Ferrer, Francesc

Gallart, Francesc

Josa March, Ramon

Latron, Jerôme

Llorens, Pilar

Poyatos, R.

Rubio, Carles Miquel

English

UPCommons

Evaluation of two methods for measuring saturated hydraulic conductivity of soils under two vegetation covers

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 Evaluation of two methods for measuring saturated hydraulic conductivity of soils under two vegetation covers   C.M. Rubio (1,2), R. Josa(1), R. Poyatos(3), P. Llorens(4), F. Gallart(4), J. Latron(4),   F. Ferrer(2)  (1) Departamento de Ingeniería Agroalimentaria y Biotecnología, Universidad Politécnica de Cataluña, Parque Mediterráneo de la Tecnología, 08860 Castelldefels (España). E-mail: carles.rubio@upc.edu (2) Departamento de Biofísica Ambiental y Suelos, LAB-FERRER Soils and Environmental Consulting, 25.200 Cervera (España). (3) School of Biological and Biomedical Sciences, University of Durham, Durham, (Reino Unido). (4) Instituto de Diagnostico Ambiental y Estudios del Aigua (IDÆA), CSIC. Barcelona, (España)    ABSTRACT The main goal of this work is to determine and to evaluate the saturated hydraulic conductivity for a silt loam soil in field and laboratory conditions. The experimental area was located in the Vallcebre research catchments, in headwaters of the Llobregat River (NE Spain). Hydraulic conductivity was measured in the field using the Guelph permeameter and field saturated hydraulic conductivity (Kfs) based on Elrick equation was calculated. The Guelph permeameter measures were made in two conditions (dry and wet) and in profiles below two vegetation covers (meadows and forest). To determine the saturated hydraulic conductivity at the laboratory (Ks) the constant head permeameter was used. The average Kfs values for the wet period was about 2 cm·h-1. During the dry period, both soil profiles presented higher values, about 7.5 cm·h-1. Under laboratory conditions, mean observed Ks values were between 12 and 25 cm·h-1. The relationship Kfs/Ks was of 0.1 cm·h-1 in wet conditions and about 0.4 cm·h-1 in dry conditions. The results indicated significant differences between both methods and between both seasons. Differences can be explained by the anisotropy of  soils as a consequence of vegetation root system that promotes preferential flows paths.   Keywords: Preferential flows, Soil water content, Constant head permeameter, Guelph permeameter  INTRODUCTION Three important physical properties govern the infiltration process: (i) the soil matric flux potential (φm)  defining the soil water retention capacity (Gardner, 1958); (ii) the hydraulic conductivity (K)  defining the flow transfer in the porous media (Hillel, 1980); and (iii) the sorptivity (S)  defining the soil capacity to absorb water when the water flow is influenced by a pressure gradient (Philip, 1957). According to the studied scale, the hydraulic conductivity can be measured by different instruments. Some of these are the constant head permeameter used to obtain measurements at the laboratory (Kessler and Oosterbaan, 1980), and the Guelph permeameter to measure the hydraulic conductivity in situ (Reynolds and Elrick, 1985, 1986).   The main goal of this work is to determine and to evaluate the hydraulic conductivity for a silt loam soil in field and laboratory conditions. This objective was split in two tasks (i) to determine soil saturated hydraulic conductivity under different vegetated covers in situ and in the laboratory , and (ii) to compare both methods for determining hydraulic conductivity at saturation regime.  MATERIALS AND METHODOLOGY TOPIC 4: WATER-SOIL-VEGETATION607 The experimental area was located in the Can Vila research catchment (NE, Spain) (Gallart et al., 2005). Four soil profiles (two under mesophylous grassland cover M-1 and M-2, and two under Scott pines cover F-1 and F-2) were used. These profiles presented high textural homogeneity, and were classified according to USDA particle size class as silt loam (profiles M-1, M-2 and F-2), and silty-clay loam (profile F-1). Mean bulk density was 1.3 g·cm-3. Calcium carbonate content presented three well-defined groups (with 5%, 30%, and about 50% respectively). Mean organic matter content on the whole of soil profiles was 4.1% (Rubio, 2005).  