A systems simulation study of irrigation policies for wheat on the light land farms of the Canterbury Plains, N.Z.: A dissertation submitted in partial fulfilment of the requirements for the Degree of Bachelor of Agricultural Science with Honours in the University of Canterbury

Abstract

A systems simulation model was set up in order to study the effect of varying the level of available soil water at which irrigation was carried out on the final grain yield of wheat, for a Lismore soil type. A soil water budget was set up using historical rainfall and pan evaporation data, and updated daily with the use of relationships derived for the purpose. The soil water budget was used to obtain a measure of plant water stress, from the difference between potential and actual evapotranspiration for the various soil moisture levels. The model calculates the final grain yield from a stress-yield relationship. Various irrigation decision rules are superimposed on the model. The derivation of each of the relationships used in the model is given. Due to inherent assumptions of the model, a linear relationship was obtained between grain yield and the amount of irrigation water applied. Maximum technical efficiency was obtained when irrigation was carried out at zero available soil water, requiring on average a single irrigation annually. Maximum net profit was obtained by irrigating at the highest available soil water level at irrigation studied (25% soil moisture), which required a mean of 6.5 irrigations annually. The results are discussed and an economic interpretation is given.