Field measurement of soil hydraulic properties characterizing water movement through swelling clay soils

Journal of Hydrology, 45 (1980) 149--158

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© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

Technical Note [4] FIELD MEASUREMENT OF SOIL HYDRAULIC PROPERTIES CHARACTERIZING WATER MOVEMENT THROUGH SWELLING CLAY SOILS

J. BOUMA

Soil Survey Institute, 6700 AB Wageningen (The Netherlands) (Received and accepted for publication June 13, 1979)

ABSTRACT Bouma, J., 1980. Field measurement of soil hydraulic properties characterizing water movement through swelling clay soils. J. Hydrol., 4 5 : 1 4 9 - - 1 5 8 . Determination of hydraulic properties in swelling clay soils is difficult due to seasonal variations in pore geometry and to soil structural heterogeneity, requiring large samples. Large in situ columns are therefore prepared to measure K~t and Kunsat near saturation, using the crust test. Soil structure descriptions can be used to estimate optimal sample size. Infiltration into dry or moist clay soil with vertically continuous cracks involves preferential vertical movement of water along the cracks. The resulting irregular moisture distribution does not allow use of many common soil physical techniques, which require homogeneous moisture contents in isotropic soil.

INTRODUCTION

Soil moisture regimes can be simulated with numerical or analytical techniques, which are based on Darcy-type flow theory, using basic hydraulic data in terms of hydraulic conductivity and moisture retentivity. Currently there are many simulation models but lack of rapid and reliable procedures to obtain representative basic hydraulic data for soils in the field limit their practical applicability. This problem is particularly relevant for swelling clay soils which are characterized by seasonal variations in hydraulic properties due to processes of swelling and shrinkage which alter pore geometry. For example, hydraulic conductivity at saturation (Ksat) of a Dutch clay soil was found to range from 50 m/day, when an initially dry soil horizon was moistened (and large cracks were still continuous}, to 1 cm/day when the soil had been allowed to swell for several months, thereby reducing the width of *Paper presented in Commission I (Soil Physics), International Society of Soil Science, Edmonton, Alberta, Canada (1978).

150 the cracks (Bouma and WSsten, 1979). Such phenomena require special testing techniques and create large variabilities, of which some aspects will be discussed.

Some techniques for measuring hydraulic properties of swelling clay soils Field measurements are, in principle, preferable to laboratory measurements because they reflect better the natural boundary conditions which govern flow processes in the field, for example in terms of overburden potentials, temperature and soil moisture regimes and other conditions imposed by surrounding soil. Laboratory measurements require sampling of cores, in which disturbances may easily occur during sampling and transportation. Clay softs are particularly sensitive to disturbances because flow processes are often governed by the dimensions and the continuity of only a few larger pores, which generally occupy a very small fraction of soil porosity (Bouma et al., 1977, 1979a). However, laboratory measurements often allow more controlled experimental conditions and may be acceptable for applied field work, but only when large undisturbed cores are used. Unfortunately, columns packed with sieved aggregates are still widely being used despite of convincing data that results are often not representative for natural soil (e.g., Cassel et al., 1974). Measurement of hydraulic properties must be separately discussed for wet and dry clay soils, because of significant experimental and theoretical differences associated with both condition~

Fig. 1. Use o f an undisturbed large soil c o l u m n for measuring Ksa t. The c o l u m n was carved o u t in situ and encased in gypsum. This assembly can also be used to measure Kunsa t by applying crusts at the infiltrative surface and using subcrust tensiometry (from Bouma, 1977).

