Department of Agricultural Engineering, University of Newcastle upon Tyne, Newcastle upon Tyne, ... factors based on ratios of calculated conductivity and mea-.
WATER
RESOURCES RESEARCH,
VOL. 16, NO. 4, PAGES 749-754, AUGUST
1980
Comparisonof Measured and EstimatedUnsaturated Hydraulic Conductivity M. E. PARKES
Departmentof AgriculturalEngineering,Universityof NewcastleuponTyne,NewcastleuponTyne,England P. A.
WATERS
Field Drainage Experimental Unit, Ministry of Agriculture,Fisheries,and Food, Cambridge,England
Most studiesof empiricalestimatesof unsaturatedhydraulicconductivityfunctionsdo not accountfor water which may be relatively immobile under the conditions in which field measurementsof conductivity are made. To investigatethis, unsaturatedhydraulic conductivitydata were obtained for three monolithlysimeters,80 cm in diameterby 135 cm deep,usingthe instantaneousprofile technique.The lysimeterscontainedwell-structured,freely draining loam soil and moisturemeasurements were made usinga neutronprobe. Conductivityestimateswere also obtainedfrom laboratorymeasurements of soil moisturecharacteristics usingthe modifiedMillington and Quirk computationalmethod.Ratios of the calculatedto measuredconductivitiesat a matchingpoint near saturationwere solargeas to suggestthat only a minor proportionof the soil pore spacewas contributingto flow through the whole profile.
tor wasused.Nielsenet al. [1973]and Bruce[1972]found likewisewhen usingeither the Millington and Quirk formulation Unsaturated hydraulic conductivitydata are required for or both. Roulier et al. [1972], Greenand Corey [1971], and mathematicalanalysisof water flow throughfield soil profiles. Carvallo et al. [1976] all recommendedthat for such calcuSince field measurementsare time consuming, alternative lated valuesthe matchingfactor shouldcorrespondto a tenmeansof estimatingconductivityhavebeeninvestigatedusing sion within the range usedfor calculations.Brutsaert[1968] data which are more readily obtained. Such methodsuse soil and Mualem [1976] have suggestedthat the effectivesaturamoisture characteristicdata to define the pore sizescontribtion (Oi/Os)shouldbe modified to accountfor moisturewhich uting to flow at variousmoisturecontents.Equation (1) is a modified formof the Marshall[1958]and Millington and is unavailablefor flow, but thiswasnot consideredin the preof unQuirk [1961] equationsin which the factor KS/KSc is used to viouslymentionedstudies.In this paper, measurements saturated hydraulic conductivity are described and compared match the calculatedand observedconductivityat saturation: with calculatedconductivities usingthe Millington and Quirk formulationwith unmodifiedsoilmoisturecontents.Matching factorsbasedon ratios of calculatedconductivityand meaKSc pg• n2 i• sured conductivityare examined at the moisture content at i-- 1,2, -.- ,n whichthe matchingconductivitywasmeasured.This givesan INTRODUCTION
K(O)i - KS30y 213••, (2j+ 1- 20hj-2
assessment of whether
where
or not a residual moisture
content
shouldbe consideredin the estimationof conductivity. K(O)i calculated conductivity for a specified content, cm/min;
¾ p g •
0 water-rilledporosity,cm3/cm3;
When the exponentp takes the value zero or 4/3, then the particular representationis equivalent to that of Marshall [1958] and Millington and Quirk [1959]. The value of the exponent reflectsdifferencesbetweenthe pore interaction terms of the original formulations. The latter have been used frequently to comparepredictedconductivityvalueswith those either
in the field or on soil cores. Greacen et al.
[ 1974]and Gradwell[1974]found satisfactoryagreementwhen usingthe Marshall formulation,providedthat a matchingfacCopyright¸ 1980by the AmericanGeophysicalUnion. Paper number 80W0215. 0043-1397/80/080W-0215501.00
METHODS
A permanent pasture site was chosen for field measurements at Rock Farm, Northumberland. This acid soil has a
surfacetensionof water, dynes/cm; densityof water, g/cm3; gravitationalconstant,cm/s2; water viscosity,g/cm s;
n total number of pore classes; hj midpoint tensionfor each pore class,cm; K$/K$c measured saturated conductivity/calculated saturated conductivity.
measured
EXPERIMENTAL
749
loam texture overlyingsandyloam, formed from glacial drift and dominated by weathered sandstone.The A horizon is well structuredwith fine subangularblocky and granular structure. The B horizon is extremely variable in terms of structureand texture. The soil structureis blocky and prismatic. Bulk densitieswithin the profile vary from 1.12 at the surfaceto 1.75g/cm3 at 1-m depth. Three monolith lysimeters,80 cm in diameterby 135 cm deep,were taken at the siteby jacking fiber glasssheathsvertically into the soil using a hydraulicjack [Cannell et al., 1973].The lysimeterswere carefullyinvertedand a 1- to 2-cm layer of coarsegravelwasaddedto eachone. Baseplateswere then fitted, which includedoutflow spouts.Finally, the lysimeterswere turned upright and installedadjacentto an undergroundenclosureso that the soil surfacewas at ground level. Lysimetersheathshad beeninsertedimmediatelyadjacent to each other acrossthe slope.A neutronprobe access tube was installedin the centerof eachlysimeterfor soil water
750
PARKES AND
WATERS:
HYDRAULIC
CONDUCTIVITY
determinations using a Wallingford neutron probe [Eeles, 1969].Augerhagsfrom each lysimeterwere usedfor determination of soil particle size distributioncorrespondingto individual lysimeters.Tensiometerswith mercury manometers DEPTH were installed in each lysimeter at depths of 20, 40, 60, 80,
1
2
3
20
A1
A1
A2
40
AB1
AB1
AB2
60
B1
AB1
AB1
80
B2
B3
AB1
100
B3
B3
B2
120
B4
B2
B4
(cm)
100, and 120 cm.
Ten 150-cm3 coreswere taken at each of the 20-, 40-, 60-, and 80-cm depthsbelow the soil surfaceat three sitesimmediately adjacentto the pit from which the three corresponding monolith lysimeterswere removed. Sampleswere taken from a rectangularpit, 0.5 m2 in area, downslopeof eachlysimeter site. The area ratio of disturbed-to-sampledsoil for the sam-
pler was 3.4% [Loveday,1974].Ten 100-cm 3 coreswere also obtained at each of the 100- and 120-cmdepthsfor the same three siteswith an area ratio for the correspondingsamplerof 8.4%. Determinations
of water
retention
data for each core
were obtained at five equilibrium moisture contentsduring desorptionfrom 0.025 to 0.5 bar using sand bath and kaolin bath apparatus[Smith and Thomasson,1974]. Four samples from each set of 10 were also usedto provide three slices,I cm deep for further determinationson pressureplate apparatus. Three equilibrium moisturecontentsover the range 1-15 bar were determinedusingthesethree sliceson pressureplate apparatus. Gravimetric moisture contents for each slice were convertedto volumetricmoisturecontentsusingthe measurement of bulk density for the whole core from which the slice was taken.
Weekly measured,paired valuesof in situ moisturetension and volumetric soil water content were recorded on the lysimetersover a period of 12 months.Both wetting and drying cyclesoccurredduring this period. These valueswere used to plot in situ water retentiondata, and they definedthe range of hystereticbehavior. A range of water flows was set up within each lysimeterusingtrickle irrigation equipment.Flows were
.or
izon_s__s_a_m_P.!e_d__
X
•
[;>
3:
AB1
•'
I•=
AB2
