The performance of travelling effluent irrigators: Assessment, modification, and implications for nutrient loss in drainage water

Abstract

and application of farm‐dairy effluent (FDE), the treatment option preferred by most regional councils, is commonly practised with the use of small travelling irrigators. Field observations indicate that FDE application by rotating irrigators to artificially drained soils can generate drainage contaminated with partially treated FDE, even when the set application depth is less than soil water deficit. The uniformity of FDE application from rotating, modified‐rotating, and oscillating travelling irrigators was determined for a range of application depths and wind conditions. The rotating irrigator produced a bimodal application profile with a two‐ to threefold difference between the highest and lowest application depths. These high application depths are likely to result in drainage of partially treated FDE in late winter and spring when soil moisture deficits are often small. A rotating irrigator was modified with splash plates or an irrigation bar that diverted more FDE to the centre of the application profile. Neither modification improved the uniformity of application. The most uniform application profiles were obtained using a new technology oscillating irrigator. The measured application profiles and a soil water balance were used to simulate the drainage and nutrient loss under each irrigator type. In early spring and late autumn, when soil water deficits were low, the more uniform application profile of the oscillating irrigator, set at its lowest application depth of 10 mm, created less risk of partially treated FDE reaching pipe drains. The simulation model estimated that when operating at a set average application depth of 25 mm the rotating irrigator and oscillating irrigator required soil water deficits of 44 and 32 mm, respectively, to avoid generating drainage. When FDE was applied at 25 mm depth with a deficit of only 18 mm, substantial quantities (30%) of partially treated effluent were estimated to have been drained from the soil no matter which irrigator was used. When application depth equalled the moisture deficit, the more uniform oscillating irrigator had a lower drainage loss (7%) compared with the rotating irrigator (14%). With only a small buffer of 7 mm between soil moisture deficit and application depth it was estimated that the oscillating irrigator achieved zero drainage. When set at their fastest travel speeds, the peak application depths of the rotating and the oscillating irrigators were similar (13 mm) and therefore these irrigators have the same number of operational irrigation days at times when soil water deficits are low.