The WUE was improved by N supply, but its effect decreased as the irrigation level increased. The WUE at W1.0, which produced the highest tuber yield, was 146 kg ha−1 mm−1 with N340 but this increased to 195 kg ha−1 mm−1 at W0.4 with N220. The NUE was affected greatly by the applied N amount and increased as the N level was decreased. The highest NUE value 176 kg yield kg−1 N was obtained at W1.0 treatment with N160 but this decreased to 136 kg yield kg−1 N with N340.