Time Domain Reflectometry Coaxial Probe for Water Content Measurements of Soil
Publication Type
Conference Paper

Water resources are utilized for many human activities. However, there is often a mismatch between the water requirements and the withdrawal actually available and sustainable. This mismatch  may become even worst as a consequence of climate change. In fact, climate change affect both water demand and water resources and it is highly probable that society will suffer increased water requirements without having a proportional increase of available water resources.

Irrigation is demanding huge withdrawals of water. One region where these problems are self evident is North-Western Italy, where the water requirements for agriculture are important in large parts of the territory. Such water demands strongly depend on meteorological variables. In addition in the whole Alps, glaciers are retreating their volume, leading in the long term to a decrease of water for irrigation. Such water requirements are strictly derived from the durations of periods with low soil moisture values. Its deficit has to be replenished by irrigation and reliable techniques has to be utilized for precisely and reliably measuring soil moisture in soils ranging from sandy to clayey ones.

Knowledge of water content is thus important for water resources management and risk prevention. Volumetric water content  of soil may be measured in situ using Time Domain Reflectometry (TDR). This technique is noninvasive and does not require sample collection for time-consuming laboratory measurements.

The aim of this work was to develop a TDR probe that can be used for loose materials while ensuring optimum working conditions in terms of ease of insertion into and contact with the surrounding porous medium, and homogeneity and confinement of electrical energy storage. The designed probe uses eight brass blades 15 mm wide and 165 mm long disposed along a cylinder and a stainless steel rod in the center. The characteristics of the new designed probe with respect to the standard three-rod probes, represent a compromise in terms of ease of insertion into the porous medium, good contact with the surrounding, and homogeneity and confinement of electrical energy storage. Signals from the new probe permits more reliable and accurate volumetric water content measurements. Once calibrated, the new probe allows measuring volumetric water content within a small error range. The performance of the new probe was tested on various porous media. A standard three-rod TDR probe and a coaxial probe were used for comparison. The criterion of mean absolute error and the associated standard deviation, calculated on the difference between the water content values measured by the TDR probes and those determined by gravimetric observations were used to determine  its performance.


Conference Title
EGUL_eo 2012
Conference Country
Conference Date
Nov. 14, 2012 - Nov. 16, 2012
Conference Sponsor