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Table Grape Irrigation (DBIRD_NT)

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					Technote
                                                                                 No. 114

                                                                                 May 2002

                                                                                 Agdex No: 241/561

                                                                                 ISSN No: 0158-2755




Irrigation Management in
Table Grapes at Ti-Tree
A. Nesbitt, Horticulturist, S. Nagarajah, Senior Horticulturist and G. Kenna, Principal Horticulturist, Alice
Springs




INTRODUCTION

The table grape industry in Central Australia is located at Ti Tree, approximately 200 km north
of Alice Springs. The climate in this area is arid with a low annual rainfall and high evaporation
rate. Irrigation of vines using sub-artesian bore water is essential for crop production. Careful
management of irrigation including scheduling and the application of the correct amount of
water is vital. Over-irrigation and under-irrigation should be avoided because they are harmful to
vines. Over-irrigation can lead to high nitrogen and salt levels in vines because the bore water
contains high levels of nitrate, sodium and chloride. High nitrogen promotes excessive vigour
and reduces fruiting capacity, particularly in rootstock vines. High levels of salt can reduce yield
and quality of fruit. Over-irrigation is also a waste of water, which increases production costs.
Under-irrigation droughts the vine and causes an accumulation of salt in the soil due to
inadequate leaching. High salt levels damage soil structure, reduce drainage and decrease
aeration, which affects vine health.

THE IMPORTANCE OF IRRIGATION MANAGEMENT IN VINEYARDS AT TI TREE

This Technote provides background information on irrigation and describes methods that can be
used to improve irrigation management, including:

•   Soil characteristics.
•   Root distribution patterns.
•   Water extraction patterns by roots.
•   Irrigation scheduling aids.
•   Maintenance of the drip system.

SOIL CHARACTERISTICS

One of our most important resources is soil. However, the properties of soil in a vineyard,
particularly at some depth, and how they relate to irrigation practices, are often overlooked.
Studies were undertaken to examine some of these properties including mechanical
composition, bulk density and infiltration rates in the Ti Tree area.
                                                      2


  A summary of the characteristics of Ti Tree soils is presented in Table 1. The figures indicate
  that in general, the soil is a deep, coarse-grained, sandy loam or loamy sand, with excellent
  drainage.

  Table 1. Summary of soil characteristics at Ti Tree

           Parameter                     Surface soil          Sub-surface soil              At depth
Northcote class1                                                Uc1.21 to Uc1.23
Texture class                            Loamy sand              Loamy sand                Sandy loam
Gravel2                                      Nil                      Nil               Coarse iron stone
                                                                                       layer exists in some
                                                                                       areas (120-150 cm)
Sand2 (%)                                      85.0                   80.0                       75.0
Silt2 (%)                                       3.0                    2.0                         2.5
Clay2 (%)                                       5.0                    9.0                       11.0
Bulk density (g/cm3)                            1.7                    1.8                         1.9
Hydraulic conductivity                                             100 mm/hr
(Infiltration at saturation)
  1
    Soil class determined by Northcote, K.H. (1992). A factual key for the recognition of Australian soils.
  Rellim Pub., NSW
  2
    Mechanical analysis of soil as developed by P. Rengasamy, University of Adelaide

  Although the soil is coarse grained with good drainage, it does possess good water holding
  capacities. Using soil water tensions (matric potential), field capacity and refill point for table
  grape production is estimated to be –5 to -10 kPa and -25 kPa, respectively.

  By understanding the basic soil properties, we are able to tailor irrigation regimes and
  recommend the most suitable scheduling aids that will work best in these types of soils.

  ROOT DISTRIBUTION PATTERNS

  Information on rooting patterns is necessary to install irrigation scheduling aids and interpret soil
  moisture data. Studies were carried out on the rooting patterns of Menindee Seedless, Sultana,
  Flame Seedless and Red Globe/Ramsey grapevines under drip irrigation. The main findings
  were:

  •    Root growth was restricted to a narrow strip, extending to about 15 cm on either side of the
       vine in the vine row. Root distribution along this strip was uniform.
  •    Rooting depth was about 150 cm, with a few roots sometimes growing below this depth.
  •    Maximum root growth was generally in the top 30 cm of the soil and decreased below this
       depth.
  •    Ramsey rootstock vines had better root growth deeper in the soil than own rooted vines.
  •    Menindee Seedless had a relatively shallow root system.

