Use of Reduced Irrigation Operating Pressure in Irrigation Scheduling. II. Effect of Reduced Irrigation System Operating Pressure on Drip-tape Flow Rate, Water Application Uniformity, and Soil Wetting Pattern on a Sandy Soil

Abstract

<jats:p>For shallow-rooted vegetables grown in sandy soils with low water-holding capacity (volumetric water content &lt;10%), irrigation water application rate needs to provide sufficient water to meet plant needs, to avoid water movement below the root zone, and to reduce leaching risk. Because most current drip tapes have flow rates (FRs) greater than soil hydraulic conductivity, reducing irrigation operating pressure (OP) as a means to reduce drip emitter FR may allow management of irrigation water application rate. The objectives of this study were to determine the effect of using a reduced system OP (6 and 12 psi) on the FRs, uniformity, and soil wetted depth and width by using three commercially available drip tapes differing in emitter FR at 12 psi (Tape A = 0.19 gal/h, Tape B = 0.22 gal/h, and Tape C = 0.25 gal/h). Reducing OP reduced FRs (Tape A = 0.13 gal/h, Tape B = 0.17 gal/h, and Tape C = 0.16 gal/h) without affecting uniformity of irrigation at 100 and 300 ft lateral runs. Flow rate was also reduced at 300-ft lateral length compared with 100 ft for all three tapes. Uniformity was reduced [“moderate” to “unacceptable” emitter flow variation (<jats:italic>q</jats:italic><jats:sub>var</jats:sub>) and “moderate” coefficient of variation (<jats:sc>cv</jats:sc>)] at 300 ft for Tape B and C compared with “good” <jats:italic>q</jats:italic><jats:sub>var</jats:sub> and “moderate” to “excellent” <jats:sc>cv</jats:sc> at 100 ft. Using soluble dye as a tracer, depth (<jats:italic>D</jats:italic>) of the waterfront response to irrigated volume (<jats:italic>V</jats:italic>) was quadratic, <jats:italic>D</jats:italic> = 4.42 + 0.21<jats:italic>V</jats:italic> − 0.001<jats:italic>V</jats:italic><jats:sup>2</jats:sup> (<jats:italic>P</jats:italic> &lt; 0.01, <jats:italic>R</jats:italic><jats:sup>2</jats:sup> = 0.72), at 6 psi, with a similar response at 12 psi, suggesting that depth of the wetted zone was more affected by total volume applied rather than by OP itself. The depth of the wetted zone went below 12 inches when <jats:italic>V</jats:italic> was ≈45 gal/100 ft, which represented ≈3 h of irrigation at 6 psi and 1.8 h of irrigation at 12 psi for a typical drip tape with FR of 0.24 gal/h at 12 psi. These results show that, for the same volume of water applied, reduced OP allowed extended irrigation time without increasing the wetted depth. OP also did not affect the width (<jats:italic>W</jats:italic>) of the wetted front, which was quadratic, <jats:italic>W</jats:italic> = 6.97 + 0.25<jats:italic>V</jats:italic> − 0.002<jats:italic>V</jats:italic><jats:sup>2</jats:sup> (<jats:italic>P</jats:italic> &lt; 0.01, <jats:italic>R</jats:italic><jats:sup>2</jats:sup> = 0.70), at 6 psi. As the maximum wetted width at reduced OP was 53% of the 28-inch-wide bed, reduced OP should be used for two-row planting or drip-injected fumigation only if two drip tapes were used to ensure good coverage and uniform application. Reducing OP offers growers a simple method to reduce FR and apply water at rates that match more closely the hourly evapotranspiration, minimizing the risk of leaching losses.</jats:p>