Laboratory of Soil Hydrology – Materials and Methods

The Soil Hydrology Laboratory at University of Naples Federico II has been operating for more than 50 years. During these decades, consolidating certain techniques and developing new measurement methods have always been a target for us and represented an excellence in the field of Soil Hydrology. Many field campaigns have been undertaken during these years with the major aim of supporting the various research activities of the different groups of our Division. All of the data contained in the database that we provided have been measured in our laboratory or in-situ. The Soil Hydrology Lab is equipped with the state-of-the-art equipment and advanced instruments used to measure the soil hydraulic properties through standard protocols as described in the SSSA Book, Series 5 (Methods of Soil Analysis), or new testing procedures. The underlined words will direct you to PDF files that describe in details the mentioned techniques. Apart from standard equipment, experimental facilities properly designed to study the movement of water in the soil and to measure input data to determine soil hydraulic properties are available as follows:

  • a dual-energy gamma-ray attenuation system (using 137Cs and 241Am as the radiation sources) and tensiometers enable water flow processes to be monitored in soil column, up to 1.5-m height and 0.20 m inner diameter, with nondestructive simultaneous measurements of bulk density, volumetric water content, and soil-water pressure potential;
  • an apparatus based on the principle of gamma-ray attenuation, along with tensiometers, is used to determine both the main wetting and drainage retention curves of undisturbed soil cores and enables to be tested soil cores up to 0.20-m inner diameter arranged in ten columns, each taking a maximum of five cores with a height up to 0.15 m.

The laboratory has also standard soil physics testing equipment to measure: particle size distribution (sieves; hydrometer method), oven-dry bulk density; soil porosity (pycnometer); infiltration characteristics (disk permeameter), soil moisture (TDR testers; capacitance probes), matric suction head (laboratory and field tensiometers; matric potential sensors). Measured soil properties and methods All soil cores analyzed in our lab are obtained by driving a steel cylinder vertically into the soil using a hand-operated device and excavating the soil around the cylinder by hand to reduce disturbances during sampling. For measuring soil bulk density and water retention we routinely use cylindrical steel samplers 7.0 cm in diameter and 5.0 cm in height. For determining soil water retention and hydraulic conductivity functions with the evaporation method we routinely use samplers 8.0 cm in diameter and 12.0 cm in height. In case of finer-textured and structured soils, we have collected undisturbed soil columns 40.0 cm in diameter and 60.0 cm in height.

Before performing the hydraulic tests, the top of each undisturbed soil core (approximately 0.03 -0.04 m) is removed and put aside for the particle-size analysis and organic carbon content, and for measuring soil water retention data points in the dry range through the pressure plate apparatus. Particle-size distribution A portion of the disturbed sub-sample is used to determine the particle-size distribution (PSD) by using standard laboratory techniques based on a set of sieves and the soil hydrometer (Gee and Or, 2002). Our particle-size data are primarily grouped into sand (<2000-0.05 µm), silt (50-2 µm), and clay (<2 µm) fractions according to the USDA classification. Particle-size data are also grouped according to the ISSS classification. The grouped data (% sand, % silt, and % clay) are used to derive the textural class of each soil sample according to the USDA or ISSS textural triangles.

Organic carbon content 

The organic carbon content is determined with the dichromate method (Mebius, 1960).  As suggested by Schulze (1849) and Russel and Engle (1928), by convention measured organic carbon is multiplied by a factor of 1.724 to obtain organic matter.

Soil hydraulic parameters at full saturationsat, Ksat)

The height of soil that remains in the core (which depends on the total length of the soil core collected) is slowly wetted from below, until saturated, using a de-aerated 0.005 M CaSO4 solution. Firstly, the individual core, with its lower side covered by a voile held in place with an elastic band, is placed on a Perspex support perforated by a series of small holes. To obtain a complete saturation under laboratory conditions, we use the following procedure: (step a) the cores are placed in a container and water is slowly poured from below into the container until a head of water is set at 0.5 cm (with respect to the lower end of the core) and this head is maintained for 48 hours; (step b) 2.0 cm of water is then gradually raised in the container so that a head of 2.5 cm is set with respect to the bottom of the core and is maintained for additional 6 hours; (step c) finally the water level in the container is raised to submerge the cores, and this level is maintained for at least 4 hours.

Saturated water content, θsat, is measured by the gravimetric method (Topp and Ferré, 2002). Saturated hydraulic conductivity, Ksat, is usually measured in the laboratory by the falling-head method, but sometimes also by the constant-head method (Reynolds et al., 2002).

Soil water retention data points measured using suction tables and pressure plate extractor apparatus

After having measured θsat and Ksat, the soil core (usually of 0.04 m in height and 0.075 m in diameter) is placed on a suction table for measuring directly data points of the soil water retention function (WRF), usually considering various steps ranging from 0.01 m to about 2.50 m of suction head. The container is built with Perspex, which has the advantages of being transparent, relatively cheap, and easily machinable, and is covered with a Perspex lid to prevent evaporation. All of the connections between the various parts that constitute the suction table consist of flexible plastic tubing (e.g., Tygon tubing). The porous material used to equilibrate the water in the soil samples with an external body of water at the desired matric head is comprised of a glass microfiber membrane of fine porosity overlain by a layer of silt–kaolin mixture. The relevant equipment and procedures are fully described in the chapter by Romano et al. (2002).

Soil water contents at higher suction pressure values, usually ranging from 75 kPa to 1200 kPa, are determined using a pressure plate extractor apparatus (Dane and Hopmans, 2002).

