Handbook for Irrigation of Wine Grapes in South Africa

If transpiration is assumed to be 2  per grapevine per day, the volume of transpired water can be converted to mm as follows: mm transpired = (  ÷ plant spacing) × (100 ÷ f w ) Eq. 2.3 where the plant spacing is (m × m) and f w is the wetted soil fraction (%). Therefore, 0.7 mm of water was extracted by transpiration when the grapevine spacing is 2.5 m × 1.2 m, and the full soil surface was wetted (Fig. 2.26A). If we assume that 5 mm is lost during the same day due to soil evaporation, the total water extraction would be 0.7 mm + 5 mm = 5.7 mm. In other words, the soil water content will decrease by 5.7 mm. If only 25% of the soil surface is wetted, the 2  used for transpiration will be extracted from a smaller soil volume. According to Equation 2.3, transpiration will extract a depth of 2.8 mm soil water from the wetted volume if the grapevine spacing is also 2.5 m × 1.2 m (Fig. 2.26B). If we assume that 5 mm is lost during the same day due to soil evaporation, the total water extraction from the smaller volume would be 2.8 mm + 5 mm = 7.8 mm. In other words, the soil water content will decrease by 7.8 mm. If the latter water use is based on the total soil surface, it would only be ca. 1.4 mm per day, which is appreciably less than 5.7 mm in the case of the total soil surface being wetted. The foregoing illustrates the dramatic effect of reduced evaporation losses from the soil on vineyard water use in the case of fractional surface wetting. In practical terms, it means that the volume of water evaporating in the case of fractional wetting is much less, compared to full surface wetting.

50 CHAPTER 2 – THE DYNAMICS OF WATER IN AND AROUND VINEYARDS