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.