  These results indicate that tensiometers installed at 30, 60 and 90 cm depths would cover most
  of the root-zone. Access tubes should be installed to a depth of at least 100 cm when using a
  neutron probe, EnviroSCAN, C-Probe, or Gopher.
                                                  3


WATER EXTRACTION PATTERNS BY ROOTS

The water extraction patterns by roots was studied using the neutron probe. Menindee
Seedless, Menindee Seedless/Sultana, Flame Seedless, Red Globe/Ramsey and
Crimson/Ramsey cultivars were used. Moisture content rapidly decreased within the first 10 cm,
mostly due to evaporation. Other results indicated that:

•   moisture extraction decreased with depth;
•   with increase in water use, extraction increased from deeper areas in the soil;
•   moisture extraction patterns were variable between cultivars;
•   in Menindee Seedless and Menindee/Sultana, moisture extraction occurred mainly in the
    surface soil;
•   more moisture extraction occurred deeper in the soil in Ramsey rootstock vines than in
    Menindee or Menindee/Sultana;
•   in Flame Seedless, moisture extraction fell between the two extremes;
•   the rooting pattern of the cultivar had a bearing on the moisture extraction pattern.

IRRIGATION SCHEDULING AIDS

An irrigation scheduling aid is a device that estimates water content indirectly. The reading
given by the device does not often have any meaning on its own, but needs to be calibrated
against actual soil water contents that have been measured directly (such as wet and dry weight
measurements combined with bulk density figures to give actual volumetric water contents).

Scheduling aids
There is an extremely large range of irrigation scheduling aids on the market today. Each has its
own advantages and disadvantages. Most of the aids that are readily available to the grower will
estimate water content in one of two ways: by suction or by measuring soil dielectric.

Suction instruments use a porous media (often ceramic) that measures how difficult it is to
extract water from soil pores held in tension. This can give some idea of how difficult it is for
roots to extract water from the soil. That is, the less moisture in the soil, the harder the plant has
to ‘suck’. These devices include tensiometers, gypsum blocks, and granular matrix sensors.

Soil dielectric is most commonly measured by capacitors, which measure the change in soil
moisture as water content affects the electromagnetic properties of the soil. These devices
include the EnviroSCAN, Diviner 2000, C-Probe, and Gopher. It must be remembered that the
proper installation and calibration of this type of equipment is critical. An excellent publication on
soil water monitoring is 'Irrigation Insights -Number 1: Soil Water Monitoring’ by Philip
Charlesworth of CSIRO Land and Water.

Tensiometers
By far the most common scheduling aid used in Ti Tree is the tensiometer. This is because
tensiometers are relatively cheap, easy to install, do not require calibration, and are reasonably
accurate from –80 to –5 kPa. Tensiometers can be used to schedule irrigation if they are
monitored regularly. In Ti Tree, irrigation should start when the 30-cm tensiometer reads –
25kPa. Irrigation can run until the 90-cm reads –10kPa. Allowing for the lag time needed for
tensiometers to equilibrate with the surrounding soil, this would result in the first 90 cm of soil to
reach field capacity.
                                                 4


Other scheduling aids
Both the C-probe and EnviroSCAN have also been used at Ti Tree. As long as they are
calibrated and maintained, they perform well. Gypsum blocks are not recommended for use on
sandy soils for vineyards as their accurate range of measurement is between –200 and –30
kPa, while Ti Tree vineyards operate between –25 and –10 kPa.

Crop factors
Scheduling aids are good at reporting on the condition of the soil, but it is often difficult to
estimate the amount of water required to bring your soil to a desired moisture level, in order to
provide the vine with its water requirements. Crop factors (or crop coefficients) estimate actual
vine water requirements based on evaporation data (from a US class A pan). The crop factors
currently used to schedule irrigation at Ti Tree are shown in Table 2. These figures are still
being researched to verify their accuracy and may need to be modified to suit individual growing
conditions, varieties, and age of vines. It is estimated that young vines (one to two years) may
have half the water requirements of mature vines.

