International Water & Irrigation

31 the soil or the plant water status and therefore requires experimental determination of plant coefficients (KC) for implementation. Crop coefficients are dependent on the condition of the system (i.e. cultivar, canopy size, soil type). The KC coefficient used for the control treatment was the best current estimate of the water requirement for top yield in the region because it was based on lysimeter and tomato irrigation trials carried out in the experimental station where the present experiment was done (Harel et al. 2012). Tension controlled irrigation is an alternate approach to deficient irrigation for saving water. In its simplest form tensiometer aided irrigation measures the soil water tension in 1-3 layers of the soil to guide a manual irrigation when a pre-determined tension is reached. In this form of operation the volume of water applied is by trial and error (Klein 1983) or by previous estimation of the required water. The present investigation reports the increased irrigation efficiency of tomato by the use of a single autonomous irrigation controller based on soil tension measurement capable of controlling irrigation precisely to any desirable depth. Measuring the inherent variability of plant water stress indicators for irrigation some conclude that soil water tension is less desirable than other plant indicators because of its high variability. The two main causes of variability are soil texture and root density distribution. To reduce variability the common practice is to monitor soil water tension in several locations in 2-3 soil layers and take into account the 40-60 cm soil layer tension as a guide for adjusting the scheduled irrigation according to the KC coefficients. Never the less monitoring tension is the preferable choice for irrigation control since the principle of water movement through the soil-plant-atmosphere continuum is governed by tension differences. Soil water potential is a measure of the readily available water for the plant and the plant water potential is the best physiological indicator for plant performance. In a high root density culture of fig grown in potted soil-less media a linear correlation of R2 = 0.98-1.00 was found between soil matric potential and photosynthesis (Klein et al. 2006 Fig 8). As expected the water saved by the tension controlled irrigation was proportional to the imposed tension (12% at - 8 kPa and 24% at -15 kPa). The autonomous irrigation controller in conjunction with the method of operation adapted overcomes the inherent variability of tensiometer controlled irrigation, as practiced traditionally. The Tevatronic irrigation controller is positioned close to a dripper at a shallow soil layer (10-15 cm from a dripper at a depth of 10-15 cm) where a dense and fibrous root system develops. Maintaining a steady soil water tension threshold at this position further encourages root development at this site. The irrigation depth by the controller is controlled precisely by a proprietary algorithm and it can be set to wet any soil depth chosen. By actively promoting an ideal environment for functioning of the tensiometer the Fig. 3. Soil water tension for the period 10/2/2016 – 18/5/2106. A - 8 kPa, B - 15 kPa. Numbers above the pre-set tension are cases when tension exceeded 2 kPa in the – 8 kPa treatment and 3 kPa in the -15 kPa treatments. Fig. 5. Tensiometer controlled irrigation of tomato as percent of control Fig 4. Cumulative irrigation of tomato comparing tension controlled irrigation at two levels with the recommended irrigation practice