The analysis of water use commissioned by Hortgro provides an interesting perspective on evapotranspiration data for pome and stone fruit. By Anna Mouton.
The lead researcher on the project, agrometeorologist Dr Caren Jarmain, collaborated with Dr Nicky Taylor, senior lecturer in the Department of Plant and Soil Sciences at the University of Pretoria. They were assisted with extracting FruitLook data by the Centre for Geographical Analysis at Stellenbosch University. FruitLook is funded by the Western Cape Department of Agriculture.
Jarmain submitted her final report in 2019. The report includes an analysis of South African data generated by platforms such as FruitLook combined with a review of local and international studies. A key conclusion was that crop water use varies substantially both between and within production regions and fruit types.
Evapotranspiration and region
Evapotranspiration is the movement of water to the atmosphere as a result of the combined effects of evaporation and transpiration. Crop production demands continuous replenishment of this water through either rainfall or irrigation. It follows that growers need to know how much evapotranspiration and rainfall to expect in order to estimate how much water is required for irrigation.
The problem is that evapotranspiration rates depend on the interaction of many factors. This complicates attempts to understand the main drivers of water use in a specific situation.
One way of comparing evapotranspiration under different climatic conditions is to use values for a reference crop. The usual reference crop is an area of actively growing grass that has a uniform height and completely covers the ground — sort of like the lawn everyone wishes they had.
The Penman-Monteith equation allows researchers to calculate evapotranspiration values — Erpm — for the reference crop — the dream lawn — from daily mean temperature, wind speed, relative humidity, and solar radiation. Figure 1 shows the difference between reference evapotranspiration and rainfall for different fruit-production regions based on Schulze long-term data.
By comparing evapotranspiration and rainfall for the reference crop in different regions, it’s obvious that regional effects are profound. Furthermore, significant variation occurs within regions. As a result, there could be times when the hypothetical lawn requires less irrigation in the Little Karoo than in the Cape Winelands.
It is essential to consider this variability when planning and managing water use, because budgeting for the mean water requirement will, in most regions, frequently result in large shortfalls.
Crop effects on evapotranspiration
Growers are understandably more interested in the water use of their trees than that of a hypothetical area of grass. It is possible to calculate the evapotranspiration value for a specific crop by multiplying the reference value with a conversion factor — known as the crop coefficient — for that crop. However, crop conversion factors are not always accurate, even when comparing apples to apples — variables such as canopy size and tree spacing make a big difference.
Rather than relying on conversion factors, Jarmain and her team instead used actual evapotranspiration data from FruitLook for the Hortgro project. Figure 2 shows the total average evapotranspiration per fruit type for the period 1 August 2018 to 31 July 2019.
As these values are for actual orchards, crop effects cannot be entirely separated from regional and other effects, such as management. The researchers point out that some of the orchards in the dataset would have been under stress. They also warn against reading too much into data from a single year — more data analysis would yield more robust results.
Statistics from near and far
The Hortgro analysis includes a review of published studies on water use of stone- and pome-fruit orchards. From these, the researchers could determine the median evapotranspiration rates for different fruit types. The median is the middle value when all the values are arranged in order from small to large. It is less affected by extreme values than is the mean.
Figure 3 shows the evapotranspiration — in most cases per growing season — for apples and pears, and Figure 4 shows the results for stone fruit. The median values are for the combined results of local and international studies, except for apricots and plums, where no local studies were available. Results from selected South African studies are shown individually. It is clear that orchard water use can differ enormously, even for the same fruit type.
The data presented in Figure 3 suggest that water use in apples is strongly affected by leaf-area index. Unfortunately, leaf-area index was not available for all the studies.
“Canopy size is an important driver of transpiration,” says Taylor. “And that plays such a big role in how much water your crop uses.” She explains that, whereas transpiration is high in orchards with large canopies, evaporation from the soil surface can be equally high in orchards with small canopies.
According to Prof. Wiehann Steyn, general manager at Hortgro Science, orchard water use is more specifically a function of leaf area. “Large, dense trees will usually have a higher leaf-area index and use more water, but high-density plantings can also have a high leaf-area index if the trees are too vigorous and the canopies overly dense.”
The South African scenario
Analysis of FruitLook data for the period 1 August 2018 to 31 July 2019 allowed Jarmain and colleagues to generate estimates of annual evapotranspiration per crop type and region. The results for apples are shown in Figure 5 and for plums in Figure 6.
The researchers caution that the FruitLook data reflect water use during restrictions imposed by drought, and that the values obtained may not represent optimal production practices in all regions. An orchard that receives little irrigation due to water shortages will show low evaporation — this might have skewed the data in some regions over the study period.
Other factors that affect the evapotranspiration figures include inaccurate identification of crop types, and the use of nets in especially the Langkloof. Nets reduce the FruitLook evapotranspiration estimates. This may be one reason why the median for apples in Figure 3 is lower than that for peaches in Figure 4.
Peaches are generally believed to use less water than apples. The lower median reported for apples could also be because the studies included in the medians were done in wetter regions — peaches are typically grown in Mediterranean climates whereas apples are grown in temperate regions.
Steyn points out that significant differences in the age profile of the orchards included in the datasets for different fruit types are additional factors that could affect the median.
Overall, the research demonstrates that there is still a lot to learn to optimise the allocation and utilisation of water. “Water-management planning typically uses just one average value for a region,” says Jarmain. “We showed that a single value isn’t representative for a region or a crop.”
For more on evapotranspiration and water use, see the September 2018 issue of Fresh Quarterly.