Irrigation water is to be provided to a crop in a field to bring the moisture content of the soil from the existing 18% to the field capacity of the soil at 28%. The effective root zone of the crop is 70 cm, If the densities of the soil and water are 1.3 g/cm3 and 1.0 g/cm³ respectively, the depth of irrigation water (in mm) required for irrigating the crop is _________.