South Africa Wine Technical Yearbook 2025

APRI L

Irrigation duration – time to give water... but how much is enough? By Gert Engelbrecht

Mountainous coastal areas like Stellenbosch, Cape Town and the Cape South Coast normally receive sufficient rain during the winter and even in the spring to fill groundwater levels to field capacity. This results in vineyards being able to grow without irrigation during October, November and even December. When the time does, however, come to irrigate, we are confronted by the question: “How much is enough”? The answer to this question is complex and dependent on a whole series of factors. In this article, we investigate the factors a producer should consider regarding irrigation duration. We also look at a few hypothetical scenarios and suggestions. In 2018, Philip Myburgh’s book Handbook for Irrigation of Wine Grapes in South Africa was published, and it serves as an excellent resource that should be on every producer’s shelf. It was also a primary source for this article. Important factors to consider Irrigation system: The delivery of the system is expressed in cubic metres (m 3 ) per hour or mm per hour. In a drip-irrigated block with a row width of 2.5 m, a drip spacing of 0.6 m and a drip delivery of 2.3 litres per hour, the irrigation system delivers 1.53 mm (or 15.3 m 3 ) per hour per hectare. System efficiency: Due to surface evaporation, drip irrigation is normally 90% effective. This means a system that delivers 1.53 mm per hour, in real terms, delivers 1.38 mm per hour, which will be available to the plant. Soil texture and water-holding ability: Coarse sand, for example, has a water-holding ability of 40 mm per metre of soil, compared to a fine sandy loam soil, which can hold 110 mm of water per metre of soil.

Stone percentage: A high stone percentage drastically decreases the effective soil volume, which naturally results in a lower water-holding capacity. Root depth: The deeper the roots, the longer it will take for irrigation water to replenish the total root zone. Soil volume and wetting percentage: Permeability and hydraulic conductivity are factors that determine how water moves once it infiltrates the soil. Due to the influence of row widths of drip irrigation, we seldom see the total soil volume being wetted, but rather a percentage wetting of 20% or 30% for example. For the purposes of this article, we will use the handy irrigation calculation tool provided by Netafim (Figure 1) in Excel format. Various parameters are entered to give the user an indication of their soil’s water-holding capacity. If the water consumption per day is also known, this tool can be used to plan irrigation scheduling. Scenarios In all three scenarios, we use an irrigation system of 2.3 litres/hour drippers with a spacing of 2.5 m by 0.6 m. Scenario 1: An Oakleaf soil form (Figure 2) with an 80 cm-deep root system, good lateral root distribution, 0% stone and a texture that can be described as fine sand/loam. In this scenario, approximately 26 hours of irrigation will be required to replenish the total root zone. Scenario 2: A Dundee soil form (Figure 3) with a 150 cm deep root system, poor lateral root distribution, 65% stone and a texture that can be described as coarse to medium sand. In this scenario, approximately four hours of irrigation will be required to replenish the total root zone. Scenario 3: A Hutton soil form (Figure 4) with a 90 cm deep root system, limited lateral root distribution, 0% stone and a texture that can be described as sandy loam. In this scenario, it will require approximately 13 hours of irrigation to replenish the total root zone. Due to the high stone fraction and sandy soil texture in Scenario 2, the argument can be made to rather consider micro-sprinklers as a more suitable irrigation system. Micro-sprinklers do not have the system efficiency of drip irrigation, however. Alternatively, lower-delivery drippers with a narrower spacing can be used in Scenario 2. This will enable the producer to wet a greater percentage of soil volume and reduce the irrigation frequency. The good water-holding ability in Scenario 1 means that

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TECHNICAL YEARBOOK 2025