Effects of irrigation frequency on ryegrass and white clover growth. 2. Improving irrigation efficiency

Abstract

Growing concerns about limited water inputs for, and deleterious environmental outputs from, irrigation of pasture make the identification of more efficient scheduling strategies imperative. A climate-driven pasture production model is used to simulate the daily soil water deficit in the topsoil and subsoil, and so pasture production under a range of irrigation strategies. Soil water content and pasture production data from a companion paper, describing a Manawatu experiment where irrigation was applied to plots at trigger deficits of 20, 40, and 60 mm, were used to evaluate model parameters. The model was then used with 30 years of meteorological data to simulate a range of irrigation strategies at Palmerston North and Winchmore. Applying 20 mm of irrigation when a 20-mm trigger deficit is reached as opposed to 60 mm of irrigation at a 60-mm deficit increased the simulated average annual pasture response to irrigation by ~80%, but this increase was at the cost of ~40% more irrigation water. A suggested alternative strategy is to apply 20 mm of irrigation whenever a 60-mm trigger deficit is reached, which in summer will be about every 5 days if no rain falls. Keeping the topsoil moist most of the time, while leaving room for rainfall in the subsoil, will increase water use efficiency (the production response per mm of irrigation) by substantially reducing the irrigation requirement and drainage excess, even though it does not maximise production.