Using Soil Moisture Sensors for Making Irrigation Management Decisions in Virginia

Prepared by: Steven J. Thomson and B. Blake Ross, Extension Agricultural Engineers, Virginia Tech

Data adapted from: Soil Moisture Sensors for Irrigation Management, Bulletin 312, University of Maryland Cooperative Extension Service, 1984; Evapotranspiration and Irrigation Water Requirements, ASCE Manual on Engineering Practice, No. 70.

Publication Number 442-024, August 1996

General Water Management Considerations

Sensor Types

Tensiometers

Gypsum Blocks

Granular Matrix Sensors

Sensor Installation

Reading the Sensors and Interpreting Their Readings

When to irrigate

Determining how much water to apply

Irrigation System Considerations

General Water Management Considerations

Across Virginia, highly variable soil and climatic conditions require different irrigation strategies. The type of irrigation system used will also dictate the timing and amount of water applied per irrigation. Ideally, a sound management plan will only replenish the active roots thereby conserving water and reducing potential leaching of nitrates and pesticides. Soil moisture sensors, if used properly, can facilitate irrigation management, conserve water, and prevent excessive chemical leaching. The following illustrates simple procedures for proper installation and use of soil moisture sensors and interpretation of their readings. Management strategies that depend on the type of irrigation system are also discussed.

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Sensor Types

Tensiometers

A tensiometer consists of a porous cup connected by a tube to a dial type vacuum gauge. The porous cup is made of ceramic and is in direct contact with the soil. A partial vacuum is created as water moves from the tube. The vacuum is an indication of the energy that a plant exerts to extract water from the soil and is commonly referred to as soil-water tension (with units of centibars (cb) or kiloPascals (kPa)). Tensiometers respond quickly to wetting and drying but can become inoperative if the soil gets too dry (a reading greater than about 80 kPa on the dial gauge). This can occur frequently with crops grown in the finer textured soils of Virginia. If the tensiometer loses water, it needs to be recharged by using a hand-held vacuum pump. Tensiometer manufacturers can supply the vacuum pump and detailed instructions on how to use it.

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Gypsum Blocks

A gypsum soil block is an "electrical resistance device" that uses gypsum as a porous material in which electrodes are embedded. Electrical resistance between the electrodes varies with soil water content and soil-water tension can be related directly to water content by the water release characteristic of the porous material. An A.C. electrical resistance meter is used to read tension. Gypsum has a characteristic much like a very heavy clay whose small pores do not begin to lose water until about 30 kPa tension. For sandy soils, more than half of the water available to the crop has already been depleted at this tension level. Thus, gypsum blocks would not be the instruments of choice for coarse soils. However, these devices are inexpensive and will work well in finer textured soils prevalent in many parts of the state and can be used over a wide tension range from about 30-1500 kPa. The gypsum blocks will dissolve, so these devices should be replaced every year.

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Granular Matrix Sensors

Another electrical resistance type sensor that has been developed in recent years is called the Watermark sensor. As with the gypsum block, the sensor's resistance varies with the electrical conductivity of solution between the electrodes. The conductivity is, in turn, related to soil water tension. Pore sizes in this matrix are larger than those of the gypsum block allowing more sensitivity in the wet range of soil moisture. Therefore, this sensor may be used in coarse soils. The Watermark sensor works in a wider range than a tensiometer (from 10-200 kPa) and does not require recharging. The matrix material is said not to dissolve, but small calibration shifts may occur with time. These sensors may be re-used the next year, but removing them from fine, compacted soils at deeper depths can be an arduous task. As with the gypsum block, the manufacturer can supply a meter to read the sensor output giving an instant readout of soil water tension.

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Sensor Installation

You want to place enough sensor stations in the field to properly represent varying conditions. Place a station in each representative location of the field that has differing terrain, soil type, or crop. For large, fairly uniform fields, you should install at least two stations. For deeper rooting crops, each sensor station should contain at least two sensors placed at different locations in the root zone. The upper sensor should be placed at the center of the effective root zone and a deep sensor should be placed near the bottom of that zone.

