Handbook for Irrigation of Wine Grapes in South Africa

Figure 2.5

Transpiration

Evaporation

Runoff

Infiltration

Root extraction

Root zone

Deep percolation Capillary rise

Unsaturated zone Saturated zone

Capillary fringe

Water table

Groundwater recharge & flow

FIGURE 2.5. Schematic representation of the water balance in a vineyard (redrawn from Hillel, 1980).

of soils (Fig. 2.5). Not only is runoff a serious water loss, but it may cause soil losses due to water erosion where soils are unstable. The gains and losses over a given period of time usually balance out to zero as indicated in the following so-called “Universal soil water balance equation”. 0 = SWC i - SWC e + P + I - D - R - ET Eq. 2.1 where SWC i and SWC e are the soil water contents at the beginning and end of the period, respectively, P is the precipitation or rainfall, I is irrigation applied, D is drainage beyond the root zone, R is surface runoff and ET is vineyard evapotranspiration. The unit for all components is millimeter. It must be noted that Equation 2.1 can be rewritten to calculate ET of vineyards as follows: ET = SWC i - SWC e + P + I - D - R Eq. 2.2 The ET calculated by means of Equation 2.2 is related to a reference ET for a given period to determine crop coefficients which can be used to estimate irrigation

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