Two Indices of Soil Structure Based on Prediction of Soil Water Processes
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DIVISION S-6SOIL & 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@example.com).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.
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) toTable 1. Criteria used for classifying site land use.fLand useclassf CriteriaDDa 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"1matric 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
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,...