The depths to place the sensors will depend on the rooting characteristics of the crop in the region you are irrigating. The rooting depth is also influenced by soil type and rainfall frequency. Table 1 indicates effective rooting depths for many crops and can be used to help indicate where to place the upper and lower sensors. For corn in a loam soil, for example, sensors might be placed at 12 inch and 24 inch depths. Early in the season, the shallow sensor will show increasing tension indicating water uptake. Later in the season, when the roots migrate downward, readings from the deep sensor could be used along with the shallow sensor to indicate when to irrigate. The deep sensor can also indicate the depth of water penetration after irrigation. Subsequent irrigations could be increased or decreased depending on readings from this sensor. Detailed strategies based on these methods are illustrated later.

Sensor manufacturers supply instructions on how to install their particular sensors. Gypsum soil blocks and Watermark sensors should be soaked for at least 1 hour before installation. A general installation principle is outlined below.

To install any of the three sensor types, a hole must be dug first. Place a hole for each sensor to be installed between plants that indicate a healthy stand. A 7/8 inch O.D. steel pipe can be used to make the hole. Grind the sides of one end of the pipe to create a sharp edge around its circumference. Wet the soil at the sensor location and allow a few minutes for the water to infiltrate. Wetting this area will prevent soil from pouring back into the hole after the hole is made. If you are installing Watermark sensors or gypsum blocks, prepare a mud slurry to refill the hole above the sensor after it is placed. Place the pipe (sharp edge down) on the soil and hammer it down to the depth you wish to place the sensor. Pull the pipe out slowly. There should be a clear hole for placement of the sensor. Using an aluminum tube to make the hole may suffice for sensors placed at shallow depths in moist soil. Our experience, however, is that aluminum tubing will bend and deform if sensors are to be placed deep in compacted soils.

Tensiometers can simply be inserted into the hole until the ceramic cup "bottoms out." Pack the soil lightly around the tensiometer.

For the Watermark sensor and gypsum block, a piece of PVC pipe of a slightly smaller diameter than the sensor should be used to place the sensor. Run the sensor wires through the pipe and push the sensor to the bottom of the hole using the pipe. Once the sensor is in place, fill the hole with the mud slurry you prepared. Tamp the soil into a slight mound to prevent puddling over the sensor. Sensor wires can be fastened to stakes placed next to the sensor using insulated staples. Be sure to mark the stakes and sketch or note sensor depths at each station.

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Reading the Sensors and Interpreting Their Readings

When to Irrigate

The main question to be answered is - when do I irrigate? The time to irrigate is simply based on the sensor readings. Soil type and differing rates of crop water use due to climate changes can all be taken into account by observing sensor readings. As indicated, the type of irrigation system you have and the depths you placed the sensors also influence this determination. Specific guidelines are outlined below for determining when to irrigate using tensiometers, Watermark sensors, and gypsum blocks.

Tensiometers can simply be read by noting their dial gauge reading. Watermark sensors and gypsum blocks come with a meter you attach to the terminals. The Watermark 30KTC meter gives an instant readout of soil water tension. The Delmhorst KS-D1 meter (for gypsum blocks) also gives a digital readout which can be converted to tension using a simple chart. All sensors should be read once a day in the morning to allow for soil-water re-distribution that may have occurred overnight. Irrigation should occur when sensor reading(s) exceed a set tension level.

Research has shown that to optimize production, irrigation should begin when tensions reach 45-70 kPa in medium textured soils and 20-35 kPa in sandy soils. These ranges are dependent on crop susceptibility to drought, growth stage, and soil hydraulic properties. Corn, for example, will require irrigation at a lower tension level during its late vegetative stage than during ripening. Tension levels to trigger irrigation for sandy soils are lower because sandy soils retain much less water than finer textured soils. Tension readings will increase much more quickly (for the same amount of water use) necessitating an earlier start. Your Extension agent or specialist is your best source for determining the values to use for your particular installation, crop, and soil type.

