Using soil morphology to develop measurement methods and simulation techniques for water movement in heavy clay soils J. Bouma Netherlands Soil Survey Institute Wageningen , The Netherlands Abstract Soil morphology data, in terms of number, size and length of large soil pores per unit surface area, are used to define: (1) optimal sample sizes for physical measurements and (2) computer simulation models for water movement in heavy clay soils. The simulation models use standar d numerical procedures to predict vertical and horizontal fluxes , while morphology data provide boundary conditions for the flow system. The morphological techniques being used require not only a description but also application of staining and/or filling of large pores with gypsum. Examples are discussed for cracks (planar voids) and cylindrical worm­ channels. Soil morphology provides data that cannot be obtained with physical methods. However, such data are only rel evant for char acter­ izing water movement when applied in a soil physical context. 1 Introduction Many heavy clay soils have relatively large cracks or root and animal burrows which occur in a fine-porous soil matrix. This matrix has a very low hydraulic conductivity (K) and significant fluxes of water and sol­ ute through the entire soil are therefore only possible when continuous large pores are present. These large pores are unstable as their dimen­ sions change upon swelling and shrinking of the soil following wetting and drying, --· - 298 A particularly complex condition is found when free water infiltrates along vertical cracks into an unsaturated soil matrix. Such processes have widely been observed in the field (e.g. Thomas and Phillips, 1979; Beven and Germann, 1982), and the name "short-circuiting" has been used to describe the phenomenon (Bouma et al., 1978). The term "bypass flow" will be used hereafter. Flow of water and solutes in heavy clay soils is quite different from flow in more sandy soils in which most soil pores contribute to water movement. The purpose of this paper is to review Dutch procedures which use soil morphological techniques for defining flow patterns in heavy clay soils. Attention will be focused on the measurement procedures themselves and on use of data in simulation models for water and solute transport. 2 Flow patterns 2.1 Introduction Flow patterns in heavy clay soils are difficult to characterize with physical methods. Measurement of fluxes provides, of course, no clues. Breakthrough curves provide information on large-pore continuity by their point of initial breakthrough (e.g. Westen and Bouma, 1979), but they do not indicate the functioning of various pores in the soil. It is sometimes important to know whether rapid breakthrough is due to flow along one large continuous pore or to flow along several pores. Also, flow along planar voids (cracks) has different dynamics tha.n flow along root- and worm-channels. Obviously, physical methods can not be used to distinguish different types of pores. Morphological staining techniques, to be discussed in the three follow­ ing subchapters, have been used successfully to define flow patterns in heavy clay soils. 2.2 Saturated flow Undisturbed samples of a clay soil that had been close to saturation for a period of several months, were percolated with a 0.1% solution of me- - 299 - the samples were freeze-dried and thin sections were made in which stained pore walls were observed next to morphologically idential pores with unstained walls (Figure 1) . Figure l. Thin secc:!.on image of a wer: clay soil in which water conduct­ ing planar voids are stained with methylene-blue (dark pore­ walls). Only continuous voids are stained. Staining indicates pore continuity which is more crucial to hydraulic conductivity than the often used pore size distribution . The studies, cit ed above, resulted in the follmving conclusions : (1) K was govern- sat ed by small pore necks in the flow system with diameters of approx:!.mate- ly 30 !m. Small changes in pore-neck sizes had a large effect _on Ksat . 1 For example, a pore neck of 22 ~resulted in a K of 5 cmd and a sat 1 neck of 30 ~m in a K of 25 cmd . (2) Water-conducting (stained) sat larger pores usually occupied a volume that was lower than 1% . Such pores should therefore be characteri zed in terms of numbers per unit area rather than in terms of relative volume. (3) Flow occurred mainly along planar voids (cracks). This contradicted the common assumption for Dutch clay soils that cracks close completely upon swelling. (4) Using morphological data, K t of six different clay soils could be calculated sa accordlng to: - 300 - (l) 3 2 in which: P =liquid density (kgm ); g • acceleration of gravity (m s ); -1 -1 2 n • viscosity (kgm s ); S =cross-sectional area of soil (m ) contain- ing a length of 1 (m) of stained plane slits with neck width d (m) and n n channels with neck radius r (m). Neck widths were calculated from the n size distribution of stained pores, using a newly developed pore-conti- nuity model (Bouma et al., 1979). 