Hydraulic conductivity was measured in the field using the Guelph permeameter, and field saturated hydraulic conductivity (Kfs) was calculated with Elrick equation (Elrick et al., 1985). In plots M-1 and F-1 Nine measurements per plot at three depths (15, 25, and 50 cm) were done. These measures were made in two conditions (i) dry season; and (ii) wet season.   At the laboratory, saturated hydraulic conductivity (Ks) was determined using a constant head permeameter. 71 unaltered soil samples were collected from different depths in all soil profiles (M-1, M-2, F-1, and F-2). Saturated hydraulic conductivity was determined using a constant water charge.   RESULTS AND DISCUSSION Table 1, presents the results of the measurements performed with the Guelph permeameter.. The values of field saturated hydraulic conductivity (Kfs) were multiplied by a factor of 2 (Reynolds and Elrick, 1985) to obtain a reasonable estimation of saturated hydraulic conductivity (Ks).  The average Kfs values for wetting season were about 2.0 cm·h-1. During the dry season, both soil profiles presented  values more than 3 times higher (about 7.5 cm·h-1). Similar values were determined by Haro et al. (1992) in soils profiles in close proximity to the Can Vila catchment. The Kfs values for all soil profiles in wet conditions decreased in depth, being this decrease more marked in the grassed profiles (Table 1).  For dry conditions, the soil profiles under forest decreased clearly in depth as a consequence of high quantity of roots at surface, which increase the porosity and the water flow into the porous media. The soil profiles under grassland presented profiles with two differentiated Ks values, the superficial values being 10 time greater than the deepest ones (Table 2).  Table 1. Mean values of Field saturated hydraulic conductivity (Kfs) measured with a Guelph permeameter under different soil moisture conditions. Std = standard deviation; CV = variation coefficient. Soil depth  n ⎯Kfs (cm·h-1) Std (cm·h-1) CV (%)  n ⎯Kfs (cm·h-1) Std (cm·h-1) CV (%) Dry conditions  Wet conditions 15 cm 3 8.3 0.4 123.4  3 4.6 0.1 26.5 25 cm 3 13.1 0.1 11.9  2 1.2 0.2 154.1 50 cm 3 1.0 0.3 85.9  2 0.1 0.6 402.1 Meadows 9 7.5 6.1 81.5  7 2.0 2.3 114.8 15 cm 3 18.2 0.4 88.4  3 2.2 1.1 834.5 25 cm 3 2.5 0.1 25.2  2 1.9 0.1 28.9 50 cm 3 2.3 0.2 38.1  1 0.5 — — Forest 9 7.7 9.1 119.1  6 1.5 1.4 82.7   ÁREA TEMÁTICA 4: RELACIONES AGUA-SUELO-VEGETACIÓN608 This difference could be related with a highest gravel and sand content above 50 cm. Sorptivity (S(ψ)) values of the soil profiles were very similar, decreasing its in depth during the wet season for the grassed plots, and increasing for the forested ones.   Table 2  shows that Ks measured at the laboratory had mean values around 18 cm·h-1. These values, higher than the measured Kfs, ranged between 0.2 and 38.6 cm·h-1 depending on depth. For all profiles Ks decreased from surface to bottom. At the grassed profiles surface Ks (5-15 cm) was about 6 times greater than at depth (50 cm). This difference was probably large in the forested profiles, but the lack of information at depth prevents to clearly affirm this tendency (Table 2).  Table 2. Mean values of Ks according to constant head permeameter CV = variation coefficient. Soil depth n Ks (cm·h-1) CV (%) n Ks (cm·h-1) CV (%) Soil profile under meadows 0-5 cm 3 13.7 15.5 3 35.7 10.3 5-10 cm 3 36.5 24.1 3 26.7 9.6 10-15 cm 3 32.2 14.3 3 18.7 31.4 25-30 cm 2 11.1 105.6 2 38.6 15.2 50-55  cm 2 5.4 21.3 2 4.2 10.7 Average 14 16.9 71.4 13 24.8 36.4 Soil profile under forests 0-5 cm 3 21.5 20.7 3 21.7 6.9 5-10 cm 3 15.0 7.6 3 28.7 5.6 10-15 cm 3 27.6 9.6 3 17.6 34.4 25-30 cm 1 8.7 — 2 1.1 450.3 50-55 cm — — — 1 0.2 — Average 10 18.2 44.9 14 11.9 98.3  A comparison of the saturated hydraulic conductivity measured by both methods (Kfs and Ks) is shown in Table 3. In both cases, Mean Kfs/Ks  was 0.10 in wet conditions, and 4 times greater in dry conditions (Table 3). These values were much lower than those found by Bouwer (1966), which established a range for this relation between 1.67 and 2.50. An ANOVA test indicated statistically significant differences between both methods. These differences could be explained considering the possible alterations suffered by the samples during the extraction. In fact, the extraction of samples to perform laboratory analyses could  involve the formation of preferential flow paths, and therefore increase Ks values.  Table 3. Comparison between field saturated hydraulic conductivity (wet and dry conditions) (Kfs) and laboratory scale (Ks).   Field     Laboratory Wet Cond. Dry Cond. Soil depth n Kfs-Wet n Kfs-Dry  n Ks Kfs/ Ks Kfs/ Ks 15 cm 3 4.6 3 8.3  3 32.2 0.14 0.26 25 cm 2 1.3 3 13.1  — — — — 50 cm 2 0.3 3 1.0  2 11.1 0.03 0.09 Meadows 7 2.1 9 7.5  5 21.6 0.10 0.35 15 cm 3 2.2 3 18.2  3 27.6 0.08 0.66 25 cm 2 2.1 3 2.5  1 8.7 0.24 0.29 50 cm 1 0.1 3 2.3  — — — — TOPIC 4: WATER-SOIL-VEGETATION609 Forest 6 1.7 9 7.7  4 18.2 0.09 0.42  Differences may perhaps be explained by some characteristics of the processes. Constant head permeameter measured Ks in a vertical direction, where preferential flows, due to the macro-porosity or the conducts produced by bioturbation and roots plants could be important. On the contrary, Guelph permeameter measured the Kfs of a wetting bulb, which included horizontal and vertical directions. In addition, the swelling-shrinking processes observed in these soils, could entail  collapses of the macrospores, and as a consequence a reduction of the hydraulic conductivity. Finally, textural homogeneity in the first 15 cm depth, determined a rapid steady-state conditions of the water flow, and fewer variations of the process.   CONCLUSIONS  The highest field saturated hydraulic conductivity of the studied soil profiles during the dry period, may probably be explained by swelling-shrinking processes, which allowed the development of macropores. The opposite process occurred during the wet period and Kfs values decreased considerably. The comparison between field and laboratory methods shown significant differences, with field saturated conductivity values always lower than laboratory ones.  REFERENCES   Bouwer, H. 1966. Rapid field measurement of air entry value and hydraulic conductivity of soil as significant parameters in flow system analysis. Water Resour. Res., 4, pp. 729-738.   Gardner, W.R. 1958. Some steady state solution of the unsaturated moisture flow equation with application to evaporation from a water table. Soil Sci., 85, pp.228-232.   Haro, S.; Fernández, J.F.; Josa, R., Gallart, F. 1992. Papel hidrológico y geomorfológico de las propiedades del suelo en una zona pirenaica de campos abandonados (Cal Parisa, Vallcebre). In: López-Bermúdez, F.; Conesa, C., Romero, M.A. (Eds.), Estudios de Geomorfología en España, 1, Sociedad Española de Geomorfología, Murcia. pp. 243-250.   Hillel, D. 1980. Application of soil physics. Academy Press, New York, 385 p.   Kessler, J., Oosterbaan, R.J. 1980. Determining hydraulic conductivity of soils. In: Drainage Principles and Applications. Vol. III. ILRI, pp. 253-296.   Philip, J.R. 1957. The theory of infiltration. 4. Sorptivity and algebraic infiltration equations. Soil Sci., 84, pp. 257-264.   Reynolds, W.D.; Elrick, D.E. y Clothier, B.E. 1985. The constant head well permeameter: effect of unsaturated flow. Soil Sci. Soc.Am. J., 139, pp. 173-180.   Reynolds, W.D., Elrick, D.E. 1985. In situ measurement of field-saturated hydraulic conductivity, sorptivity and α-parameter using the Guelph permeameter. Soil Sci., 140, pp. 292-302.   Reynolds, W.D., Elrick, D.E. 1986. A method for simultaneous in-situ measurement in the vadose zone of field-saturated hydraulic conductivity, sorptivity and the conductivity-pressure head relationship. Groung Water Monit. Rev., 6, pp. 84-95.   Rubio C.M. 2005. Hidrodinámica de los suelos de un área de montaña media mediterránea sometida a cambios de uso y cubierta. PhD Tesis, Universitat Autònoma de Barcelona, 195p   ÁREA TEMÁTICA 4: RELACIONES AGUA-SUELO-VEGETACIÓN610

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Evaluation of two methods for measuring saturated hydraulic conductivity of soils under two vegetation covers

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