Tuesday, July 5, 2022

Irrometer Watermark Series: Construction Guide

Mississippi State Extension put together a publication covering the Irrometer Watermark series. The assembly guide below depicts the Irrometer Watermark 200SS, a granular matrix sensor commonly used in Mississippi for scheduling irrigation. This publication provides a step-by-step guide to proper Watermark sensor construction. Following these steps will make the sensors easier to install at the intended depths and easier to remove at the end of the season.

Tools & Supplies

The tools and supplies pictured below will be used. Fifteen feet of sensor wires is usually convenient.

One set of 6-inch, 12-inch, 24-inch, and 36-inch sensors will require one 10-foot stick of ½-inch Class 315 PVC pipe. Select the correct PVC specifications to avoid frustrations later.

ABS-PVC transition cement is best for joining the ABS sensor collar to a primed PVC section. Using a different cement can increase the risk of sensor-PVC separation during removal.


Step 1: Using the PVC cutter, cut a PVC length that is 10 inches longer than the intended sensor depth to simplify sensor removal. Following the color code below for that sensor depth, wrap a ring of colored electrical tape 4 inches from one end of the cut PVC section.

Sensor depth

PVC length

Tape color













Step 2: Using the electric drill and a 3⁄16-inch drill bit, make a weep hole ¼ inch from the untaped bottom end of the PVC section.

Step 3: Apply PVC primer at least ½ inch inside the bottom end of the PVC section. Be ready for drips. 

Step 4: After a few minutes of drying, thread the sensor wires from the bottom end of the PVC section to the top end until the sensor collar meets the bottom end. Bundle the extra wire.

Step 5: Carefully apply an appropriate amount of ABS-PVC transition cement to the sensor collar.

Step 6: While aligning the sensor weep slot with the drilled weep hole, push the sensor collar fully into the bottom end of the PVC section.

Ensure that the weep hole will allow water to drain out.

Step 7: Wrap a ring of black electrical tape so that the distance between the bottom of the tape and the middle of the attached sensor equals the intended sensor depth. After installation, the bottom edge of the black electrical tape should be flush with the ground.

Placing a rubber washer around the PVC section can reduce water flow down the installation hole.

Step 8: Slide a rubber cap onto the top end of the PVC section. The construction is now complete!

This publication is a contribution of the National Center for Alluvial Aquifer Research (NCAAR), the Mississippi State University Extension Service, and the Row-Crop Irrigation Science Extension and Research (RISER) initiative. NCAAR is supported by the Agricultural Research Service, United States Department of Agriculture (USDA), under Cooperative Agreement number 58-6001-7-001. RISER is supported jointly by Mississippi Soybean Promotion Board, Mississippi Corn Promotion Board, Mississippi Rice Promotion Board, Cotton Incorporated, Mississippi Peanut Promotion Board, and by the Conservation Innovation Grants program at USDA’s Natural Resources Conservation Service under award number NR203A750008G007.

This information and photos were provided by the Mississippi State University Extension Service; Jacob Rix, Extension/Research Associate; Himmy Lo, PhD, Assistant Extension/Research Professor; Drew Gholson, PhD, Assistant Professor; and Mark Henry, Extension Associate, Delta Research and Extension Center.

For more information, please contact Extension Irrigation Specialist Drew Gholson at drew.gholson@msstate.edu or (662) 390-8505.

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