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Fig. 2. Picture of a horizontal cross-section through a clay soil with "smooth" prismatic peds (air-dry soil peel). The width of the large cracks is strongly reduced upon swelling; resulting in a strong reduction of Ksat. The vertical hydraulic conductivity of clay soils in The Netherlands is important to determine whether or n o t subsoil tile drains are effective in lowering seasonally high water tables. The auger-hole m e t h o d for measuring Ksat was f o u n d to be unsuitable because of puddling of the walls of the test hole during augering which resulted in unrealistically low Ksat values. A field test was therefore devised to measure Ksat by carving out in situ a large cylinder of soil with a diameter of 30 cm and a height of up to 40 cm (Bouma et al., 1976; Bouma, 1977) (Fig. 1). The column can contain one or more horizons. Steady infiltration rates into the column, which is encased in gypsum and detached from the underlying soil thus allowing free outflow and calculation of Ksat, are measured with an infiltrometer. This procedure avoids disturbances associated with pushing cylinders into clay soil. Results from some 80 measurements, made in early spring to ensure complete swelling, demonstrate the variability of Ksat in soil between 30 and 70 cm below the surface (Bouma et al., 1979b). Ksat values ranged from 1 c m / d a y in some clay soils with smooth prismatic peds (Fig. 2) ( " p e d s " are natural aggregates) to 5 m / d a y when some worm- and root-channels were present (Fig. 3) (Bouma and WSsten, 1979). Hydraulic conductivity at pressure potentials near saturation, where structural pores such as cracks and worm- and root-channels most dominantly affect the

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Fig. 3. Picture of a horizontal cross-section through a clay soil with "rough" prismatic peds and large, vertical root- and worm-channels (air-dry soil peel). The width of the cracks is strongly reduced upon swelling but flow along the channels remains. A high Ksat is maintained.

flow regime, can best be measured by the crust test which also uses the large in situ columns already discussed (e.g., Bouma, 1977). A crust composed of specific mixtures of sand and gypsum is placed on t o p of the column, creating unsaturated conditions in the subcrust soil. The steady flux through the crust, as measured by the infiltrometer, is equal to Kunsat at the measured subcrust pressure potential, when the hydraulic head gradient is equal to 1 cm/day. This steady-state m e t h o d proved successful up to potentials o f - 40 cm water or so. Other methods must be used at lower potentials, because the crust test then requires t o o much time. The in situ instantaneous profile method, which can also be used in the laboratory, has limitations for measuring K-curves of clay soils because of slow drainage and evaporation rates. In fact, water in these soils is only effectively removed by growing vegetation. Also, slow drying of a wet clay soil results in an increase of the evaporation rate with time, rather than a decrease, as should be expected according to flow theory. This phenomenon, which complicates calculations for the instantaneous profile method, is due to the formation of vertical cracks upon drying, which increase the evaporative surface. Also, shrinkage upon drying results in poor contact between tensiometer cups and surrounding soil. Measurement of retentivity curves should preferably be made in situ using neutron or gamma probes for determining moisture contents and tensiometers with transducers for measuring the corresponding pressure potentials. If soil

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cores are used, they should be large and a flexible coating (e.g. of Saran ® plastic) can be applied to allow a determination of changing volumes upon shrinkage (Grossman et al., 1968). Existing flow t h e o r y does, as such, not apply to infiltration into dry, cracked clay soil where water may move rapidly downwards along vertical cracks in unsaturated soil ("short-circuiting") even when applied in relatively low quantities and intensities. A separation should be made between flow into and through the often large peds and (free) flow into and through the larger vertical cracks, which can only occur if t h e y are open at the infiltrative surface. Rather than to measure infiltration rates into the entire dry, cracked soil under ponding conditions, it is advisable and more realistic to measure them into the peds only, considering the (limited) capacity of the peds to accept liquid as one critical factor governing free flow through the cracks. Other critical factors are quantity and intensity of the water, applied as rain or sprinkling irrigation. A small infiltrometer with a height and diameter of 7.5 cm has been used successfully to measure infiltration rates into single soil peds (often large prisms)

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Fig. 4. Schematic diagram of the measurement of the infiltration rate into a p e d (from Bouma et al., 1978).

