Handbook for Irrigation of Wine Grapes in South Africa

Chapter 8

Practical irrigation scheduling

8.1 INTRODUCTION Irrigation scheduling can simply be defined as the answers to “When to irrigate?” and “How much water must be applied?” In order to answer these questions, measurements or calculations are required. Direct measurements can be soil or plant based. Soil measurements include soil water content and matric potential, whereas leaf water potential, infrared thermometry and remote sensing by means of satellite imagery can be used to assess grapevine water requirements. On the other hand, a reference evaporation in combination with crop coefficients may be used to calculate irrigation requirements indirectly. QUALITATIVE ASSESSMENT OF ROOT SYSTEMS The first requirement for correct irrigation scheduling of vineyards is to qualify the root system. This information is essential to decide where to install instruments and to know the root depth to where irrigation must be applied to. In addition to anchoring grapevines, roots absorb and transport nutrients and water, store reserves and produce hormones. Digging soil pits is a simple, effective way to assess grapevine root systems. Soil pits must be made between two grapevines and to the middle of the work row as indicated graphically in Figure 8.1. Evaluate root systems where the majority of the roots occur. Single, deep roots do not make a significant contribution, and water will be wasted if irrigation is applied to depths where these isolated roots occur. The four most important criteria for evaluating grapevine root systems are as follows: 1. Lateral distribution: The ideal root system should have roots throughout the soil between two grapevines (Fig. 8.1). In the case of full surface irrigation, roots should occur up to the middle of the work row as indicated by “A” in Figure 8.1. In the case of strip wetting, e.g. drip irrigation, roots should occur over the width of the wetted soil volume as indicated by “B”. 2. Depth distribution: The depth of the root system, as indicated by “C” in Figure 8.1, should be at least 60 cm deep if good quality irrigation water is abundant, 80 cm deep if irrigation water is limited and 1 m deep for dryland (rainfed) vineyards. 8.2

236 CHAPTER 8 – PRACTICAL IRRIGATION SCHEDULING