Placement of Tensiometers

discussion by Ted Sammis

Placement of tensiometers for scheduling irrigation's in drip irrigation systems is different than for a surface irrigation system. Tensiometer in surface irrigation systems should be placed at 63 percent of the depth of the roots in order for it to be placed in the depth of the root zone representing the average extraction level. Several researchers have suggested the following recommendations for positioning of tensiometers in drip systems.

Don Pitts
E-mail djp@icon.imok.edu

"Placement of tensiometers is very important both in respect to the depth of the root zone and in respect to the emitter and plant. For vegetable crops (line source emitter) we generally place the tensiometer in the plant row. For tree crops we place the tensiometer within the tree drip-line and within the emitter wetted area. Placement of tensiometers with respect to crop root depth is also critical. For very shallow rooted (less than 1 ft) vegetable crops, only one tensiometer may be required. It should be placed centrally in the crop root zone, but at least 4 to 6 inches below the surface. For moderately rooted crops, such as drip-irrigated tomatoes, two tensiometers (6 and 12 inch) should be used at each measurement site. For citrus and tree crops we often recommend three tensiometers (6,12, and 18 inch) at each site. The tensiometer should not be placed directly under the emitter (dripper), but it should be totally in the area wetted by the emitter. The distance from the emitter will vary with soil texture and hydraulic properties".

Alan S. Wicks
Kennewick, WA

"Tensiometers should be put one in the area of the active root zone and generally to one side of the irrigation line. The chosen root zone is midway between emitters. This area is used to indicate when to start irrigating since it has the highest concentration of active roots and the water content will decrease the fastest. I also place a tensiometer below the root zone between the emitters. I watch this one during irrigation so that I know when the root zone is nearly full. Additional irrigation simply wastes water and fertilizer. This second tensiometer does not have to reach zero since there will be some drainage into that area after the water is turned off. The exact cut off point is a function of soil but this methodology has worked well for me in vegetables with SDI as well as for grapes with above surface emitters. "