Pear tree transpiration and implications for irrigation

By Graeme Thomson, Ian Goodwin, David Cornwall and Steve GreenPips logo 130

Water is an issue for many growing regions, here we find out what affects transpiration rates in pear trees and ways to ensure your pears get enough water without wasting any.

There is no point in providing trees with more water than they need. Surplus irrigation wastes water, is an unnecessary expense and could have detrimental effects on both tree and soil health such as through water-logging and salinity.

Optimising water use

Key points

  • Maximum mid-summer transpiration in two micro-irrigated high-density pear orchards was 4.5mm/day.
  • The total crop water requirement from bud burst to leaf fall was just over 500 mm (= 5 Ml/ha).
  • Growers can calculate day-to-day irrigation needs using weather data and tree size where the fraction of the orchard floor shaded by the trees (effective area of shade, EAS) is a good measure of tree size.

Lack of water for irrigation due to changing climate is emerging as the biggest threat to the viability and sustainability of the pear industry in Australia. A better understanding of pear tree water requirements under different environment and farm management conditions is needed to maintain productivity and manage eco-efficient, water saving strategies like regulated deficit irrigation.

In general, and without considering potential benefits of induced water stress, a tree only requires enough water to match the amount it loses from transpiration. Most of this loss occurs through small pores in the leaf surface and sunny, hot, windy weather with low relative humidity tends to generate most water loss via transpiration.

If growers directly measure the transpiration of trees and allow for rainfall, irrigation requirements can be fairly accurately calculated. However, measurement of transpiration is complex. The work reported here aimed to find an easier, more practical way of determining irrigation input than is currently used.

Also, because micro-irrigation is an increasingly important irrigation technique in Australia, it is essential to improve understanding of this practice. In micro-irrigated orchards, irrigation requirement is dominated by transpiration because of minimal evapotranspiration from the irrigation wetting pattern on the orchard floor (Bonachela et al., 2001).

Tree water requirements

We investigated tree water requirements in high-density pear orchards by relating transpiration measured using a sap-flow method to reference-crop evapotranspiration (ETo) and effective area of shade (EAS) in two commercial orchards of ‘Williams’ Bon Chretien’ pear grafted on D6 rootstock at Shepparton East (Orchard 1) and Ardmona (Orchard 2).

Canopy radiation interception was estimated from the amount of photosynthetically active radiation (PAR) detected by sensors installed at ground level under the tree canopy.

Planting date Tree structure Spacing Irrigation
Orchard 1 2000 Central leader 4.5×2 m microjet
Orchard 2 2007 2-leaderOpen Tatura 4.5×1 m drip
Table 1: Details of two orchards measured to determine tree water requirements.

Results showed that there was considerable variation in transpiration from day-to-day and between sites. Seasonal changes saw transpiration increase after full bloom to reach a maximum in mid-summer of approximately 4.5 mm/day in both orchards, and then decline in late summer and autumn.

At its mid-summer peak, transpiration corresponded to an average of 40 and 20 litres/tree/day in Orchard 1 and 2, respectively. As expected, low values of daily transpiration were recorded on overcast days with rainfall.

To measure transpiration by sap flow method, two sets of heat pulse probes are installed in the trunk of each tree.

To measure transpiration by sap flow method, two sets of heat pulse probes are installed in the trunk of each tree.

Canopy radiation interception was estimated from the amount of PAR at ground level under the tree canopy and the amount of incoming PAR above the canopy. EAS was estimated as the mean value of the fraction of radiation intercepted at solar noon, solar noon -3h and solar noon +3h.

We found that EAS remained relatively constant at 0.6 in both orchards for most of the season apart from the first 30 days after full bloom and at leaf fall.

Daily transpiration data were compared with daily ETo weighted for EAS for pre- and post-harvest periods. ETo was calculated every 15-minutes using the FAO-56 standardised Penman-Monteith equation (Allen et al. 1998) using weather data collected from a nearby portable station located in a cleared area adjacent to the pear orchards. Excluding data from the withholding irrigation period in Orchard 2, we found a similar linear relationship in the pre- and post-harvest periods.

Calculating irrigations needs

Given the nature of these relationships, the results of our study suggest that pear tree transpiration and irrigation requirements prior to harvest can be estimated simply from 1.1 x EAS x ETo, where EAS accounts for daytime changes in radiation interception associated with tree size, row orientation, training system and leaf area density.

A grid of photosynthetically active radiation (PAR) sensors on the orchard floor are used to measure canopy radiation interception.

