefficient of rank cor-relation = 0.87) when sites with saturation-excess runoffare omitted from the comparison.

Rankings based on surface Sw or A horizon Em weredissimilar to RRI rankings. The influence of sorptivityon predicted infiltration is known to decrease as ante-cedent soil water content increases. Hence, it is unlikelyto relate well to predicted infiltration given the relativelylarge initial soil matric potentials specified in the defini-tions of the two runoff indices. The A horizon Em valuesdid not relate closely to predicted infiltration either.Porosity available to store water in the soil profile doesaffect saturation-excess runoff. However, the Em valuesare not good indicators of this storage capacity. The Emonly quantifies the volume of porosity in pores up to asize of 0.1-mm equivalent spherical diameter. It cannotaccount for differences in A horizon depth that affectthe volume of porosity available for storage, nor doesit necessarily relate to B horizon porosity. The A hori-zon £100 values do not relate closely to hydraulic conduc-tivity, as they are based on measurement of pore volumeand do not measure pore continuity.

Soil sampling requirements differ between the pro-

o 10 3020RRI ranking

Fig. 4. Comparison of site ranking based on the Runoff RecurrenceIndex (RRI) and the Em index of macroporosity.

posed indices and the less complex measures. In orderto apply the proposed runoff indices, data are requiredfor each soil horizon characterized. While the less com-plex soil structure parameters require less sampling andmeasurement effort, they do not provide a similar evalu-ation of soil structural form.

Within-Site Variability in A Horizon SoilStructural Form

The PRI and RRI results for each of the six measure-ment replicates (sampled 1992-1993) at Sites 19, 116,and 125, each of which is a Palexeralf (Soil Survey Staff,1996), are shown in Table 5. The variability of the indexvalues at each site arises from variability in measuredA horizon soil hydraulic properties and not from theclimate input, which is assumed spatially uniform acrosseach site, or from the simulation procedure, or fromvariability in B horizon hydraulic properties. It was nec-essary to assume B horizon hydraulic properties to bespatially uniform because replicated B horizon hydrau-lic data were not available. Thus B horizon hydraulicproperty values measured in 1990-1991 were assumedto apply at each sampling location.

The runoff generation processes at each site deter-mine whether site variability primarily reflects spatialvariation in A or B horizon hydraulic properties. WhereHortonian overland flow is predicted, then variabilityin each index will result mainly from spatial variationin A horizon hydraulic properties. Variation due to Bhorizon hydraulic properties would be evident if runoffgeneration is by the saturation-excess mechanism. Site

Table 5. Within-site variability in calculated values of the Poten-tial Runoff Index (PRI) and the Runoff Recurrence Index(RRI).

Site

19

116

125

Land l-in-20-yr ARI SampleLocation usef 30-min rainfall replicate

mmGrenfell TTa 31.2 1

23456

MeanStandard error

Wagga Wagga DDa 29.1 123456

MeanStandard error

Yerong Creek DDb 28.8 123456

MeanStandard error

LSD 0.05*

PRI

mm18.618.018.518.618.118.218.30.112.90.00.60.00.12.41.00.530.33.22.34.30.82.42.20.611.8

RRI

y<i<i<i<i<i<i_-7

201672201024.29.8

19795

158

10.62.2

22.1

t DDa, cropped using direct drilling, stubble retained until late bum; DDb,cropped using reduced tillage and stubble incorporation or early burning;TTa, cropped using multiple-pass tillage with tined implements, stub-ble incorporated.

t Applies to comparison between Sites 116 and 125 only.

Page 9: Two Indices of Soil Structure Based on Prediction of Soil Water Processes

GEEVES ET AL.: TWO SOIL STRUCTURE INDICES 231

19 exhibits substantial saturation-excess runoff and thesmall variability in index values is because of the as-sumption of uniform B horizon hydraulic propertiesacross the six sampling points.

Mean RRI for Site 116 is 24.2 yr (standard error 9.8yr), for Site 125 is 10.6 yr (standard error 2.2 yr) andfor Site 19 is <1 yr. Site 116 has undergone 15 yr ofcontinuous cropping with direct drill sowing and stubbleretention, whereas Site 125 had been cropped from 1984to 1992 with direct drill sowing but without stubbleretention. Site 19 had been subject to intensive croppingwith multiple-pass tillage prior to 1985 with a mixtureof reduced-tillage cropping and short pasture leys from1986 to 1992. Sites 116 and 125 are not significantlydifferent in terms of mean PRI or mean RRI values atthe 0.05 level (least significant difference). There areonly small differences between these sites in the rainfallamount and intensity for an event with a particularaverage recurrence interval. While these differences arerepresented in the index values, the statistical compari-son between Sites 116 and 125 reflects soil structuralform more than local climate. The A horizon soil struc-tural form at Site 116 is very good given the 15 yr ofcontinuous cropping without pasture ley. The farmerminimizes tillage, tills at appropriate soil water contents,and maximizes residue retention.

Soil structural form is known to exhibit significantspatial variability (e.g., Shouse et al., 1995) and hencethe recommended use of the indices described is tosample a sufficient number of replicates for hydraulicproperty measurement in both the A and B horizonsand then calculate the PRI and RRI for each replicate.This approach allows sites to be compared statistically.

CONCLUSIONSSoil structural form should be evaluated in ways that

relate explicitly to the important limitations or con-straints it may place on soil use. There are advantagesin doing this by mechanistically predicting response insoil water processes using measured soil hydraulic prop-erties and arbitrary initial and boundary conditions. Twoindices that use this approach have been proposed andused to assess soil structural form at sites in the cerealbelt of southern New South Wales and northern Victo-ria. These quantitative indices reflect a mechanistic inte-gration of the physical properties of different soil layersand horizons, the interaction between these soil layersand horizons, and the variation in regional rainfall char-acteristics.

Mean ranking on the basis of the runoff recurrenceindex was significantly greater (more favorable) forwoodland sites than for the least conservative agricul-tural land uses. Heavily grazed pasture sites had thesmallest (least favorable) mean ranking, being similarto sites that had been continuously cropped for a num-ber of years with multiple-pass tillage and little residueretention. A number of grazed sites appeared in poorerstructural condition than sites under intensive tillage.Of the agricultural land uses, the best rankings were forpasture that was not overgrazed, and for sites cropped

using direct drilling or reduced tillage. Saturation-excessrunoff generation was predicted, indicating that a soilstructure analysis considering both A and B horizon soilproperties is important for assessing where to targetstructural amelioration. In 12 of the 37 sites assessed,there was initial evidence for prioritizing B horizonstructural amelioration (on the basis of infiltration).Other indicators of A horizon structure, evidence ofseedling emergence problems, for example, should alsobe considered before deciding on ameliorative action.

The proposed indices of soil structural form shouldbe used together with sufficient replicate soil hydraulicproperty measurements to characterize the inherentspatial variation of the area being assessed. The simula-tion of bypass flow processes as they affect infiltrationand redistribution of soil water is a necessary futuredevelopment of the indices proposed. A prerequisite isefficient methods by which to measure the hydraulicproperties controlling these bypass flow processes.

ACKNOWLEDGMENTS

Financial support for this project (CDS4) from the Landand Water Resources Research and Development Corpora-tion and the CSIRO Land and Water Care Program is ac-knowledged. We thank the landowners who allowed us accessto their properties, Rod Drinkwater for expert technical assis-tance, and Peter Ross for making the SWIM programavailable.

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232 SOIL SCI. SOC. AM. J., VOL. 62, JANUARY-FEBRUARY 1998


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