by Linzy Carlson

As water restrictions become greater and dry conditions persist, careful soil moisture monitoring and irrigation scheduling will continue to be an important aspect of irrigated agriculture.  Automatic soil moisture monitors, in combination with AgriMet crop water use data, can be a great tool in assisting irrigators with this area of their operation.  This article deals mainly with the Hansen AM400 moisture monitor, but much of the information can be generalized to other brands of monitors.

How Do the Monitors Work?
Each monitor is attached to Watermark sensors, which consist of cylindrical gypsum blocks (plaster of Paris) covered by a metal mesh.  The sensors dry out and wet up with the soil throughout the course of the irrigation season.  Each sensor has two wires that connect to ports in the Hansen AM400 data logger.   Essentially, the logger measures electrical resistance in the sensor and converts that to usable information.  Each 6"x8"x2" logger can monitor up to two different sites, with each site containing three sensors buried in one hole at different depths, usually 8", 18", and 30" for shallow rooted crops and 12", 24", and 48" for deep rooted crops.  Sensors can be located up to 1000' away from the logger and connected by telephone cable.  Each sensor represents a foot or foot and a half section of the soil profile, so the sum of the readings from the three sensors gives the total moisture content of the soil profile.

What Do the Monitor Readings Mean?
The graphic at the right shows a moisture monitor screen.  The "S3" indicates which sensor the machine is showing data from, in this case Sensor #3.  The "14C" indicates soil temperature in oC.  The monitors do not give direct moisture content readings in inches. Rather, they give readings in centibars (cB), which is a unit of pressure that measures how much "suction" the plants must exert to pull water from the soil.  On the Hansen meters, a larger reading means the soil is drier, requiring greater suction by the plant.  To give some perspective, a soil at 0 cB is at field capacity, meaning that after an irrigation or rain, gravity has drained away all the water it can.  Basically, a soil at 0 cB is very wet.  Conversely, a soil at 1500 cB is at permanent wilting point, meaning it is too dry for plants to extract water.  Most plants operate successfully in the range from 0-200 cB, depending on the soil type and the plant.  The "16CB" on the graphic is the pressure reading, so this particular soil is still quite wet.  The graph on the monitor screen covers the last 5 weeks, with peaks corresponding to  irrigations or rain and valleys indicating drier times.  The scale on this graph ranges from 0-25 cB, but as the crop dries out and the pressure readings increase, the scale will adjust to show those values.

Converting Centibars to Inches
The readings from the moisture monitors represent the force with which water is held in the soil and are not readily usable in terms of irrigation scheduling.  Fortunately, the relationship between moisture content and pressure is well documented, so it's not a big step to convert the pressure reading from the monitor to inches of plant available water.  The following table specific to shallow rooted crops, such as grains or beets, grown in silt loam or clay loam soils converts the readings from the monitor for each of the three sensors to inches of available water.

Conversion of Centibars to Inches of Available Water in a Silt Loam or Clay Loam Soil
If the Reading (in centibars, cB) in the Upper Right Corner of the Soil Moisture Monitor is:
Sensor #1 at 8" (Represents moisture contents of the 0-12" depth)
Sensor #2 at 18" (Represents moisture content of the 12-24" depth)
Sensor #3 at 30" (Represents moisture content of the 24-36" depth)
10cB or Less
You have 1.9" of water in the 0-12" depth.
You have 1.9" of water in the 12-24" depth.
You have 1.9" of water in the 24-36" depth.
11-20 cB
1.65"
1.65"
1.65"
21-30 cB
1.45"
1.45"
1.45"
31-40 cB
1.2"
1.2"
1.2"
41-50 cB
1.0"
1.0"
1.0"
51-60 cB
0.8"
0.8"
0.8"
61-70 cB
0.55"
0.55"
0.55"
71-80 cB
0.35"
0.35"
0.35"
81-85 cB
0.15"
0.15"
0.15"
86 cB or More
You have no water left.
You have no water left.
You have no water left.

