Handbook for Irrigation of Wine Grapes in South Africa

2.4.1.1 FACTORS THAT AFFECT EVAPORATION Soil texture: For each soil, the rate of E s

losses depends on a so-called β value

which describes the relation between E s and a reference evaporation (Myburgh, 2015). The β value is related to soil texture (Fig. 2.7). Due to the high number of coarse pores, water evaporates quicker from sandy soils, compared to heavier soils following rainfall or irrigation, particularly during Phase I. This explains why sandy soils generally dry out faster than heavier soil under the same conditions. Measuring E s in a pot experiment showed that the effect of soil texture can be substantial, e.g. 32 mm water evaporated from the coarse sand over 12 days following an irrigation, compared to only 15 mm from the clay loam soil (Fig. 2.8). Consequently, vineyards in sandy soils need to be irrigated more frequently, compared to heavier soils under the same atmospheric conditions. Since there is a concomitant high E s peak with each irrigation, seasonal evaporation losses from frequently irrigated vineyards in sandy soils will be substantially higher, compared to less frequently irrigated vineyards in heavier soils. Therefore, it is essential that root systems of grapevines in sandy soils must be deep to allow the intervals between irrigations to be as long as practically possible. Such root systems can only be achieved by deep tillage and amelioration of chemical limitations.

6

5

(i)

(vi)

4

(iv)

(v)

2 ȕ value (mm 0.5 ) 3

(ii)

(iii)

1

y = -0.113x + 4.877 (R 2 = 0.7861)

0

0

5

10

15

20

Clay (%)

FIGURE 2.7. Relationship between β values and clay content for (i) coarse sand, (ii) fine sandy loam, (iii) coarse sand clay loam, (iv) fine sandy loam, (v) fine sand and (vi) fine sandy loam (Myburgh, 2015).

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