Simultaneous determination of soil water retention and hydraulic conductivity functions using the evaporation method (direct and inverse methods)

Soil water retention and hydraulic conductivity data of a soil core are simultaneously determined from laboratory evaporation experiments by analyzing the measurements with a modified Wind’s method (Wendroth et al., 1993; Peters and Durner, 2008) or using an optimization approach (Ciollaro and Romano, 1995; Romano and Santini, 1999).

The evaporation experiment starts from an initially saturated soil core of length L and involves the measurements with time of matric head profiles and soil core weights. Starting from a condition of hydrostatic equilibrium, with a nearly zero matric head at the bottom of the soil core, hL, the evaporation experiment is performed by draining the core with a small fan placed near the top and sealing completely the lower end of the core. In some cases, the small fan is removed and evaporation occurs at room temperature. During the transient flow event, the total weight of the soil sample, Pj, (by a load cell) and the matric head, hij, (by horizontally inserted micro-tensiometers) at different soil depth zi, where z=0 is the top of the soil sample, are measured at frequent but irregular time tj. All data were recorded automatically and processed using a data-logger and a personal computer. The selection of the size of the soil sample as well as of the number and locations of the tensiometers were determined based on the works of Ciollaro and Romano (1995) and Romano and Santini (1999).

Direct evaporation experiment

In brief, the soil core subjected to the evaporation experiment is split into two (or more) sections centered around each micro-tensiometers. An iterative calculation is set up to determine the water retention function.

First, an approximated water retention function is fitted by a polynomial regression on the basis of the average water contents measured for the entire core by the load cell, and the mean values of the measured matric heads. Then, a polynomial function is assigned to each section of the soil core to predictwater contents at the measured matric heads. By comparing at fixed times the mean predicted water contents and the measured  water contents, the iterations are updated until no improvement was detected from a statistical viewpoint. After determining the water retention function, the hydraulic conductivity function is computed using a modified instantaneous profile method. Readers wishing further details are directed to the works by Wendroth et al. (1993) and by Peters and Durner (2008).

Inverse evaporation experiment

The water flow in the soil core induced by the evaporation experiments is simulated by the Richards equation, while parametric relations (such as, for example, those of van Genuchten-Mualem, or of Brooks and Corey, or some more complex bimodal analytical relationships) are assumed to describe the soil hydraulic properties, i.e. the soil water retention and hydraulic conductivity functions. Estimation of the unknown hydraulic parameters and the associated uncertainty is carried out by employing a Maximum Likelihood (ML) approach. Additional details of this inversion technique and the minimization algorithm can be found in the paper by Romano and Santini (1999).

Bulk density and porosity

At the completion of the hydraulic tests, we measure the oven-dry bulk density of the specific soil core being tested. For most of the soil cores of our database, total porosity was calculated from the measured oven-dry bulk density assuming that soil particle density is 2.65 g/cm3. In some cases, however, soil particle density is measured using the pycnometer method.

References cited

Ciollaro, G., and N. Romano. 1995. Spatial variability of the hydraulic properties of a volcanic soil. Geoderma 65:263-282.

Dane, J.H., and Hopmans, J.W., 2002. Soil Water Retention and Storage - Introduction. In “Methods of soil analysis, Part 4, Physical Methods” (J.H. Dane and G.C. Topp, eds.), pp. 671-674, SSSA Book Series No. 5. SSSA, Madison, WI, USA.

Gee, G.W., and Or, D., 2002. Particle-size analysis. In “Methods of soil analysis, Part 4, Physical Methods” (J.H. Dane and G.C. Topp, eds.), pp. 255–293, SSSA Book Series No. 5. SSSA, Madison, WI, USA.

Mebius, L.J., 1960. A rapid method for the determination of organic carbon in soil. Anal. Chim. Acta. 22:120-124.

Peters, A., and W. Durner. 2008. Simplified evaporation method for determining soil hydraulic properties. J. Hydrol. 356:147-162.

Reynolds, W.D., Elrick, D.E., Young, E.G., Booltink, H.W.G., and J. Bouma, 2002. Saturated and field-saturated water flow parameters: Laboratory methods. In “Methods of Soil Analysis, Part 4, Physical Methods” (J.H. Dane and G.C. Topp, eds.), pp. 826–836, SSSA Book Series N.5, Madison, WI, USA.

Romano, N., and A. Santini. 1999. Determining soil hydraulic functions from evaporation experiments by a parameter estimation approach: Experimental verifications and numerical studies. Water Resour. Res. 35:3343-3359.

Romano, N., Hopmans, J.W., and Dane, J.H., 2002. Water retention and storage: Suction table. In “Methods of Soil Analysis, Part 4, Physical Methods” (J.H. Dane and G.C. Topp, eds.), pp. 692-698, SSSA Book Series N.5, Madison, WI, USA.

Russel, J.C., Engle, E.B., 1928.  The organic matter content and color of soils in the central grassland states.  In: Deemer, R.B. (Ed.), Proceedings and Papers of the First International Congress of Soil Science, June 13–22, 1927, Washington, D.C., USA.  International Society of Soil Science, Washington, DC, USA.

Schulze, F., 1849.  Anleitung zur untersuchung der ackererden auf ihre wichtigsten physikalischen eigenschaften und bestandtheile.  Praktische Chemie 47, 241–335.

Topp, G.C., and Ferré, P.A., 2002. Water content. In “Methods of Soil Analysis, Part 4, Physical Methods” (J.H. Dane and G.C. Topp, eds.), pp. 417–545, SSSA Book Series N.5, Madison, WI, USA.

Wendroth, O., W. Ehlers, J.W. Hopmans, H. Kage, J. Halbertsma, and J.H.M. Wösten. 1993. Reevaluation of the evaporation method for determining hydraulic functions in unsaturated soils. Soil Sci. Soc. Am. J. 57:1436-1443.

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Università degli Studi di Napoli "Federico II"

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