## 1 Introduction  
The production of deciduous fruits is practiced widely across the world. The South African deciduous fruit industry was valued at R 15.67 billion (~$795 million) during the 2020/21 crop year, with plums accounting for roughly 10.7% of its total gross production value [3 rd behind apples and pears] [1]. Globally, South Africa is the sixth largest exporter of fresh plums and second in the Southern Hemisphere behind Chile. Plum production in the country has steadily grown since 2006, reaching a peak of 101,969 metric tons in 2021 [1]. [PERSON] et al. [2] attributed the increased production volumes of deciduous fruits in recent years to the use of improved plant material [e.g., scion-rootstock combinations] and orchard management practices [e.g., increased orchard densities and better cultivar selection]. Intensive, high-yielding plum orchards (~$30 tons ha\({}^{-1}\)) are becoming a common occurrence in the South African agricultural landscape. [PERSON] and [PERSON] [3] noted that increased crop yields were associated with higher crop water requirements. Plums, along with other high-value fruit crops, are grown under irrigation. As such, water availability is crucial for the sustainability and growth of the South African fruit industry. Despite this, there is a relative paucity of knowledge on the seasonal water requirements of plums. In a time of changing climatic conditions [characterized by increased drought frequency and rainfall variability] and dwindling water resources, the aforementioned gap in the literature poses an imminent threat to the long-term sustainability and global competitiveness of the South African plum industry. The agricultural sector is the country's largest water consumer, with irrigated agriculture accounting for 62% of its total consumption [4]. [PERSON] et al. [5] emphasized that the goal of modern-day agriculture is to utilize less water for irrigation (improve efficiency) without decreasing fruit yield and quality. These sentiments have become more prevalent in recent years, where increased competition from municipal and commercial sectors has intensified the stress put on the agricultural industry to improve its water use efficiency. Therefore, it is imperative to equip water managers, policy makers and farmers with tools for accurately quantifying the unstressed crop water requirements of full-bearing (3-6 years), high-yielding plum orchards. This would assist in the allocation and licensing of water resources, as well as promoting improved water use efficiency (WUE) within the agricultural space through the development of water-saving strategies. Improved WUE and water-saving strategies are necessary given that South Africa is one of the driest countries globally [6] and that almost 98% of its available water resources have already been allocated [7]. Therefore, from a farmer's perspective, accurate information on crop water requirements would provide a framework for developing accurate irrigation scheduling programs and determining orchard water budgets. [PERSON] [8] revealed that less than 20% of South African farmers use science-based approaches to irrigation scheduling, with the majority implementing a knowledge-based approach, often leading to over-irrigation and increased non-beneficial orchard water use. Although there is a multitude of methods for estimating the evapotranspiration (ET) and crop water requirements (CWR) of various field crops, limited experiments have been conducted on plums. Thus, there is little information on the seasonal water requirements of plums and the associated methodologies utilized that could inform water allocation to farms. The South African Department of Agriculture, Forestry and Fisheries [9] proposed a set of monthly water use estimates for full-bearing plum orchards (ranging from 32.2 mm month\({}^{-1}\) in April and August to 193.8 mm month\({}^{-1}\) in January).