Handbook for Irrigation of Wine Grapes in South Africa

Figure 8.42

30

Ambient Canopy

c

29

b

28

Temperature (°C) 27

a

26

Wet

Drying

Dry

FIGURE 8.42. Midday ambient and canopy temperature of Shiraz grapevines measured on 24 January 2004 near Nuriootpa in Australia, where “Wet” = daily irrigation, “Drying” = irrigated 7 days before measurements and “Dry” = not irrigated since 1 December (redrawn from Loveys et al. , 2008). Figure 8.43

A

B

FIGURE 8.43. Measuring canopy temperature by means of (A) a hand held infrared thermometer (B) that measures canopy, as well as ambient temperature.

Although the principle seems to be simple, there are some constraints when measuring Δ T. Unfortunately, Δ T is affected by the prevailing atmospheric conditions, particularly VPD. In practice this means that Δ T will increase as the VPD increases, i.e. as the air becomes drier. However, the effect of VPD can be reduced by using a Δ T vs VPD “baseline” for a particular crop. This baseline is obtained by measuring Δ T for well-watered plants under a range of VPD values under field conditions (Fig. 8.44). It is interesting to note that Δ T for well-watered grapevines appears to be appreciably more sensitive to VPD than peppermint. The slope and the intercept of the baseline can then be used to determine a so-called crop water stress index as follows: CWSI = [(Tc-Ta) - (a + bVPD)] ÷ [b((es - es ′ ) - VPD)] Eq. 8.8 where Tc and Ta are leaf and air temperatures (°C), VPD (kPa) is at Ta, es is the saturated vapour tension, es ′ is the saturated vapour tension at the equivalent temperature (Ta + a), a is the intercept in °C of the baseline and b is the slope of . ΔT (°C) VPD (kPa) 1 0 4 2 0 2 -2 -4 -1 -3 -5 -6 ΔT = 1.32 - 0.98VPD (R 2 = 0.68; MSE = 0.546) 0 2 3.0 4 1.5 0 0.5 2.0 2.5 1.0 VPD (kPa) ΔT = 4.198 - 2.699VPD Figure 8.44

ΔT (°C)

-2

-4

6

274 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING