South Africa is currently the 7th biggest exporter of apples (Malus domestica Borkh.) in the world. The main Southern hemisphere competitor is Chile, which is ranked third in terms of export. In South Africa, most apples are grown in the Eastern and Western Cape Provinces. The Western Cape experiences Mediterranean climatic conditions and most of the rain falls outside the fruit growing season in winter. This region is projected to experience severe water shortages in future due to the rapidly increasing population, increasing agricultural and industrial activities and climate change (Midgley and Lötze, 2011). Accurate information on water use by apple orchards is therefore critical for the sustainability of the multi-billion Rand deciduous fruit industry. This information will be useful for, among other things: (1) developing irrigation scheduling guidelines for the apple industry, (2) water allocation and planning, and; (3) developing strategies for coping with drought whose frequency and severity is expected to increase in the near future in the South Western parts of the country. Severe water deficits imposed by a lack of irrigation not only reduce the current year's yields, but they also negatively affect production in subsequent years, enhance alternate bearing and damage or kill trees (Fereres et al., 2012). An important information gap currently exists regarding the water requirements of high yielding (≥ 100 t ha-2) apple orchards which have become the norm in this region in recent years. There is also no published information on how water use varies with orchard growth stages from planting until full-bearing. Although several studies have quantified water use by apple orchards (Braun et al., 2000; Volschenk et al., 2003; Dragoni et al., 2005; Gush and Taylor, 2014) these studies were not done in exceptionally high yielding orchards. The specific objective of the study was to quantify how transpiration of apple orchards change with canopy cover, to facilitate the development of irrigation guidelines. An example of a programme in which the fruit industry could directly benefit from the data generated in this study is the recently launched Validation and Verification of Lawful Water Use (V&V) programme. This programme has been rolled out throughout South Africa by the Department of Water and Sanitation (DWS) mainly to confirm how much water may be used lawfully for commercial purposes (including irrigation) to facilitate water use licensing in line with the country's Water Act of 1998. Therefore, this study sets a benchmark on the water requirements of the country's most productive apple orchards.

The maximum daily solar radiation was 30.02 MJ m-2 d-1 recorded on 17 December 2014 and the lowest was 6.9 MJ m-2 d-1 measured on 2 November 2014. December 2014 had the highest average daily solar radiation of approx. 28.3 MJ m-2. The maximum and minimum air temperatures were 37.3 and 1.4 ∘C reached in December 2014 and June 2015, respectively. The maximum air temperature exceeded 30 ∘C in seven of the nine months. The relative humidity ranged from 99.9 % to as low as 5.4 % on hot dry days with the vapour pressure deficit of the air peaking at 3.1 kPa (Fig. 1). The reference evapotranspiration (ETo), which is a measure of the atmospheric evaporative demand, was calculated as the evapotranspiration from a short grass reference using the modified Penman-Monteith equation according to Allen et al. (1998). The seasonal total reference evapotranspiration was 1205.4 mm. Transpiration for the full bearing “Golden Delicious” trees peaked at 5 mm d-1, while the similar size trees of the Cripps' Pink cultivar peaked at 3.5 mm d-1 at the end of January 2015. The non-bearing “Golden Delicious” and Cripps' Pink trees transpiration peaked at 1.7 and 2.02 mm d-1 (Fig. 2) on 25 January 2015 and 6 December 2014, respectively. The seasonal total for the full-bearing “Golden Delicious” was 770 mm while for “Cripps' Pink” it was 587 mm. The non-bearing “Golden Delicious” trees transpired a seasonal total of 187 mm while “Cripps' Pink” transpired 238 mm for the season. Variations in transpiration rates in full-bearing orchards were a result of differences in canopy management practices between the “Golden Delicious” and “Cripps' Pink” orchards. “Cripps' Pink” canopies are kept open e.g. by pruning and spraying shoot growth retardants such as Regalis^® to expose the fruit to solar radiation for anthocyanin synthesis to occur and to promote the development of the red fruit colour. The leaf area index (LAI) for the full bearing “Golden Delicious” was ∼3.5 while for the “Cripps' Pink” it was ∼2.8. For non-bearing orchards it was 0.8 for “Golden Delicious” and 1.0 for “Cripps' Pink”. Canopy cover seems to be the main driver of water use, the bigger the canopy, the more water the tree uses. The daily total transpiration was linearly related to the daily solar radiation (Fig. 3a and b). However, the relationship between transpiration and VPD (Fig. 3c and d) was non-linear, suggesting that the VPD is the most limiting variable for apple trees transpiration.

This study provides first estimates of the water use characteristics of apple trees considering newly planted and mature trees. For mature trees, it appears that “Cripps' Pink” trees tend to use less water than “Golden Delicious”. This is because they tend to have more open canopies due to management practices e.g. the spraying of chemicals such as Regalis^® to reduce shoot growth. Dense foliage is not favourable for the development of the red colour on fruit in red and blushed cultivars and these canopy management practices appear to have water saving benefits. In non-bearing trees, the Cripps' Pink cultivar seems to be using more water than the “Golden Delicious”. Transpiration was linearly related to the solar radiation, while VPD seemed to be a limiting variable for water use.