When to water: Dual measurements solve the mystery

When to water: Dual measurements solve the mystery

Dual measurements solve the mystery of when to water

Though water potential is a better indicator of plant available water than water content, in most situations, it’s useful to combine the data from both sensors. This is because the intensity measurement of water potential doesn’t translate directly into the quantity of water stored or needed.  Water content information is also required in applications such as irrigation management and water balance studies.

Dual measurements simplify irrigation decisions

The value of dual measurements can be illustrated with data from the Brigham Young University Turf Farm, where researchers are investigating the optimization of turfgrass irrigation.  Because the research plots were located in a sandy soil where water was freely available, the researchers measured both water potential and water content. Figure 1 illustrates why.

Figure 1. Turf farm data: water potential only

Early water potential data look uninteresting, showing adequate water availability most of the time, however, they don’t indicate if too much water is applied.  In addition, at times when water potential begins to change, the soil reaches a stress condition quickly.  Within a couple of days, the turfgrass is in danger of going into dormancy.  Water potential data are critical to understanding when it is crucial to water, but because the data doesn’t change until it’s almost too late, water content data is also required.

Soil moisture sensors complete the picture


Figure 2. Turf farm data: volumetric water content only

Unlike water potential, the water content data (Figure 2) are more dynamic. Soil moisture sensor data not only show subtle changes due to daily water uptake, but they also indicate how much water needs to be applied to maintain the root zone at an optimal level.  However, with water content data alone, it’s impossible to identify an optimal level.  For example, if there were large changes in water content over four or five days, researchers might assume, based upon on-site observations, that it’s time to irrigate. In reality, they know little about the availability of water to the plant. Thus, it’s useful to put the two graphs together (Figure 3).

Figure 3. Turfgrass data: both water potential and volumetric water content together

Figure 3 illustrates the total soil moisture picture.  Researchers can observe where water content declines and at what percentage the plants begin to stress.  It’s also possible to recognize when the soil has too much water: the water content is above where water potential sensors start to sense plant stress.  Using this information, researchers can identify the turfgrass optimal range at 12%-17% volumetric water content.  Anything below or above that range will be too little or too much water.

Soil moisture release curves explain total water availability

Figure 4. Turfgrass soil moisture release curve (black). Other colors are examples of moisture release curves for different types of soil.

Dual measurements also enable the creation of in situ soil moisture release curves like the one above (Figure 4), which detail the relationship between water potential and water content.  Scientists can evaluate these curves and understand many things about the soil, such as hydraulic conductivity and total water availability.

For more in-depth information about measuring water potential, read “Why soil moisture sensors can’t tell you everything you need to know” and “Why measure water potential?“.

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