2.3 Bypass flow Two case studies are discussed: (1) A solution of methylene blue in water was used for sprinkling irri­ gation on a dry, cracked clay soil (Bouma and Dekker, 1978). Soil below the experimental plot (1.0 x 0.5 m) was excavated and visual observations were made of the infiltration patterns of the water, which consisted of 5 to 7 mm wide vertical bands on ped faces. The total number and surface area of bands were determined in soil below the 0.5 m2 plot for each 10 em depth increment down to 100 em below surface. Five sprinkling inten­ sities were tested in four different clay soils. The total surface area of bands, to be called "contact area" (S) hereafter, is an important characteristic as it defines the area which is available for lateral in­ filtration into the (dry) peds. The contact area increases up to 200 cm 2 as sprinkling intensity becomes higher (Bouma and Dekker, 1978), but the stained fraction of the total vertical surface area of cracks remains low in all cases. A coarse prismatic structure with peds of 10 em cross­ section has (per 10 em thickness increment) a contact area of 20 000 cm 2 2 2 in a plot of 0.5 m • The maximum stained contact area of 200 cm repre­ sents, therefore, only 1% of the potentially available vertical surface of infiltration. (2) Methylene blue in water and a gypsum slurry were used to trace in­ filtration patterns in a silt loam soil with vertical worm-channels (Bouma et al., 1982). Excavation of the soil allowed an evaluation of sizes, and shapes of the vertical channels. - 301 - 2. 4 Horizontal cracking Vertical cracks or worm-channels may result in bypass flow. However, soil shrinkage also causes the formation of horizontal cracks which strongly impede upward flow of water in unsaturated soil (Bouma and De Laat, 1981). A method was devised to stain air-filled horizontal cracks at different moisture contents and corresponding (negative) pressure heads. A cube of soil (30 em x 30 em x 30 em) is carved-out in situ (Figure 2). The cube is encased in gypsum and is turned on its side. methylene blue em tensio­ meters .l'x% 10 Kmacro= K micro • ( ~00 x) Figure 2. A schematic representation of the method for measuring the area of air-filled horizontal cracks as a function of the pressure head (see text). The upper and lower surfaces are opened and two sidewalls of the turned cube are closed. Methylene blue in water is poured into the cube and will stain the air-filled cracks. The surface area of these stained cracks is counted after returning the cube to its original position. A separate cube is needed for each (negative) pressure head. The K-curve for the peds (Figure 3) is ''reduced" for each pressure head measured in a cube. When. for example. 50% of the horizontal cross sectional area is stained, K for upward flow is 50% of the K at the same pres- unsat unsat sure head in the peds • .- 302- 10 1 10-4 -10 -to2 Figure 3. K curve for a heavy clay consisting of the regular curve which defines water movement in the peds and a Kmacro curve (see Figure 2) which is used to define upward, unsaturated flow from the water-table. Three cubes of soil were used to obtain the three reduced K values that are indicated. 3 Methods 3 . 1 Introduction Several methods will be descr ibed that have particularly been developed for clay soils with large structural elements ("peds") and large pores. All methods have one feature in common . which is use of large. undis­ turbed samples. The size of samples is a function of soil structure. Use of standard sample sizes in different soils is incorrect {Bouma , 1983) . - 303 - ,~, . ("; 3.2 Cube and column method Measurement of K in clayey soils presents the following problems: (i) sat smearing of the walls of bore-holes may yield unrealistically low K sat values for the auger-hole method, which are in any case an undefined mix- ture of K (hor) and K (vert); (ii) small samples give poor results sat sat because of unrepresentative large-pore continuity patterns (e.g. Bouma, 1981) and (iii) water movement occurs only along some pores which occupy less than 1% by volume. These pores can be easily disturbed by compac­ tion which may occur when sampling cylinders are pushed into the soil. The cube method (Bouma and Dekker, 1981) avoids these problems and uses a cube of soil (25 em x 25 em x 25 em) which is carved out in situ and encased in gypsum on four vertical walls.