Since sensors are typically placed at two or more depths for proper water management, you need to know which sensors to read. Sensors at different depths will, of course, read differently depending on growth stage. When the crop is young, only the upper sensor will show an increase in tension as the soil dries. Sensors in the lower soil horizons will not show a tension increase because the roots will not have penetrated deeply enough to absorb water in those zones. In that case, use the upper sensor only to determine when to irrigate. As the crop grows and the roots migrate downward, the lower sensor(s) will begin to show water use. In this case, an average reading should be taken and compared to the single trigger level to determine if irrigation is warranted. The examples outlined next illustrate how this should be done. This method was devised based on field experience and permits conservative irrigation.

Example 1: -Corn is at an intermediate stage of growth and sensors are installed at one station. The desired point to trigger irrigation is 60 kPa. The following sensor readings were noted:

	_________________________________________
			Day 1		Day 2
	12" sensor 24" sensor		35 kPa 12 kPa		80 kPa 25 kPa

The upper sensor reading is clearly greater than 60 kPa but the lower sensor shows water uptake in the lower zones. As a rule of thumb, average the two readings on day 2 if the reading at the 24" level has increased by more than 10 kPa. In this case, the lower sensor increased by 13 kPa over the day before. The average of the two sensor readings would be (80+25)/2 = 53 kPa. Irrigation would not yet be recommended because the composite reading is still less than 60 kPa.

Example 2: Peanuts are at an early stage of growth and sensors are installed at one station. The desired point to trigger irrigation is 70 kPa. The following sensor readings were noted:

Day 1

Day 2

_________________________________________

12" sensor

24" sensor

35 kPa

10 kPa

80 kPa

12 kPa

In this case, the lower sensor did not register a great change in its reading indicating little or no water use at that level. The 80 kPa reading would be taken by itself and irrigation would be recommended.

The same procedures as outlined above should be followed for additional stations in the same soil type and crop and an average tension value can be determined across all stations.

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Determining how much water to apply.

The amount of water to apply can be adjusted by observing the wetting response (after irrigation) of the deepest sensor that showed a previous drying response. Example 1 above, for instance, shows the 24" sensor drying while example 2 shows that the 12" sensor was the deepest one to show drying. Two more examples will illustrate how you might adjust subsequent irrigations according to sensor readings:

Example 3: Corn was irrigated with 1.2" of water when the soil dried enough for irrigation to be recommended. The following readings were observed the next morning (Day 2) after irrigation:

Day 1

Day 2

_________________________________________

12" sensor

24" sensor

90 kPa

40 kPa

4 kPa

15 kPa

In this case, the proper amount of water was applied because the lower sensor showed re-wetting, but not as strongly as the upper sensor. This indicates water was largely confined to the rooted volume.

Example 4: Peanuts in example 2 were irrigated with 1.5" of water when the soil dried enough for irrigation to be recommended. The following readings were observed the next morning (Day 2) after irrigation:

	_________________________________________
			Day 1		Day 2
	12" sensor 24" sensor		90 kPa 12 kPa		12 kPa 5 kPa

In this example, the lower sensor showed complete re-wetting. Example 2, however, showed that it is only permissible to re-wet the upper 12" since the lower sensor did not indicate significant roots at that depth. In this case, you should decrease the irrigation by a set amount so that only the top sensor responds. Irrigation on the next cycle might be cut back by a large amount (0.5") in this case and sensor responses could be observed again.

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Irrigation System Considerations

The above situations assume that the irrigation system applies water uniformly with time. This may be the case for drip systems or solid-set sprinkler systems that irrigate field sections represented by their own sensors. Traveling gun or center pivot systems prevalent in Virginia, for example, do not apply water uniformly with time. A traveling gun system may take up to one week for a complete run. In cases like these, the system should be started early so the crop in the last part of the field irrigated is not water-stressed. This may lead to over-application in the first part of the field to prevent crop stress in the last part of the field.