 

Example: Assume Sensor #1 reads 45 cB, Sensor #2 reads 65 cB, and Sensor #3 reads 55 cB.   Add up the moisture content (blue, bold-faced) numbers to get the total water in the profile available to the plant (1.0" + .55" + .8"=2.35").

Go to AgriMet
Now, it's time to go to AgriMet to find out how long that 2.35" will last.  The AgriMet network, run by the Bureau of Reclamation, supplies real time plant water use figures for a variety of crops around Montana.   Since AgriMet gives specific numbers for crop water use, and the soil moisture monitors give a specific amount of water held in the soil, it's quite simple to determine how long that water will last. 

To continue our example, assume that the crop is alfalfa, and yesterday it used .28" according to AgriMet.  Divide the total available water by the daily use to figure out the days before the next irrigation (2.35"/.28"=8.39 days).  Of course, the number of days can change if the weather changes, but watching the AgriMet data helps keep producers on track.  Most of the producers who tried the Hansen monitors this season had an AgriMet station within a few miles of the monitor. 

Hind Site's 20/20
The Hansen moisture monitors have built in data loggers that store all of the daily soil moisture information for the season.  The data from a monitor can be downloaded at the end of the season and graphed to show the variation in moisture content for the entire growing season.  The graph shows the readings from all of the sensors and gives a clear picture of the soil moisture trends.  Although this information is not available until after the growing season, it can be valuable for making future decisions about irrigation water management. 

Many irrigators underestimate the amount of water in their fields and irrigate accordingly.  In such a situation, the crop does not have time to use the water available to it, and much of the water applied soaks through the wet profile and is lost to deep percolation.  Most crops can withstand moisture contents as dry as 200 cB, but often producers apply water long before it reaches that point, which does not make the most efficient use of water.  Producers using the Hansen meter should use 50-100 cB as a guide, depending on the soil.  Drying the profile down to this point leaves enough moisture to be safe while using water efficiently. The following graph is from a small grain field under a center pivot.  None of the sensors in the soil profile got any drier than about 25 cB between June 28th and July 18th.  At 25 cB, a silt loam soil still has 1.45" of available water per foot of profile, so this 3' profile probably had from 4-4.5" of available water at any point during this time period.  Deep percolation is a likely fate of much of the water added at this moisture content. Each peak indicates an irrigation, so the pivot was running almost constantly.  It would have been a better use of water and power to eliminate 5 or 6 of these irrigations in order to allow the crop to use the available moisture, develop a fuller root system, and uptake nutrients from deeper in the soil profile.  The remaining irrigations must have a larger volume to compensate for the longer time period between them.  Of course, some pivots do not have the capacity to fill the profile after it is dried down like this, so the system's output must be a consideration when scheduling irrigations. 

 

This example, which  comes from a beet field under flood irrigation, exhibits a nearly ideal draw down pattern.  There was a 19-day time span between the irrigations on the 11th and the 30th, which gave the crop ample time to use the available water. The irrigation on the 30th was timely in that the crop had used most of the water in the top of the profile and some of the water in the lower depths prior to watering.  By this time, the drier soil had a greater capacity to absorb and retain the irrigation water, resulting in less loss to deep percolation.  Note that the top sensor dries out much more quickly than the deeper sensors.  This is logical since the majority of the crop's root mass lies un the upper part of the profile.

 

Where to Get Information about Moisture Monitors
If you would like more information about the Hansen AM400 moisture monitors, you can contact the company directly.  Visit their web page at http://www.mkhansen.com, or send an email to mikeh@nwi.net.    The Watermark sensors are made by the Irrometer company, and their web page is located at http://www.irrometer.com.  Their phone number is (909) 689-1701.

In Closing
The technology of the 21st century has affected all areas of human life, and agriculture is no exception.  The potential these tools create for energy and water savings is tremendous, and as people become more aware of the finite nature of the world's natural resources, such tools will have an even greater place in irrigated agriculture.