Table 2. Current knowledge of Crop Factors for use in Ti Tree vineyards

       Growth Stage                        Month                          Crop Factor
       Post-harvest                        January                            0.4
                                           February                           0.3
       Leaf Fall                           March                              0.2
                                           April                              0.2
       Pruning                             May                                0.1
                                           June                               0.1
                                           July                               0.1
       Bud Burst                           August                             0.1
       Flowering                           September                          0.2
       Veraison                            October                            0.5
       Harvest                             November                          0.55
       Harvest and Post-harvest            December                           0.4

Using crop factors
A crop factor simply represents the proportion of water needed to replace that which has been
lost by the plant through transpiration. For example, a crop factor of 0.5 means that transpiration
(or water loss) is 50% of evaporation (from the US class A pan). To estimate the amount of
irrigation required to replace this loss, we simply multiply the amount of evaporation for the day,
week, or month by the corresponding crop factor and subtract any rainfall. The first 10 mm of
rain is usually discounted. Rainfall above this is termed effective rainfall. The final result gives
the amount of irrigation to be applied, in millimetres. To turn that to megalitres per hectare
simply divide by 100. Put simply:

Water use (ML/ha) =      [Crop Factor x Evaporation (mm)] – Effective Rainfall (mm)
                                               100

Or

Water use (litres per vine) = [(Crop factor x Evaporation) – Rain] x Row space (m) x Vine
                              space (m)

Then, using individual irrigation outputs, we can work out hours of irrigation based on emitter
spacing and outputs. This will provide the vine with its optimal water requirements. Modifications
                                                  5


need to be made sometimes for optimal water use. This is to allow for leaching programs or
irrigation regimes that use regulated deficit irrigation. It is suggested that leaching programs add
around 10% to your annual water cost. These should be applied during particular growth
periods when salt in the soil is known to increase to levels that are detrimental to the vines’
health (i.e. mid-April, mid-October and mid-February). See Technote No. 113 ‘How to Reduce
Potential Problems of Salt in Grapevines at Ti Tree’ on reducing potential salt problems.

No matter how you schedule irrigation, monitoring aids like those mentioned above should
always be used to back up regimes. Even though the use of crop factors has been suggested,
individual needs of vines can vary because of:

•   different varieties and different rootstocks;
•   the age of the vine (younger vines have a much smaller root system and lose less water
    through smaller canopies);
•   vigour (large leaf areas have higher transpiration, although correct irrigation management
    will help control vine vigour);
•   soil types (different soil types lose water through evaporation at different rates).

Scheduling aids such as tensiometers should be checked every couple of days, while the more
expensive type of equipment, like C-Probes, can automatically transmit readings to your
computer at regular intervals. All aids should be well calibrated and maintained.

Monitoring water use
One other useful method of checking water use is to actually monitor the amount of water
pumped using flow meters. Daily, weekly or monthly readings can be used to calculate water
applied per hectare or even per vine. It is useful to compare these types of values with
estimated crop water use based on crop factors, to gauge the effectiveness of your irrigation
regime.

MAINTENANCE OF THE DRIP SYSTEM

In order for scheduling irrigation to be effective, regular maintenance of the actual irrigation
system is critical. Particularly, filters and dripper outlets require the most attention. Scheduling
based on crop factors requires accurate figures on emitter outputs and spacings. If only a few
blockages persist, then calculating water use on a large scale for the vineyard can result in
inadequate watering of vines. Methods such as acid or chlorine injection, and regular (weekly)
cleaning of filters can greatly reduce most of these problems and help avoid under-irrigation.

SUMMARY

•   Irrigation management is one of the most important vineyard management issues in Ti Tree.
    Over or under-irrigation can be harmful to the soil and vines.
•   Soils at Ti Tree can be described as deep, well-drained, loamy sands or sandy loams with
    good water holding capacities.
•   Vine root distribution is limited to a narrow strip 15 cm either side of the vine butt down the
    row, with roots to a depth of 150 cm under drip irrigation. Maximum root growth is generally
    in the top 30 cm.
•   Water extraction patterns differ with variety and rootstock, with most of the extraction taking
    place in the first 30 cm for most cultivars.
                                                       6


•   Irrigation scheduling aids should monitor moisture levels to at least 100 cm, or with
    tensiometers placed at 30 cm, 60 cm, and 90 cm.
•   A large range of scheduling aids exists in the market, with most requiring accurate
    calibration and good maintenance. Tensiometers and the C-probe are currently used with
    good results at Ti Tree.
•   Crop factors are a useful tool in scheduling irrigation, but should always be backed up by
    soil moisture monitoring devices.
•   Maintenance schedules should form a part of any irrigation scheduling program.




Please visit us on our website at www.primaryindustry.nt.gov.au




Published: Friday 10 May 2002.




While all care has been taken to ensure that information contained in this Agnote is true and correct at the time
of publication, the Northern Territory of Australia gives no warranty or assurance, and makes no representation
as to the accuracy of any information or advice contained in this publication, or that it is suitable for your
intended use. No serious, business or investment decisions should be made in reliance on this information
without obtaining independent/or professional advice in relation to your particular situation.

				
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