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as a function of time (Bouma et al., 1978) (Fig. 4). A single dry or moist prism is sampled, the infiltrometer is carefully placed on top and gypsum is applied all around for support. Initially high infiltration rates into the prisms, may allow absorption of relatively high intensity rainfall (il) for a short period t < tl (or low intensity rainfall (i2) for a longer period t < t~) (Fig. 5). However, the infiltration rate into the prisms decreases and continued rain cannot entirely be accepted by the prisms as soon as the application rate exceeds the infiltration rate (t > tl for i~ and t > t2 for is, respectively). Then, water will flow into a crack along narrow bands on vertical faces of prisms which were made visible by dyes (Fig. 6) (Bouma and Dekker, 1978). The n u m b e r of bands was a function of application intensity and duration. Measurements of infiltration into dry clay soils in The Netherlands have shown that the observed depth of penetration was about six times deeper than predicted with classical flow t h e o r y assuming the presence of homogeneous soil. The measured moisture content in the wetted zone was m u c h lower than the predicted one (45 vol.% rather than 60 vol.%, with corresponding pressure potentials o f - 4 a n d - 0 . 0 0 2 bar, respectively). Infiltration rate or rainfall

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These results applied to application rates between 8 and 30 mm/h, which were well below measured Ksat values. Short~circuiting, as described, is not restricted to dry soil but occurs even more strongly in moist soil as long as large vertical pores are continuous. The

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Fig. 6. Samples from vertical prism faces aIong cracks showing 5 mm wide stained bands at a depth of 50 cm below the surface of a dry cracked clay soil. The stains resulted from spraying colored water on the soil surface at an intensity of 30 ram/hr., during 30 min. and are representative for infiltration patterns of rainfall or spray irrigation into dry clay soil. Infiltration into the cracks occurs when the application rate exceeds the vertical infiltration rate into the upper surface of the prism. p h e n o m e n o n has major practical implications because rainfall or water applied by sprinkling irrigation may rapidly move beyond rooting depth, whereas deep leaching of fertilizers, pesticides or waste products can result in unexpected groundwater pollution. Unfortunately, m a n y c o m m o n in situ physical methods for determining soil moisture conditions (tensiometry, neutron probe, etc.) cannot be applied satisfactorily due to the preferential flow patterns of the water, which result in a highly heterogeneous moisture distribution. Empirical procedures m a y be most suited at this time to obtain relevant data for practical applications. However, a simulation model has been proposed (Hoogmoed and Bouma, 1980).

Variability of measurements Variability may be due to inadequate experimental procedures using small samples or to inherent properties of the soil. Small core samples cannot

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adequately represent (wet) soil horizons in clay soils which may consist of peds having diameters of up to 10 cm or even more. Reference should always be made to the size of the elementary unit of structure, requiring soil structure descriptions prior to sampling. The elementary unit will be a single grain in a sand, but may be a large ped in an aggregated clay soil. Any sample, whether it is measured in situ or in the laboratory, should in principle consist of at least a minimum number of elementary units, which can be determined by statistical analysis of sampling data. This implies the need for different sample sizes among soils and among different horizons within the same soil. A standard 100-cm 3 core of a coarse sand contains approximately 65,000 individual grains {elementary units). A horizon consisting of (small) subangular blocky peds with a diameter of 1 cm would have to be characterized (when applying the hydraulic c o n d u c t i v i t y

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same standard) by a sample of at least 65 It Perhaps this size is unrealistically large but more effort should be made to define optimal sample sizes for different soil horizons, also at different moisture contents. As discussed in the previous section, subsampling and characterization of single peds may be needed in slightly moist or dry horizons when peds are mutually separated by large, continuous voids. Sample height of pedal soils is crucial when Ksat is to be measured, because of the effect on vertical large-pore continuity. Ideally, samples should be as high as the horizon to be measured. Errors may result if samples are shorter. 5 cm high samples of a subangular blocky silty clay loam soil horizon (sampled when wet) yielded an average Ksa t of 670 cm/day, which was reduced to 130 cm/day (sample height: 10 cm) and 80 cm/day (height: 15 cm) (Anderson and Bouma, 1973) (Fig. 7). The probability of large pore continuity decreases as sample height increases and Ksat is therefore bound to decrease as well. A planar void interaction model, which was developed to predict vertical large-pore continuity and Ksat, was used successfully to explain the observed differences (Bouma and Anderson, 1973; Bouma et al., 1979a). Variability among test results in the same clay soil may still be considerable even when large samples have been used. Variability depends on the type of test For example, at random extraction of soil moisture with small suction cups may result in variable data even in large samples due to the undefined location of the cups, which may or may not intercept preferential flow paths of water. Identical problems may occur with tensiometry or with moisture content measurements. Attempts should then be made to develop selective sampling

