Handbook for Irrigation of Wine Grapes in South Africa

A

B

FIGURE 2.22. In the case of (A) drip and (B) furrow irrigation, water evaporates primarily from the relatively small wetted fraction of the soil surface.

FIGURE 2.23. Grapevines in the foreground were irrigated by means of micro-sprinklers, whereas those in the background were drip irrigated at the same time.

The effect of fractional surface wetting will be explained by means of schematic diagrams. First, it must be noted the soil water content is seen as the depth of water, e.g. mm per meter of root depth, and that 1 mm = 10 m 3 = 10 000  per ha. The depth of water in mm, lost by evaporation from containers of different diameters, will not differ (Fig. 2.24). Likewise, the depth of water in mm, lost by E s from a vineyard, will not be influenced by the size of the wetted soil area. This means that, under the same conditions, the same depth of water will be lost from 1 hectare as from 0.25 hectare. However, the volume of water evaporating from the smaller wetted area will be substantially less, compared to full surface wetting (Fig. 2.24). On the other hand, if a given volume of water is removed from containers with the same height, but differing in diameter, the water level will decrease much less in a wider container, compared to one having a smaller diameter (Fig. 2.25). Vineyards trained onto the same trellis system and with the same leaf area will transpire the same volume of water, i.e. if none of them experience serious water constraints. Therefore, the water extracted by transpiration from a large wetted soil volume will reduce the mm soil water content less than extraction from a small wetted volume

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