Determining Effective Area of Shade in Orchards and Vineyards to Estimate Crop Water Requirement

Note Number: AG1383
Published:August 2009

 

This Agnote provides information on assessment of the amount of shade cast by trees or vines to determine how much water is transpired to the atmosphere and hence how much water (rain and irrigation) is required to maintain the tree or vine in a non-stressed state.

Introduction

Effective area of shade (EAS) is a simple and practical estimate of the actively transpiring leaf area in an orchard or vineyard. EAS is determined from measurements of the percent shade cast by the trees or vines at 3 key times a day (3½ h before solar noon, at solar noon and 3½ h after solar noon). Taking 3 measurements per day accounts for changes in the percent shade during the day due to sun angle, foliage extent (i.e. training system and tree/vine size), planting arrangement (i.e. row orientation and tree spacing) and leaf area density. EAS is calculated from the average of the 3 measurements.

EAS can be used to estimate crop water requirement. Results from peach and apple experiments have shown strong relationships between EAS and tree transpiration, enabling accurate prediction of crop water requirement. Current research is aiming to determine the relationship between EAS and grapevine transpiration.

Method

The percent shade in an orchard or vineyard can be measured by sophisticated light interception instruments that require considerable expertise and are expensive. Alternatively percent shade can be visually assessed using a ruler and an estimate of the patchiness of the shade. A visual assessment of percent shade is the most practical method for growers to use.

• Select six representative regions within an irrigation unit or block. The range in plant vigour throughout a block should be represented by the selected regions (i.e. high, low and moderate vigour regions should be selected). Areas with missing or unhealthy plants should be avoided.
• Observations are made under clear sky conditions (i.e. the shadow cast by the plants is clearly delineated).
• Measure three times during a day at 3½ hours before solar noon, at solar noon and 3½ hours after solar noon (in central Victoria this corresponds to 9 am, 12:30 pm and 4 pm AEST or 10 am, 1:30 pm and 5 pm summer time).
• Calculate EAS = average of the percent shade from the measurements in the morning, at solar noon and in the afternoon.

Shade measurements

• Go to the first representative area in the orchard or vineyard block.
• Place the ruler (or graduated pipe as shown in Figure 1) on the ground, perpendicular to the row, so that the shade is cast onto the ruler. The ruler should be level - a brick may be needed to raise one end of the ruler, particularly if rows are hilled.
• Looking down the row, record the length of shade perpendicular to the direction of the row.
• Visually estimate the gap fraction within the length of shade. This describes the patchiness of shade and accounts for gaps within the canopy and gaps between plants. The gap fraction is scaled from 0 to 1, with 1 equivalent to a solid band of shade with no patches of light.
• Repeat visual estimate of percent shade at five other locations in the block (i.e. a total of six regions per block: two high vigour, two medium vigour and two low vigour).

Calculations

• Calculate percent shade:

 

• Calculate mean percent shade for the morning, at midday and the afternoon from the 6 visual estimates in the block.
• Calculate EAS (%) = average of the percent shade in the morning, at solar noon and in the afternoon:

 

Data checks

Graphing mean percent shade for individual locations from measurements in the morning, midday or in the afternoon can provide a quick check of the data (Figure 3). The plotted data may be U-shaped or flat, depending on the row orientation, training system and tree/vine size.

Crop water requirement

A simple estimate of peach tree transpiration (ET peach) is fundamental for determining crop water requirement according to the principles outlined in the Food and Agriculture Organisation of the United Nations Irrigation and Drainage paper 56 (www.fao.org/). ETpeach can be calculated from EAS and reference crop evapotranspiration (ETo):

 

where ETo is equivalent to water use of well-watered short grass and is calculated from standard measurements of wind speed, solar radiation, temperature and relative humidity. Historical and current ETo can be obtained from the Bureau of Meteorology.

Further References

Goodwin, I. and OConnell, M. (2004). Estimating effective area of shade to improve irrigation. Summer Fruit Australia Quarterly 6(2), 13-14.

Contact/Services available from DPI

Ian Goodwin

Senior Irrigation Scientist, Tatura

Email: ian.goodwin@dpi.vic.gov.au

Phone: 03 5833 5240

Acknowledgements

We acknowledge the contribution to this Agnote of projects jointly funded by the Victorian Department of Primary Industries and the Grape and Wine Research and Development Corporation.

This Agnote was developed by Ian Goodwin, Future Farming Systems Research. August 2009.