Modelling the effects of irrigation reliability on pasture growth in a dairy system in Canterbury

Abstract

The Canterbury Plains is an area of low rainfall, and large pasture growth responses to irrigation have been demonstrated. As the demand for irrigation water has increased, questions about water allocation impacts on dairy farm production and profit have arisen. This study used farm systems modelling to predict the impacts of water allocation on dairy farming. The model farm was based on a spray irrigated farm, fed from the Rangitata River. A multi-year approach with rainfall, temperature and water supply all affected by weather highlighted both average impacts and the variability in impacts between years. Shifting from dryland to irrigated increased yield and decreased annual variability. A 20% range in water allocated within the four irrigated scenarios gave a 7% range in pasture yield, but there were no differences in annual variability between scenarios in either water supplied or pasture yield. Analysis of simulation results indicated that clusters of poor years and the relationship between dryland and irrigated yields may be important sources of variability for dairy farmers to consider. Interactions between local climate, the seasonal water supply pattern from different irrigation schemes, soil type and irrigation hardware will influence the impacts of restricting water availability on individual farms. The results of this study emphasise the need to consider the role of water storage, climatic patterns, feed storage, use of run-off blocks and the regional balance of water demands as important factors in reducing production variability on-farm and in the region. Keywords: farm systems, pasture yield, Whole Farm Model, yield variability