Water use in the Valetta Irrigation Scheme, Canterbury, New Zealand

An analysis of historical data and a detailed field survey were used to define the pattern and efficiency of water use in the Valetta Irrigation Scheme in Canterbury, New Zealand. In this scheme of 7050 ha, pasture is irrigated using river water, distributed in unlined open channels, and applied by flooding border strips. Statistical analysis of historical water use and climatic data shows that the response of the scheme water use to climatic factors, particularly rainfall, is constrained by the limited channel capacities and delivery schedule in the scheme. Up to 80% of water diverted from the river was delivered to the farms during the 1979-80 irrigation season. Average on-farm water use is 200 mm per application, corresponding to an on-farm use efficiency of 26%. Seasonal water use averages 1171 mm (1979-80 - 1982-83), whereas nearby farms using spray irrigation use, on average, 74 mm of water per application and 160 mm per season. The greatest potential for water savings on this scheme lies with more efficient application of water on-farm, although some savings would be obtained by lining off-farm distribution channels.