A grid of photosynthetically active radiation (PAR) sensors on the orchard floor are used to measure canopy radiation interception.

Given this, irrigation requirement, assuming negligible understorey water use, can be calculated from ETo and routine measures of orchard EAS. We found that post-harvest full irrigation requirements appear to be less and can be estimated from 0.8 x EAS x ETo.

This is a simpler procedure than current FAO recommendations based on ETo that require a look-up table of basal crop coefficient values for different growing periods and further adjustments for vegetation cover on the orchard floor (Allen et al. 1998).

The EAS-weighted ETo proposed here provides an objective estimate of transpiration for which EAS can be readily established by routine measurements of radiation interception by irrigation consultants, extension officers, or growers.

EAS need not be measured using a grid of sensors. Light bars (handheld or mounted on a vehicle) or approximations of the fraction of the orchard floor that is shaded by the tree foliage in the morning, at midday and in the afternoon can be employed (to learn how to do this refer to AgNote 1383 on the old DEPI website).

Four or five measures of EAS during the irrigation season are probably adequate because our measurements showed that EAS remains relatively constant for most of the season. EAS changes following bud burst but in northern Victoria there is no need to irrigate at this time because of sufficient stored soil moisture and frequent rain. In addition, it is unlikely that EAS for a mature orchard block will change from season-to-season so that once EAS is defined, irrigation requirement will vary mostly with ETo.

Seasonal changes in daily pear tree transpiration in Orchard 1 (left) and Orchard 2 (right). The vertical arrows show harvest in each orchard. The horizontal arrow in Orchard 2 shows the period when irrigation was withheld (WI).

Seasonal changes in daily pear tree transpiration in Orchard 1 (left) and Orchard 2 (right). The vertical arrows show harvest in each orchard. The horizontal arrow in Orchard 2 shows the period when irrigation was withheld (WI).

Example calculation

An example of how to calculate how many litres to apply per tree once you have all the necessary data:

EAS = 0.40

ETo = 8 mm (hot summers day)

Tree spacing = 2 m

Row spacing = 4.5 m

Irrigation = 1.1 x 0.4 (EAS) x 8 (ETo) x 2 (Tree spacing) x 4.5 (Row spacing) = 32 litre per tree

Daily tree transpiration. Left: Orchard 1: Green dots show from full bloom to harvest and blue dots from harvest to the start of leaf fall. INSERT above right graphic: Orchard 2: Red dots show from full bloom to the start of irrigation, green dots show from the start of irrigation to harvest and blue dots show from harvest to the start of leaf fall.

Daily tree transpiration.
Left: Orchard 1: Green dots show from full bloom to harvest and blue dots from harvest to the start of leaf fall.
Right: Orchard 2: Red dots show from full bloom to the start of irrigation, green dots show from the start of irrigation to harvest and blue dots show from harvest to the start of leaf fall.

Acknowledgements

Quick guide

To determine your pear orchard’s irrigation needs:

  1. Determine your orchard’s effective area of shade (EAS) by following AgNote 1383.
  2. Determine your orchard’s reference crop evapotranspiration rate (ETo) using data from your closest weather station.
  3. Calculate pre-harvest irrigation needs by using the equation 1.1 x EAS x ETo, which will give you the mm depth of water to apply in your orchard.
  4. Convert the depth of water to the litres of irrigation to apply to each tree by multiplying by the tree spacing in metres and the row spacing in metres.

Sound too tricky? 

Your local agronomist can help you make these calculations. Get them out to your orchard and put them to work to help you save water and improve your pear trees’ performance!

This project was funded through the Productivity Irrigation Pests and Soils (PIPS) research program by Horticulture Innovation Australia Limited using the apple and pear industry levy, voluntary contributions from industry and matched funds from the Australian Government. Additional financial support was provided by the Department of Economic Development, Jobs, Transport and Resources, Victoria.

References

Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. (1998) Crop evapotranspiration. Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, FAO, Rome.

Bonachela, S., Rogaz, F., Villalobos, F.J. and Fereres, E. (2001) Soil evaporation from drip-irrigated olive orchards. Irrigation Science 20: 65-71.

About the authors

Graeme Thomson, Ian Goodwin and David Cornwall are from Department of Economic Development, Jobs, Transport and Resources, Victoria, and Steve Green is from Plant and Food Research, New Zealand.

For more information contact Ian on 03 5833 5240 or ian.goodwin@depi.vic.gov.au.

By |January 13th, 2015|PIPS, Soils, nutrition and irrigation|

About the Author:

Department of Economic Development, Jobs, Transport and Resources, Victoria