Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.5

ET = k c

x ETo

ET = (k s

+ k cb

) x ETo

Transpiration: Leaf area per grapevine Canopy orientation Plant spacing

Atmospheric conditions: Reflects in the reference evapotranspiration

Evaporation: Soil texture % Wetted area

FIGURE 8.5 . Schematic illustration of the dual crop coefficient approach where k c

is divided

into k s

and k cb .

Figure 8.6

Since ET calculation becomes complicated if all the above-mentioned variables need to be considered, the VINET model was developed by ARC Infruitec- Nietvoorbij at Stellenbosch for estimation of ET in South African vineyards. The model is based on the dual crop coefficient concept that distinguishes between evaporation and transpiration (Myburgh, 1998). The E s is estimated by means of a simple parametric model (Boesten & Stroosnijder, 1986; Stroosnijder, 1987). Daily E s for clean cultivated soil is calculated using either ETo, or E p , and a soil specific parameter, the so-called β -value. The β -value also depends on canopy orientation, i.e. horizontal vs vertical (Myburgh, 1998). Phase 1 E s is also adjusted according to vineyard canopy changes over the growing season (Myburgh, 2015). At the single grapevine level, transpiration is related to total leaf area per grapevine, canopy orientation and ETo (Myburgh, 2016). Therefore, leaf area, canopy orientation, plant spacing and ETo are used to calculate vineyard transpiration by means of regression models (Myburgh, 1998). Whole grapevine sap flow measurements were carried out to develop the transpiration model. Total leaf area per grapevine is estimated from the cane mass per grapevine at pruning in winter. The model requires long term mean daily ETo or E p for each month from the nearest weather station (mm/d). Since the number of weather stations for which long term mean ETo is available is limited compared to E p , the latter is needed as input by the model in its present configuration. Other readily available inputs are the size of the vineyard, or rather irrigation block (ha), plant spacing (m), row spacing (m), canopy orientation, cane mass at pruning (k/grapevine), root depth (m), available water over the root depth for the specific soil (mm/root depth), and emitter spacing of the irrigation system along the grapevine row (m). The estimated wetted soil volume (%), as well as the target PAW depletion during each physiological phase

242 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING