Thirst trap

How much water do high-performing apple orchards need? By Esté Beerwinkel.

A lingering drought and remarkably high-yielding orchards compel farmers to give their irrigation practices a second thought. A four-year project set on establishing how much water apple orchards need concludes that it all boils down to orchard management.

Water is a critical, but finite resource. The world revolves around it, and farmers know this acutely.

At the beginning of this decade, the United Nations Food and Agriculture Organisation reported South Africa’s total water withdrawal at 12 496 million m³, with irrigation accounting for 62% of it.

Since then, farmers have had to reconsider their water-use practices. In 2015 a drought tightened its grip on the Western Cape. Also, the water requirement for apple orchards of 6 000 m³ per hectare, based on past research, may no longer be accurate due to changes in planting systems and a general, steady increase in yields. Exceptionally high-yielding apple orchards, producing more than 100 tonnes per hectare, have become commonplace locally and elsewhere in the world.

This is why, four years ago a team of researchers in the deciduous-fruit industry embarked on a mission to figure out how much water apple trees need.

Research team leader, Dr Sebinasi Dzikiti of the Council for Scientific and Industrial Research, determined that apple trees in a high-production orchard “drink between 80 and 136 glasses” of water a day—or 30 000–45 000 litres per hectare per day depending on leaf area. “But it’s more complex than that as other factors such as the local microclimate, rootstock, crop load, etcetera also influence orchard water use,” according to Dzikiti.

When they set out to establish the water requirements of apple orchards, the researchers had the following focus areas:

1) to determine the maximum unstressed water requirements for high-yielding orchards

2)to quantify how orchard water use changes from planting until full-bearing age

3) to establish whether fruit quality is affected by how much water a tree receives.

 

A checklist was designed to select qualifying orchards for this study:

a) Orchards needed to be well-managed.

b) Orchards needed to be located in the Elgin-Grabouw-Vyeboom-Villiersdorp (EGVV) or Koue Bokkeveld regions.

c) Orchards needed to be either Golden Delicious or Cripps Pink. Golden Delicious was chosen because it’s the major cultivar in South Africa, and bears high yields. Cripps Pink was chosen due to its high yields, high value, and long growing season, which may increase seasonal water requirements.

d) The irrigation system needed to be micro-sprinklers as this is the industry norm.

e) Orchards needed to have a deep soil profile for installation of soil-moisture probes, and cover a large area for accurate measurement of evapotranspiration.

f) Trees needed to be on the industry standard M.793 rootstock.

g) Mature orchards needed to yield an average of 100 tonnes/ha or more.

Dzikiti says finding suitable orchards was the most challenging aspect of this study.

“It proved harder than we initially thought to source suitable orchards. Sometimes we also dealt with unsuitable rootstocks, small orchard sizes, and stony soils in the EGVV. However, the cooperation from the farmers was a lekker highlight.”

After finding suitable orchards the research team set out to quantify the actual tree water use every hour throughout the growing season using sap-flow sensors installed on three to six trees per orchard. Since tree sizes differ between orchards, the tree-specific data was then extrapolated to a per hectare basis.

“We also measured the soil water content in the root zone, orchard microclimate, tree and fruit growth, yield quality and quantity, irrigation volumes, and orchard evapotranspiration,” Dzikiti says. “In addition to tree transpiration we also directly measured orchard floor evaporation in some instances to get an overarching picture of orchard water use.”

Dzikiti and company also monitored newly planted orchards in addition to the full-bearing orchards.

This was all done using the following techniques and technologies that collected the data hourly throughout the growing season for some variables:

a) sap-flow sensors measured tree transpiration

b) eddy-covariance and FruitLook for orchard-scale evapotranspiration

c) weather stations monitored orchard microclimate

d) dendrometers measured stem and fruit growth

e) pressure chambers measured plant water stress levels

f) infrared gas analysers measured gas exchange (leaf transpiration and photosynthesis)

g) time domain reflectometers measured soil water content

h) leaf-area index meters measured canopy development and seasonal changes.

The research team concluded that orchard leaf area is the main factor that determines how much water an apple orchard needs. While other key determinants of how much water apple trees need are climate and crop load, leaf area plays the bigger role. This fact, Dzikiti says, is met with some contention.

“Some in the industry have argued against our results which show leaf area rather than crop load as the main determinant of orchard water usage. They argue that the bigger the crop, the more food the tree will have to generate for the fruit. While this is true, you can also have an orchard with a larger leaf area and less fruit that uses the same amount of water as the orchard with the larger crop load but smaller leaf area.”

Dzikiti continues to explain this, using findings from their study to illustrate. “For example, mature Cripps Pink orchards, which had more open canopies and lower leaf areas for reasons such as red colour development, transpired 5 900 to 6 300 m³/ha/season. On the other hand, mature Golden Delicious orchards, which had more closed canopies and a higher leaf area to protect the fruit against sunburn used 7 600 to 7 900 m³/ha/season. However, the Cripps Pink orchards which had a relatively low water use had higher yields averaging 110 tonnes per hectare compared to around 98 tonnes per hectare for the Golden Delicious which had higher water-use rates”.

The seasonal total evapotranspiration, which equals the orchard water requirements, ranged from 9 500 m³/ha/ season in the full-bearing Cripps Pink to around 10 500 m³/ha/season in the Golden Delicious.

Dzikiti furthers that in young orchards, more than 60% of the evapotranspiration in summer came from the orchard floor compared to around 18% in full-bearing Golden Delicious and 32% for Cripps Pink at maximum leaf area. “This is because younger orchards have fewer leaves, thus more water goes to the cover crop or evaporates from the orchard floor; mature orchards have more leaves so the trees, not the orchard floor, use the water.”

On whether the study’s results also apply to other cultivars such as Royal Gala, or other pome fruit such as pears, Dzikiti says maybe, but further research is needed to confirm this.

Whether fruit quality is affected by trees receiving less water, Dzikiti says the jury is still out.

The study did not address this. The literature shows that there are potential negative effects, depending on the level and timing of stress.

“Farmers should be wary of overloading Golden Delicious trees; these trees indicate a limit to economic water productivity (R/m³) resulting from small fruit size at very high yield numbers.” Physical water productivity (kg of fruit per m³ of water used) increased with increasing yield. However, economic water productivity levels off.

According to Dzikiti producers can use the results from this study as a guide for improving irrigation practices (for example irrigation scheduling, water allocation and so on), and how to do more with less.

“By using water saving techniques such as drip irrigation to reduce evaporation from the orchard floor, shade nets to reduce evaporation and tree transpiration, and by choosing dwarfing rootstocks to reduce unnecessary shoot growth, producers can get more fruit with less water—the key is effective orchard management.”

Bonus: Useful water-related definitions

Transpiration. Water moves through the plant to the atmosphere.

Evaporation. Water moves from ground or water surface to the atmosphere.

Evapotranspiration. Water moves from earth’s surface, and through the plant to the atmosphere.

Irrigation efficiency. Fraction of water applied that is used in transpiration

Physical water productivity. Yield (kg of fruit produced) per m³ water used by the plant. An orchard with a yield of 100 tonnes per ha will have a higher physical water productivity than another orchard that uses the same amount of water but only yields 50 tonnes per ha. However, physical water productivity does not consider fruit quality.

Irrigation efficiency. Fraction of water applied that is used in transpiration

Economic water productivity. Rand income generated per m³ water used. This ratio brings fruit quality into the equation, but the values is affected by market-related issues. Hence, it’s best to consider the economic water productivity over a number of years. When comparing the economic water productivity of farms, regions, fruit types, and so on, it is important to remember that socio-economic factors should also be considered. Note that these productivity measures do not account for wasteful water application. However, both indicators can also be calculated for irrigation.

Water-use efficiency. From a plant perspective means photosynthesis divided by transpiration. It is often erroneously used when people are actually talking about physical water productivity.

Bonus: Which uses more water: leaf area versus crop load?

Scenario one: You have two Golden Delicious orchards with the same crop load, but one has more leaves than the other — the orchard with more leaves will use more water.

Scenario two: You have two orchards with the same leaf area, but the crop load differs — the orchard with the larger crop load will use more water.

Probability one: Leaf-area effect in terms of water use is larger than that of the crop load.

Probability two: If I lessen my crop load during drought conditions, I’ll use less water; provided I don’t stimulate any further growth.

Probability three: If I reduce both the crop load and leaf area, the water conservation effect will be greater.