Canopy drives water use

Gordon Brown reviews South African research on water use in Cripps Pink and Golden Delicious highlighting the importance of understanding water use when planning orchard systems.

The Western Cape region of South Africa experiences Mediterranean climatic conditions where most of the rain falls during winter. Like many apple growing-regions in Australia this region experiences severe water shortages due to competition for water and climate change.

South African researchers Zanele Ntshidi, Sebinasi Dzikiti, and Dominic Mazvimavi set out to quantify how transpiration of apple orchards changes with canopy cover, to facilitate the development of irrigation guidelines for high-yielding, modern orchards.

In their research report Water use dynamics of young and mature apple trees planted in South African orchards: a case study of the Golden Delicious and Cripps’ Pink cultivars the authors state that while many studies have quantified the water requirements of apple orchards the information for new high-density high yielding (greater than 100tonne/hectare (t/ha)) is sadly lacking.

water use

Cripps Pink trees tend to use less water than Golden Delicious due to more open canopies designed to maximise light.

Method

This study used data from two orchards each with adjacent 6ha blocks of mature Cripps Pink and Golden Delicious trees on M793 rootstocks with micro sprinkler irrigation systems. Each block had consistently yielded above 100 t/ha and was planted with 1667 trees/ha. Tree transpiration rate was monitored using sap flow measurements. The seasonal reference evapotranspiration was 1,205mm.

The authors do not specify how canopy cover or leaf area was measured and appear to use the leaf area index as a measure of canopy cover. This is odd as canopy cover is a critical aspect of the research objective. The leaf area index is a measure of tree leaf area expressed over the planting square such that a dense canopy will have a small canopy cover compared to the same leaf area index in an open canopy. Hence a more accurate objective for this paper would be to quantify how transpiration of apple orchards changes with leaf area index. Better still the authors should have related transpiration to practical measurements of canopy cover and light interception as has been done in Australian on pears and peaches.

The seasonal transpiration water requirement for the 2014/15 season was 770 and 587 mm for Golden Delicious and Cripps Pink respectively. This equates to 7.7 Megalitres per hectare (Ml/ha) for Golden Delicious and 5.9 Ml/ha for Cripps Pink. In these trials, with tree density at 1667/ha, the individual trees transpired 4619L (Golden Delicious) or 3500L
(Cripps Pink) over the season.

The peak tree water use occurred at the end of January at 5 and 3.5 mm/day for Golden Delicious and Cripps Pink. This equates to individual Golden Delicious trees requiring up to 30 litres of water a day compared to 21 litres for Cripps Pink trees. Average daily water use requirement was relatively stable for both cultivars for the months of November, December and January at about 24 litres per tree for Golden Delicious and 17 litres per tree for Cripps Pink. Hence, if using four-litre-per-hour drippers with one dripper per tree and assuming 10 per cent water loss due to irrigation inefficiencies, it will be necessary to run them for 6.5 hours per day for Golden Delicious trees and 4.5 hours a day in the Cripps Pink. Similar calculations can be made for micro sprinklers although additional allowances would need to be made for inefficiencies in water delivery to the root system.

The variations in transpiration rates between the two cultivars were a result of different canopy management practices. Cripps Pink canopies were kept open by pruning and Regalis® use to enhance red colouration of the fruit.

This resulted in a leaf area index of 2.8 for the Cripps Pink compared to 3.5 for the Golden Delicious, indicating 25 per cent more leaves on the Golden Delicious trees, in line with the additional water use of this cultivar.

Research conclusions

The authors concluded from this that: ‘canopy cover seems to be the main driver of water use, the larger the canopy, the more water the tree uses’.

‘This study provides first estimates of the water use characteristics of apple trees considering newly planted and mature trees,” they reported. ‘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.’

water use 2

Knowledge of how much water your crop requires is essential when designing and managing your orchard.

Take home messages

Australia is not the only country experiencing water shortages in their apple growing regions.

Knowledge as to how much water your crop requires is essential when designing and managing your orchard.

A mature Golden Delicious orchard in South Africa needs a total of close to 8Ml/ha (i.e. 800 mm) per season easily available to the root system. This can come from rainfall or irrigation, however, water delivery inefficiencies (both rainfall inefficiencies and irrigation inefficiencies) mean that more than 8Ml needs to be actually applied.

A larger canopy with a high leaf area density uses more water than smaller canopies or canopies with a lower leaf area density. Hence total water use can be influenced by canopy management practices.

Further reading

(1) Water use dynamics of young and mature apple trees planted in South African orchards: a case study of the Golden Delicious and Cripps’ Pink cultivars

(2) APAL Irrigation page

 

About the author:
Dr Gordon Brown, Director, Scientific Horticulture and Dry Ideas
03 6239 6411
gordon@scientifichorticulture.com.au

 

About the Author:

APAL is an industry representative body and not-for-profit membership organisation that supports Australia’s commercial apple and pear growers.