Author
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Guber, Andrey |
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Submitted to: Theory of Methods of Soil Physics
Publication Type: Book / Chapter Publication Acceptance Date: 5/11/2007 Publication Date: 9/1/2007 Citation: Guber, A.K. 2007. Unsaturated soil hydraulic conductivity: The field infiltrometer method. Theory of Methods of Soil Physics, Sein E.V. and Karpachevsky (eds.), Grif and Company Publishing House, Moscow, Russia. pp. 270-273. Interpretive Summary: Unsaturated hydraulic conductivity is one of the most important soil hydraulic parameters as it characterizes the ability of soil to conduct water when soil pore space is not fully filled with water. The unsaturated hydraulic conductivity depends on soil water content, and therefore its values have to be known at various soil water contents. Because different soil water contents can be created in soil by applying different suctions or negative pressures, measurements at different negative pressures provide a series of values of the unsaturated hydraulic conductivity at different soil water contents. A simple method is described that allows one to do such measurements in the field. A Mariott device is used that automatically maintains the water level constant in a tube with a low-conductivity porous membrane at one of the tube ends. The membrane is placed parallel to the soil surface on a thin sand layer. Because the conductivity of the membrane is lower than the conductivity of soil, water flow through the membrane cannot saturate the soil. Soil sucks water in faster than it comes from the membrane, and therefore the soil remains unsaturated. Eventually, the suction ability of soil equilibrates with the conducting ability of the membrane and the flow stabilizes. A mathematical model of such flow is fitted to the data to obtain the unsaturated hydraulic conductivity; a parameter in this equation provides the best fit. An example of field measurements for the B1 horizon of Greyzem soil illustrates the calculation procedure. Measurements are repeated at different heights of water in the Mariott device to obtain the unsaturated hydraulic conductivity at different soil water contents. This simple, inexpensive and flexible method is an important component in the soil physicist's toolbox. Technical Abstract: Theory: Field methods to measure the unsaturated soil hydraulic conductivity assume presence of steady-state water flow. Soil infiltrometers are desired to apply water onto the soil surface at constant negative pressure. Water is applied to the soil from the Marriott device through a porous membrane and infiltrates into the soil forced by soil capillarity. Measurements at different negative pressures, maintained in the Marriott device, provide data for unsaturated hydraulic conductivity calculations. Protocol: An infiltrometer has to be installed on a 3-5 mm sand layer above soil surface. A negative pressure of -25 cm is created in the Marriott device and the water influx into soil is initiated. Water loss is measured every 30 minutes in the Marriott device. Measurements continue for four to eight hours until the relationship between infiltrated volume and time becomes linear and four ‘time-volume’ data pairs are obtained of this linear dependence. Then negative pressure in the Marriott device changes and measurements are repeated at pressure -15, -10, -5, -2.5 and -1 cm as described above. Values of soil hydraulic conductivity are calculated based on Wooding analytical solution for infiltration from a cylindrical source combined with the Gardner equation for relationship between unsaturated hydraulic conductivity and soil water pressure. An example: An example of field measurements for B1 horizon of Greyzem soil (author’s data) illustrates the calculation procedure. |
