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United States Department of Agriculture

Agricultural Research Service

Title: Infiltration Measurement Using a Vertical Time-Domain Reflectometry Porbe and a Reflection Simulation Model

Authors
item Timlin, Dennis
item Pachepsky, Yakov

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 1, 2001
Publication Date: June 1, 2002

Interpretive Summary: Infiltration of water into soil is an important hydrological process. The ability to calculate crop water budgets, water, pesticide runoff, and infiltration of fertilizers and chemicals depends on the ability to quantify infiltration. The ability to quantify infiltration also depends on knowledge of the saturated hydraulic conductivity of soil. This property can vary greatly from location to location in a field. Over time, simple methods to measure infiltration of water can provide a useful tool for hydrologists and soil scientists to quantify saturated hydraulic conductivity and resultant infiltration for a large number of locations in a landscape. Time Domain Reflectometry (TDR) is a method used to measure water content in soil. The TDR method employs a probe using three steel rods 1/8" in diameter, 12" long. We developed a method to track the change of water content along the rods as water infiltrates into the soil. The method gave excellent estimates of wetting front depth as a function of time. This method can be automated to track the wetting front advance at a large number of locations. The result of this research will be a better understanding of the distribution of infiltration rate in a landscape.

Technical Abstract: Experimental methods are needed to simultaneously measure infiltration at a number of locations during rainfall or irrigation. The objective of this study was to test the feasibility of using a Time Domain Reflectometry (TDR) probe installed vertically into the soil to track the progress of the wetting front during infiltration. We used a numerical method to simulate wave traces. The dielectric constant above the wetting front and probe characteristics were known. The trace simulation method was coupled to a nonlinear optimization program to fit the apparent lengths of the TDR probe above and below the wetting front and the dielectric constant of the soil below the wetting front. The optimization program employed a genetic algorithm. The progression of the wetting front into the soil was recorded as a function of the apparent length of the section of the TDR probe above the wetting front. Direct measurements of the wetting front advance were obtained from observations of the wetting front in a clear acrylic cylinder packed with soil and water. The method gave excellent estimates of wetting front depth as a function of time.

Last Modified: 10/1/2014