Handbook for Irrigation of Wine Grapes in South Africa

Irrigation amounts are most commonly indicated as mm water applied. In contrast, irrigation volumes are expressed as m 3 or M  of water per unit land. The latter is preferred in some overseas countries. It must be noted that 1 mm equals 10 m 3 per ha, and that 1 M  equals 1 000 m 3 or 100 mm. The rate at which irrigation water is being applied, is referred to as the precipitation rate. The latter is usually in millimeters per hour (mm/h). Calculation of precipitation rate for the various irrigation systems is discussed below. DRIP There are basically two types of drippers, i.e. non-pressure compensating (Non- PC) and pressure compensating (PC). Pressure in the irrigation lines may vary according to the topography, length of the laterals, etc . Therefore, PC drippers are preferred to ensure more even water application within an irrigation block. Drippers can be attached to the laterals, or pre-molded drippers can be inserted inside the laterals while they are being extruded in the factory (Fig. 4.2). The latter are referred to as “in-line” drippers. Conventional “high” dripper discharge rates vary from 1.6 to 4  /h. However, drippers with discharge rates of less than 1  /h are also available. These “low flow” drippers are primarily used where permanent row crops, e.g. citrus, are continuously irrigated during daytime. The discharge rates most commonly used for vineyard irrigation are 3.5 to 4  /h for sandy soils and 2 to 4.2 4.2.1 SURFACE IRRIGATION SYSTEMS

FIGURE 4.2. Examples of a dripper attached onto the lateral (top) and the orifice of an in- line dripper (bottom).

74 CHAPTER 4 – IRRIGATION SYSTEMS