In The Netherlands drought causes occasional reductions in yield and quality of potatoes, especially on sandy soils. The mean annual yield loss determines the investments that can be made to prevent the losses. The mean annual loss is determined by the water deficit and the yield reduction per mm water deficit. Experiments which compared well-watered controls with transient drought treatments showed that the average yield reduction per mm water deficit amounted to 117 kg tubers ha-1 on a fresh weight basis and to 36 kg ha-1 on a dry weight basis. The water deficit is determined by the precipitation deficit integrated from 1 May to 1 September, and the net amount of water that the crop can extract from the soil during that period. From the distribution function of precipitation deficits, mean annual water deficits can be calculated. In this way mean annual reductions in tuber yield (fresh weight) were estimated to range from 8.6 t h-1 for zero mm soil water to 1.1 t ha-1 for 140 mm soil water; corresponding figures on a dry weight basis are 2.7 and 0.3 t ha-1.
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