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Irrigation water management – soil water sensors
By Gary Stone, UNL Extension Educator, and
Dr. Gary Hergert, UNL Soil and Nutrient Management Specialist
Soil water sensors are instruments placed in a field to monitor soil water content and crop water use from a growing crop. They can also be placed in fallow or just-harvested dryland winter wheat fields to monitor off-season soil water content from trapped precipitation.
A number of different soil water sensors can be used to manage irrigation water, including individual resistance type and capacitance type,
Capacitance type soil water sensors monitor soil water content at varying depths, from very near the soil surface to as deep as five feet. This type of sensor is contained in one tube placed in the soil. Generally, it is connected to a satellite-internet system where a producer can get the relatively instant soil water content of a field via a PC, laptop, or smart phone. Usually a producer does not own this type of soil water sensor, but leases it for the growing season. It can be expensive.
Individual resistance soil water sensors are more basic. These sensors are installed at given depths, depending on the relative root zone of the crop. A producer can take readings with a hand meter, install a monitor to record the readings throughout the growing season, or connect via a satellite-internet system and get the relatively instant soil water content of a field from a PC, laptop, or smart phone.
A producer owns this type of sensor and monitors. They can be relatively inexpensive, but there is a service charge for the satellite or cell phone hookup.
These types of sensors can last up to five years or more with proper care.
Both types of sensors do a good job of monitoring soil water content. Which type of sensor to use depends on how much a producer wants to spend and how much convenience they want. There may be cost share for either of these types of soil water sensors from the U.S. Department of Agriculture Natural Resources Conservation Service, local natural resources districts, electrical power supply company, or area conservation district.
Resistance-type soil water sensors, more commonly referred to as Watermark™ soil water sensors, are glued to a length of PVC tubing for the depth the sensor is to be installed. The sensor is placed in a hole in the soil created by a soil probe. It’s important to mark the sensor depth on the tube or cap.
The deeper sensors would show if the producer is over-applying irrigation water, causing deep percolation that goes past the roots of the growing crop (the deep sensor stays wet), or if the producer is utilizing the available soil water in profile and the deep sensors gradually go dry.
In the field, sensors are usually placed under a center pivot along the access road to the pivot point, somewhere between the second and third tower in from the outside. Placement should be in a representative part of the field with the same overall soil type.
To determine what soil type in that field, go to the USDA NRCS Web Soil Survey. Click on the green “WSS Start” icon to get started. Select your state and county, refine your selection to determine your field location, and then load the soil maps to determine the soil types and locations in a field.
If a producer is using a hand meter to take readings from the soil water sensors or has installed a monitor to record the readings, the producer can take those readings and go to the UNL Nebraska Agricultural Water Management Network web site.
- At this site, clicking a link titled “Watermark Sensor Reading Calculator” brings up a calculator page, on which the producer will select the basic soil type and enter the readings from his soil water sensors.
- Clicking the “Calculate” icon will take him to a page with the results for his soil water sensor readings. This page displays how much water has been depleted by the crop at each sensor depth in the soil and in total inches, between 100 percent capacity and 50 percent field capacity. It also displays how much water is remaining in the soil profile, in inches, between 100 per cent capacity and 50 percent field capacity.
- The page will also provide the producer with an average soil water sensor reading and link to determine if the producer should apply water to the crop.
- Clicking the link “Table - WatermarkTM sensor centibar reading versus soil texture water (inches)” will take them to a new page with irrigation trigger points for different soil types. Selecting the soil type displays the “*Suggested range of irrigation trigger point” box for that soil type. If the calculated average soil water sensor reading is near or above the trigger point, the producer should apply water to his crop.
The UNL Nebraska Agricultural Water Management Network web site has more information on assembling the soil water sensors with PVC tubing and installing them in the field. The web site also has links to UNL Extension publications on soil water sensors and the “Crop Water App” for the Watermark Sensor Reading Calculator for smart phones.
For more information, contact Gary Stone, Extension Educator, at 308-632-1230.