Water use in plums

A new study investigates water use in full-bearing Japanese plum orchards, and how water availability impacts fruit yield and quality. By Engela Duvenage.

The project is co-funded by the Water Research Commission and Hortgro. It is led by Prof. Nebo Jovanovic of the Department of Earth Sciences and the Institute for Water Studies at the University of the Western Cape.

“We would eventually like to know what the water consumption of plums is, and the volume that is needed to irrigate such orchards properly yet still receive a maximum income from it,” says Jovanovic.

Most export-quality plums are grown in the Little Karoo including the Breede River Valley and the Berg River area. Not much is known about the water needs of South African plum orchards. In fact, internationally there is also a dearth of research on the topic. Most related studies are from Spain.

“Plum production in South Africa is totally reliant on irrigation,” states Jovanovic. “Water availability is the single most important risk to sustainable fruit production, given the increased frequency of droughts in regions such as the Western Cape.”

Research is being conducted in an African Delight and a Fortune orchard on each of two commercial farms in Wellington and Robertson. The study includes microsprinkler- and drip-irrigated orchards.

Both farms, says Jovanovic, have decades’ worth of experience and are operating at nearly their maximum yield potential. He believes that data gathered will therefore be extremely valuable for informing optimal water allocation.

The two farms are roughly 100 kilometres from each other, yet they experience vastly different climatic conditions. According to Jovanovic, the first three months of rainfall data collected as part of the study showed that Robertson only measured 52 mm of rain between August and October 2021, whereas Wellington received more than 300 mm. He highlights that these are important months in a plum orchard when flowering and fruit growth start.

Robertson and Wellington both received higher than average rainfall in 2021. Irrigation therefore only began after mid-September, some two to three weeks later than usual. “Not all years are like this one, and therefore one has to prepare for risks,” says Jovanovic.

Two Master’s students from the University of the Western Cape are involved in the project. As part of his research, Nonofo Motsei is evaluating the use of the FruitLook remote-sensing platform, which is already used widely by Western Cape farmers, to gather water-related data in plum orchards via satellite images. He will also assess an evapotranspiration model called SEBS — short for surface energy balance system.

Ubaidullah Mathews is validating the HYDRUS soil-water balance model for plum orchards as one key aspect of his research. Since its development in California, HYDRUS has been widely used for different water-management applications. One of its advantages, says Jovanovic, is that evaporation from the soil and transpiration from plants can be estimated separately.

“It works especially well in orchards that received localised irrigation, through for instance drip or microjet irrigation around trees.”

Measurements of soil-water balance, canopy growth, crop yield and quality, and evapotranspiration are ongoing. The project is set to continue until 2024.