Subsurface drip irrigation (SDI) is known as one of the most effective irrigation methods capable of improving water use efficiency through providing small amounts of water at short irrigation intervals and causing little or no water loss in terms of deep percolation, runoff and soil evaporation. However, temporal waterlogging within the root zone during and after irrigation events adversely affects root respiration, water and nutrient uptake and consequently plant growth. Therefore, irrigation of plants with hyper-aerated, or oxygenated, water could alleviate the impacts of waterlogging in the rhizosphere. Recent studies reportedly showed that aeration of irrigation water by means of venturi air injector in SDI systems (known as oxygation) enhanced crop performance in hypoxic soils. However, there was evidence of non-uniform improvement in crop yield along lateral pipes which might be ascribed to non-uniform distribution of air flow along irrigation pipes. Moreover, under pipe inclinations ranging from 5º to 15º, preferential flow of air was reportedly observed in branching pipe systems (containing no emitters) for ratio of water to air flow in the range of 0.1 – 0.3. In the current study, preliminary investigation on preferential flow of air into branching horizontal pipe layouts suggested that delivery of air bubbles from the first emitter on the lateral pipe closest to the junction of main pipe and manifold might have formed a zone of relative low pressure. It was speculated that the low pressure zone was responsible for occurrence of preferential air flow in the branching pipe systems.