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Close, M.E. Simulating the effects of the Waiau irrigation scheme on recharge and groundwater levels

The Waiau Irrigation Scheme. North Canterbury. commenced operation in late 1980. A water balance approach is used to calculate recharge to groundwater from 1976 to 1984, and assess the effect on recharge estimates of variations in soil water-holding capacity and percolation. Increases in water-holding capacity of 30 mm cm-responded to a decrease of approximately 30 mm in mean annual recharge. with increases at low water-holding capacity having a greater effect than at high water-holding capacity. Without irrigation. annual recharge for 1976 84 would have averaged 338 mm with values for individual years ranging between 143 and 548 mm. When full irrigation development of the scheme was simulated for 1977-84, mean annual recharge was 703 and 751 mm for water applications of 80 and 100 mm/irrigation, respectively.
Pre-development groundwater levels were used to identify and calibrate a time-series model which relates groundwater levels to calculated recharge. The effects of the irrigation scheme were estimated by simulating the groundwater levels that would have occurred as a result of rainfall recharge only. and comparing them with the observed groundwater levels for the period 1980 to I984. The change in water levels has increased from 7cro in 1980! 8I to a 1.4 m rise in the 1983.4 irrigation season. reflecting progressive implementation of the scheme. The mean rise in groundwater levels following full development of the irrigation scheme is predicted to be 0.7 m, with a spatial range of 0.4 to 1.8 m, which would lead to waterlogging in some areas.

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