Handbook for Irrigation of Wine Grapes in South Africa

Chapter 4

2.3  /h for heavier soils. The spacing between drippers along the lateral lines should be set according to the soil texture. In heavier soils, water will spread wider under a dripper, compared to sandy or gravelly soils. Therefore, the basic principle is that the spacing between drippers should be narrower as the soil becomes sandier. For example, the spacing between drippers would usually be 1 m for a clay loam soil and 0.6 m for a sandy soil. Drip precipitation rates on the small wetted areas are substantially higher, compared to irrigation systems where the total area is wetted, e.g. overhead sprinklers or micro-sprinklers. Consequently, the water precipitation rate could easily exceed the water infiltration rate of most soils. That is why water generally accumulates in small puddles under drippers irrespective of the soil texture (Fig. 4.3). Since the puddles are usually round, and assuming that most of the water infiltrates from the puddles into the soil, the precipitation rate under drippers can be estimated as follows: Precipitation rate (mm/h) = Discharge rate (  /h) ÷ Area of puddle (m 2 ) Eq. 4.1 For example, if the diameter of the puddle is 20 cm and the discharge rate of the dripper is 3.5  /h, the precipitation rate is 3.5  /h ÷ [(22 ÷ 7) x (20 cm ÷ 2 ÷ 100)] 2 = 111.4 mm/h. In the case of ridged soil, water runoff may occur under drippers which will cause ineffective soil wetting (Fig. 4.4). In such cases, it would be better to use drippers with lower discharge rates. If the dripper in the example is replaced with a 2.3  /h dripper, the precipitation rate reduces to 73.2 mm/h. For the purpose of irrigation scheduling, the precipitation rate of drippers can be based on the total area or the fraction of the land that is being wetted. For one hectare of grapevines, the drip precipitation rate is calculated as follows: Precipitation rate = (Discharge rate x (100 2 /(d R x d D )) ÷ 100 2 ) Eq. 4.2 where the discharge rate is in  /h, d R is the row spacing (m) and d D is the spacing between the drippers (m). For example, if the row spacing is 2.4 m and 3.5  /h

A

B

FIGURE 4.3. Examples of water puddles under drip irrigation in (A) clayey soil and (B) sandy soil.

IRRIGATION OF WINE GRAPES 75

Figure 4.4