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Time Domain Sensors


The other type of sensors that respond to soil electrical properties are those that measure the travel time of an electrical pulse along a probe in contact with the soil. The electrical engineering equations that describe how these sensors operate do not include a geometric factor, so the shape and extent of the EMF permeating the soil do not affect the travel time. Travel time is a linear function of water content in most agricultural soils but with some calibration differences between soils due to soil texture. Soil BEC and temperature have minor effects on the travel time, although very high BEC values can prevent measurement of the travel time. Until recently, the time domain sensors were mostly used only by scientists due to the complex and expensive time domain reflectometry, or TDR, equipment needed. However, the ubiquity of high frequency electronic chips in the cell phone industry has now allowed miniaturization of the TDR circuit to fit into the head of a sensor that can be inserted into the soil. This greatly reduced expense and eliminated the complex coaxial cabling and multiplexers needed for traditional TDR. Direct coupling of the TDR circuit and the sensor electrodes (metal rods) improved the accuracy of the method and expanded the range of BEC values over which the TDR method can reliably measure travel time and sense water content. These sensors are accurate enough for MAD-based scheduling and can be installed at depths as needed (see fig. 3).


Figure 1


Capacitance Sensor EM Field Geometry


 Sensor in uniform soil uniform geometry 


 But capacitance sensors obey Gauss’ law: C = ga


0


 Sensor in soil with structure and more or less conductive (wetter or drier) areas  geometry (g) changed


 Both soils have same mean water content but readings differ.


Drier


Electrode Electrode


Wetter


Uniform soil


Electrode Electrode


Figure 2


The MAD Irrigation Scheduling Paradigm 1 –


Management


0.8 – 0.6 – 0.4 – 0.2 – 0 –


Figure 3


0.4 0.3


Steven R. Evett, PhD, is a research soil scientist with the


USDA Agricultural Research Service, Conservation and Production Research Laboratory, Bushland,


Texas. Evett also serves as the ARS research coordinator for the Middle


East Regional Irrigation Management Information Systems Project, which


has research and extension partners in Israel, Jordan and the Palestinian


Authority; and he co-leads the Water Saving Technologies Flagship Project – a USDA-China cooperative effort in advanced irrigation technologies.


200 202 204 206 208 210 Day of year, 2016


Photo credit: Steve Evett, USDA ARS


This work was supported in part by the Ogallala Aquifer Program, a consortium between USDA-Agricultural Research Service, Kansas State University, Texas AgriLife Research, Texas AgriLife Extension Service, Texas Tech University and West Texas A&M University.


0.2 1


0.8 0.6 0.4 0.2 0


4 in. 8 in. 12 in. 18 in. 28 in. 40 in. FC WP f-Dep.


– allowed depletion


Fairly narrow range of water contents, e.g., 0.25 to 0.33


Saturation Field capacity Refill point  Permanent wilting point


irrigationtoday.org 15


Water content {in./in.}


Soil volume {in./in.}


Fractional depletion (–)








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