Irrigation Scheduling UsingTensiometers - I

Irrigation Scheduling

Irrigation scheduling is defined by the frequency of the irrigations and the amount of water applied in each irrigation. There are various approaches to irrigation scheduling, including those based on water balance approach, soil moisture measurements, plant stress etc. Irrigation scheduling saves water, prevents crop stress, prevents fertilizer leaching and saves energy costs.

Good irrigation scheduling means applying the right amount of water at the right time, in order to reach the appropriate soil moisture, so that water is available to the crop whenever it is needed. In this report, we focus on tensiometers as tools for irrigation scheduling. Advantages of using tensiometers are their ease of use and the immediate results.
 

What Is A Tensiometer? 

"Tensiometer" literally means "tension measurement". In order to absorb water from soil, the plant has to overcome the suction tension of the soil. This tension is measured by the tensiometer, thus giving indication of the soil moisture at the depth in which it was placed.

A tensiometer consists of a vacuum gauge and a sealed tube with a porous ceramic cup. The ceramic cup simulates the flow water through the soil. As the soil around the ceramic cup of the tensiometer dries up, water is sucked out of the sealed tube, creating a vacuum and driving the tensiometer gauge reading up. The drier the soil the higher the tensiometer reading will be. As the soil gets wet, water enters the tube, dropping the reading closer to zero.

The same tensiometer reading in different soils indicates different moisture content. This is because each soil has its own water holding characteristics and at the same tension different soils hold a different amount of water. Thus, heavy soils contain more water than sandy soils. Therefore, irrigation scheduling for sandy soils requires more frequent irrigations at higher readingst of the tensiometer . For most soils a tensiometer reading of below 10 cbar (or kPa) is an indication of wet soil, and above 50 means the soil is dry. 

Selecting a Suitable Location for Installing Tensiometers

Before placing tensiometers in soil, the ceramic cup should be soaked for 24 hours in water with disinfectant. Then the tube should be completely filled with water, so that all air is removed.

The tensiometers should be installed in locations that represent the field conditions. The number of tensiometer stations depends on the variance in the field. For example, different crop types, different irrigation systems, different soil types etc. 

The tensiometers should be installed where the plant takes up the water, i.e. at the same distance from drippers, neighboring plants etc.

 
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