One way to help might be to speed up the system so it arrives at the last part of the field sooner. This will apply slightly less water per irrigation but will help prevent over-application in the part of the field irrigated first. Alternatively, the system could be slowed down to apply more water toward the end of the cycle. Sensors should be placed at both the first and last parts of the field irrigated and should be monitored as above. If soil variations exist in the irrigated field, the irrigation system should apply less water to soils with lower water holding capacities and more water to soils with greater water holding capacities. If you can, start the system over soils that have lower water holding capacities first. A Soil Survey map for your county (published by the USDA, Forest Service and SCS in cooperation with Virginia Tech) can indicate predominant soil types in your irrigated field. Field experience will indicate the best compromise between sensor responses and speed patterns of the system.

Table 1. Effective Root Zone Depth

_________________________________________________ Field Crops
	Crop Type	Effective Root Zone (Inches)
	Barley Corn Field Cotton Flax Oats Peanuts Rye Sorghum Soybeans Sunflower Tobacco Wheat	24 24 24 24 24 24 24 24 24 24 18 24
	Crop Type	Effective Root Zone (Inches)
_________________________________________________ Forage Crops
	Alfalfa Bluegrass Bromegrass Ladino Clover Orchardgrass Red & Sweet Clovers Sudan Grass Ryegrass Bermuda Grass Tall Fescue	24 18 24 18 24 24 24 24 18 18
	Crop Type	Effective Root Zone (Inches)
_________________________________________________ Vegetable Crops
	Asparagus Beets Broccoli Cabbage Cantaloupes Carrots Cauliflower Celery Corn (Sweet) Cucumbers Kale Lettuce Lima Beans Onions, Bunch Onions, Dry Peas Peppers Potatoes Radish Snap Beans Spinach Squash Tomatoes Watermelons	24 12 12 12 18 12 12 12 24 18 18 6 18 6 12 18 18 18 6 18 6 18 18 24
	Crop Type	Effective Root Zone (Inches)
_________________________________________________ Fruit Crops
	Apples Blueberries Cane Fruits & Grapes Peaches Pears Strawberries	24 18 18 18 18 6
	Crop Type	Effective Root Zone (Inches)
_________________________________________________ Turf
	Athletic Fields in Active Use Athletic Fields Not in Active Use Golf Greens & Fairways Grass Sod Being Established or in Preparation for Immediate Sale Grass Sod (Lawns and Sod Being Held for Sale)	6 12 6 6 12
	Crop Type	Effective Root Zone (Inches)
_________________________________________________ Flowers and Nursery Plants
Flowers
	Annual Flowers Ericaceous Ornamental Plants (Azalea, etc.) Gladioli, Peonies, Irises Other Bulb or Corm Plants	6 12 12 12
Nursery
	Bedded Plants after Propagation Finished Landscape Plants (Ready for Sale) Ground Cover Plants (Vinca, Ivy) Lining-Out Plants Pernnial Ornamentals, Trees, and Shrubs (Conifers and Flowering Shrubs)	6 18 to 24 6 12 24

Root depth was based on: 1) the depth of soil to which the larger portion of the total root system has developed when the marketable part of the crop is being produced or when the loss of water from turf and ornamental plants is greatest, 2) research and experience regarding the overall water needs of each crop for maximum quality as well as yield or growth, and 3) the kind of soil in which some crops are grown. The depth of irrigation while the crop is developing its root system should be determined by the actual root depth at that time.

Disclaimer: Commercial products are named in this publication for informational purposes only. The authors, Virginia Cooperative Extension, and Virginia Polytechnic Institute and State University do not endorse these products specifically and do not intend discrimination against other products which are not mentioned but which might also be suitable.

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