Handbook for Irrigation of Wine Grapes in South Africa

0.8

a

0.6

b

0.4

I g

c

0.8

0.8

0.2

a

a

0.6

0.6

0

Wet

Drying

Dry

Another practical, though cumbersome, application of infrared thermometry is based on the canopy temperature difference between the vineyard and a well- watered reference plot (Van Zyl, 1986). Previous studies have shown that Δ CT relates reasonably well to the difference in stomatal resistance ( Δ Rs) between the vineyard and the reference plot (Fig. 8.46A). As a consequence, Δ CT was also related to the difference in soil water content ( Δ SWC) between the vineyard and the reference plot (Fig. 8.46B). According to this particular study, the onset of water constraints in Colombar grapevines occurred when PAW depletion reached 50 to 70% (Van Zyl, 1986). At this stage, the canopy temperature was 1.16 to 1.62°C higher compared to the reference plot. ΔCT (°C) 0 2.0 3.0 1.0 ΔCT (°C) 0 2.0 3.0 1.0 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). b c 0 0.4 0.2 I g Dry Drying Wet b c 0 0.4 0.2 I g Dry Drying Wet FIGURE 8.45. Midday I g

y = 0.28082x + 0.01225 (r = 0.65)

-1.0

-1.0

y = 0.8892x + 0.2728 (r = 0.78)

-1.0 0

1.0

2.0 3.0 4.0 5.0

0

2.0

4.0

ΔSWC (%) 6.0

8.0 10.0

A

B

ΔRs (s/cm)

3.0

3.0

3.0

3.0

2.0

2.0

2.0

2.0

1.0

1.0

1.0

1.0

ΔCT (°C)

ΔCT (°C)

ΔCT (°C)

ΔCT (°C)

0

0

0

0

y = 0.28082x + 0.01225 (r = 0.65)

-1.0

y = 0.28082x + 0.01225 (r = 0.65)

-1.0

-1.0

-1.0

y = 0.8892x + 0.2728 (r = 0.78)

y = 0.8892x + 0.2728 (r = 0.78)

-1.0 0

1.0

2.0 3.0 4.0 5.0

-1.0 0

1.0

2.0 3.0 4.0 5.0

0

2.0

ΔSWC (%) 6.0 4.0

ΔSWC (%) 6.0

8.0 10.0

0

2.0

4.0

8.0 10.0

ΔRs (s/cm)

ΔRs (s/cm)

FIGURE 8.46. Relationship between (A) Δ CT and Δ Rs, as well as (B) Δ CT and Δ SWC where a well-watered plot was used as a reference (redrawn from Van Zyl, 1986).

When using infrared thermometry for irrigation scheduling in vineyards, it is important to standardize measurements, i.e. irrespective of the basic method. Measurements should preferably be taken between 11:00 and 14:00. The angle at which the infrared thermometer is held with respect to the canopy must be kept constant (Van Zyl, 1986). In this regard, it will be useful if the instrument is clamped to a stand. Since

276 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING