ORIGINAL PAPER Analysis of factors affecting the availability of air bubbles to subsurface drip irrigation emitters during oxygation M. Torabi • D. J. Midmore • K. B. Walsh • S. P. Bhattarai • L. Tait Received: 1 September 2011 / Accepted: 6 April 2012 Ó Springer-Verlag 2012 Abstract In this study, we sought to define system features that impact air delivery in subsurface drip irrigation sys- tems. A 58 % decrease in emitter cross-sectional area caused a 71 % reduction in the water flow rate, but the 37 % decrease in emitter air flow rate was statistically non- significant. For emitters with symmetric connectors, delivery of air from the emitters depended on the length of the connector relative to the air layer flowing at the top of the pipe. Emitters with asymmetric connectors yielded greater air flow rates compared to those with symmetric connectors, a result ascribed to a low-pressure region at the tip of asymmetric connectors. When symmetric connectors were used, an increase in pipe diameter was directly related to an increase in the uniformity of emitter air flow rate distribution expressed by the Christiansen uniformity coefficient (CUC), and inversely related to the efficiency of air bubble delivery and mean emitter air flow rate. In contrast, when asymmetric connectors were used, an increase in pipe diameter was associated with a decrease in CUC, and an increase in the efficiency of air bubble delivery and mean emitter air flow rate. Abbreviations SDI Subsurface drip irrigation ID Internal diameter CSA Cross-sectional area AE Efficiency of air bubble delivery CUC Christiansen uniformity coefficient Introduction Subsurface drip irrigation (SDI) in common with other methods of irrigation is able to expel soil air around the root zone during and following irrigation events, thereby impairing root function and crop performance (Bhattarai et al. 2005). It has been shown that SDI can be adapted to provide the root zone with air and therefore oxygen by coupling an air injector venturi to the pressurized irrigation line (Goorahoo et al. 2002), a technique based on the Bernoulli principle. This method of supplying oxygen to the root zone through the SDI delivery system has been termed ‘oxygation’ (Bhattarai et al. 2005) or ‘AirJection Ò Irrigation’ (Goorahoo et al. 2006). Oxygation has led to significant enhancement in growth parameters for a number of crop species (Bhattarai et al. 2004, 2006, 2008a, b, 2010; Essah et al. 2009; Goorahoo et al. 2007a). Nevertheless, in an oxygation trial carried out on bell peppers, there was evidence indicating a Communicated by S. O. Shaughnessy. M. Torabi  D. J. Midmore  K. B. Walsh  S. P. Bhattarai  L. Tait Faculty of Science, Engineering and Health, Centre for Plant and Water Science, Central Queensland University, Rockhampton, QLD 4702, Australia e-mail: d.midmore@cqu.edu.au K. B. Walsh e-mail: k.walsh@cqu.edu.au S. P. Bhattarai e-mail: s.bhattarai@cqu.edu.au L. Tait e-mail: l.tait@cqu.edu.au Present Address: M. Torabi (&) Esfahan Agricultural Research Centre, Post Box 81785-199, Amir Hamzeh Town, Esfahan, Iran e-mail: mctorabi@yahoo.com 123 Irrig Sci DOI 10.1007/s00271-012-0337-1