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procedures in situ, for example o n t h e basis of a soil morphology description. Measurement of fluxes (as in the crust test) respond better to using large samples due to the averaging effect of the increased upper surface area of the sample. But even then, statistical procedures are needed to interprete variability among replicate measurements in the same type of soil occurring within an area. The probability of K being at or above (or below) a certain level should be expresses rather than some "average" value. For example, the K-curve of several silty clay loam B horizons was measured by the author in Wisconsin (U.S.A.) with the crust test and data indicate, for example, an average K at a pressure potential o f - 1 0 cm of 6 c m / d a y (Fig. 8). However, the 95% confidence interval of the log-normal distribution was between 0.7 and 30 cm/day (Baker and Bouma, 1975). Defining K-curves and moisture retentivity data in such terms allows the user to choose his own required degree of accuracy which will depend on the type of application and, particularly, the risks and costs involved. REFERENCES Anderson, J.L. and Bouma, J., 1973. Relationships between hydraulic conductivity and morphometric data of an argillichorizon. Soil Sci. Soc. Am. Proc., 37: 408--413. Baker, F.G. and Bouma, J., 1975. Variability of hydraulic conductivity in two subsurface horizons of two siltloam soils.Soil Sci. Soc. Am. J., 40: 219--222. Bouma, J., 1977. Soil survey and the study of water movement in unsaturated soil.Soil Surv. Inst.,Wageningen, Soil Surv. Pap., No. 13, 107 pp. Bouma, J. and Anderson, J.L., 1973. Relationships between soilstructure characteristics and hydraulic conductivity. In: R.R. Bruce (Editor), Field Soil Moisture Regime. Soil Sci. Soc. Am., Spec. Publ. No. 5, Ch. 5, pp. 77--105. Bouma, J. and Dekker, L.W., 1978. A case study on infiltrationinto dry clay soil,I. Morphological observations. Geoderma, 20: 27--40. Bouma, J. and W~sten, J.H.M., 1979. Flow patterns during extended saturated flow in two undisturbed swelling clay soilswith different macrostructures. Soil Sci. Soc. Am. J., 43: 16--22. Bouma, J., Dekker, L.W. and Verlinden, H.L., 1976. The vertical hydraulic conductivity at saturation of some Dutch " k n i k " clay soils. Agric. Water Manage., 1: 67--69. Bouma, J., Jongerius, A., Boersma, O., Jager, A. and Schoonderbeek, D., 1977. The function of different types of macropores during saturated flow through four swelling soil horizons. Soil Sci. Soc. Am. J., 41: 945--950. Bouma, J., Dekker, L.W. and Wbsten, J.H.M., 1978. A case study on infiltration into dry clay soil, II. Physical measurements. Geoderma, 20: 41--51. Bouma, J., Jongerius, A. and Schoonderbeek, D., 1979a. Calculation of hydraulic conductivity of some saturated clay soils using micromorphometric data. Soil Sci. Soc. Am. J., 43: 261--264. Bouma, J., Dekker, L.W. and Haans, J.C.F.M., 1979b. Drainability of some dutch clay soils: a case study of soil survey interpretation. Geoderma, 22(3): 193--203. Cassel, D.K., Krueger,. T.H., Schroer, F.W. and Norum, E.B., 1974. Solute movement through disturbed and undisturbed soil cores. Soil Sci. Soc. Am. Proc., 37: 36--38. Grossman, R.B., Brasher, B.R., Franzmeier, D.P. and Walker, J.L., 1968. Linear extensibility as calculated from natural clod bulk density measurements. Soil Sci. Soc. Am. Proc., 32: 579--573. Hoogmoed, W.B. and Bouma, J., 1980. A simulation model for predicting infiltration into cracked clay soils. Soil Sci. Soc. Am. J. (in press).