Handbook for Irrigation of Wine Grapes in South Africa

FIGURE 4.4. An example where drip precipitation exceeded infiltration rate causing water running off ridged soil.

drippers are spaced 0.6 m apart on the laterals, the precipitation rate of the system will be 2.43 mm/h on the total area. However, if only 20% of the soil is wetted, d R must be divided by 5. The precipitation rate on the wetted fraction will then be 12.15 mm/h. The water distribution patterns under drippers are primarily a function of soil texture, irrigation duration and dripper discharge rate. Sandy or gravelly soils contain more coarse pores than loamy or clayey soils. Therefore, the gravitational water flow usually exceeds the capillary flow in sandy soils. This results in deep, but narrow distribution patterns over time (Fig. 4.5A). In the case of heavier soils, where smaller pores dominate, capillary flow is usually higher than gravitational water flow. Therefore, the distribution patterns tend to have “onion-like” shapes as the irrigation proceeds (Fig. 4.5B). Due to the high variability in the factors that determine water distribution from a point source, the prediction distribution patterns by means of mathematical calculations is complicated. Consequently, such calculations are not commonly applied in practice. If there is uncertainty about the expected distribution patterns, it will be better to carry out in-field tests using drippers that have different discharge rates.

76 CHAPTER 4 – IRRIGATION SYSTEMS