In this section:
Salinity in Riverland Grapevines
Managing Salinity in Riverland Grapevines
Stevens, R.M. (1996), Murray Pioneer, 9th, 16th and 23rd July.
Salinity has been associated with irrigation for thousands of years. This strong link exists because salt is water soluble and moving water also moves salt.
When rain falls in the upper catchment of the Murray River it contains very little salt. For example a megalitre (equal to 1000 kilolitres) of rain at Albury contains only five kg of salt. As we move down river, both the diversion of fresh water and the inflow of saline water increase the concentration of salt. At Loxton, a megalitre of irrigation water with an EC of 480 contains 261 kg of salt.
Upstream in Sunraysia, where the river salinity is lower than the Riverland, Dr Nagarajah of Agriculture Victoria surveyed the salt content of sultana grapevines following periods when the river salinity was around 300 EC units. In a quarter of the vineyards, he found that the salt concentration in the vines was high enough to be injurious. Even with low river salinity, inappropriate management practices may cause a high salt concentration in the vine.
Action by irrigators can influence the salt concentration in the vine and river salinity. Before considering such actions I will discuss how salinity is measured.
Measuring salinity
When salt dissolves in water it forms particles which carry an electric charge. They are called ions. The greater the concentration of ions the easier the passage of electricity through water. This passage is measured as the electrical conductivity (EC) of water (conductance is the inverse of resistance). Locally EC is measured in microSiemens per centimetre (µS/cm) and for the remainder of this article this unit will be used when referring to EC. Another common unit (and the internationally preferred unit) used to express the EC is deciSiemens per metre (dS/m) which is identical to an older unit the millimhos per centimetre (mmho/cm); one dS/m equals one mmho/cm. A thousand µS/cm is equal to one dS/m. For example an EC reading of 500 µS/cm could be expressed as 0.5 dS/m.
Water salinity may also be measured by weighing the amount of dissolved salt. The most common unit in use with this measurement is parts per million (ppm) which is equivalent to milligrams per litre (mg/L) as a litre of water weighs a million milligrams. An older unit. grains per gallon, can be converted to ppm by multiplying by 14.3. The exact relationship between conductivity of river water and the mount of total dissolved solids depends on the types of dissolved salts. Multiplying the value of conductivity in µS/cm, that is the local EC units, by 0.64 will give the approximate concentration in mg/L or ppm of a solution.
Effects of salinity on the grapevine
When applied to plants, the term salinity refers to the poor conditions for plant growth caused by exposing the plant to too much dissolved salts. Undissolved salts do not affect plants. In grapevines the initial visual symptoms of salinity are leaves with blackened or dead margins. They are most often seen after the onset of hot weather in late spring. Salinity has two effects on the grapevine growth
Droughting effect
This arises because adding salts to the soil makes it harder for grapevine to suck water from the soil. The higher the concentration of salts the harder it is to suck water. This effect is similar to that caused by droughting the vine. Technically this effect is known as the osmotic effect. All types of soluble salts will cause this problem and it will occur without the salts entering the plant.
Vines exclude about 99% of the salt added in irrigation water. As vines withdraw water from the soil the concentration of the excluded salts in the water remaining in the soil increases. Two modifications to irrigation management can reduce the droughting effect.
(I) Each irrigation dilutes the salt concentration in the soil water around the roots. Increasing the frequency of irrigation can reduce the amount of time that roots spend exposed to soil water with a high salt concentration. Increasing the frequency of irrigation will lengthen the proportion of the season over which the surface soils are wet. This will increase the seasonal losses of water by evaporation from the soil and the annual depth of irrigation will probably need to be increased to account for this extra loss.
(ii) Applying more water than is necessary to replenish the rootzone can wash excluded salts out of the rootzone - this practice is known as leaching the salt. Prerequisites for successful leaching are a freely draining soil and an irrigation system with good distribution uniformity, that is a system which delivers the same amount of water to all the vines in an irrigation shift. If the natural soil drainage or that due to the installation of drains is poor then the leaching component of an irrigation can become a liability. For, rather than draining away, it accumulates in the sub-soil slowly causing a perched watertable to form. This leads to temporary or permanaent waterlogging in part of the rootzone. Waterlogging should be avoided as it reduces the ability of the vine roots to exclude any toxic salts which are present.
Toxic effect
Some types of salt are toxic to grapevines. The toxic effect occurs when such salts enter the grapevine and causes it to become sick. Locally the most common toxic salt is sodium chloride. Uptake of toxic salts depends on a number of factors including their concentration in soil water, the texture of soils in the lower rootzone, the type of rootstock, the aeration of the rootzone, the timing of saline irrigation and the type of irrigation system.
Uptake of sodium and chloride by the grapevine roots is proportional to the concentrations of these salts in the rootzone. Successful leaching irrigations (see previous section) reduce the salt in the rootzone.
Across the Riverland soil texture is highly variable. Ms Prior and co-workers of NSW Agriculture at Dareton investigated the response of field-grown own-rooted sultana vines to irrigation with a range of saline waters (between 300 and 3500 EC units) at a site which contained a number of soil textures. They found that salinity caused a bigger yield loss in areas where the soil in the lower rootzone was high in silt and clay. The result demonstrates that losses to salinity will be less at sites where the deeper soils in the rootzone have a lighter texture that is they are low in clay and silt, and high in sand.
Researchers looking to find rootstock vines with a greater tolerance to nematodes also noted that some rootstocks had a low chloride content. At CSIRO Merbein, Dr Walker and co-workers irrigated sultana vines on their own-roots and Ramsey rootstock with saline water (3500 EC units) throughout the entire season for four consecutive years. They found that with saline irrigation the depression in the yield from vines on Ramsey rootstock was less than the in vines on their own-roots. The use of rootstocks which restrict the entry of toxic salts can reduce the effect of salinity on vine yield.
At Loxton Centre we investigated the effect that irrigation management can have on the ability of rootstocks to restrict the entry of toxic salts. Potted sultana vines growing on their own-roots or on a range of rootstocks (including Ramsey) which excluded toxic salts were assigned to two groups. Both groups were irrigated with saline water. In one group, we maintained a free draining rootzone and, in the other, we waterlogged the rootzone by placing the pots in a buckets of water for one week in every two. Waterlogging increased the leaf chloride in both own-rooted and rootstock vines. In the group with free draining rootzones, the use of a rootstock reduced the concentration of chloride in the leaf by 60%. In the group with waterlogged rootzones, the exclusion of toxic salts by the vines on rootstocks was markedly reduced; the use of a rootstock only reduced the concentration of chloride in the leaf by 18%. Temporary waterlogging reduced both the ability of vines to exclude toxic salts and the benefits for salinity tolerance likely to be gained by investing extra capital in a rootstock which excludes toxic salts. Vines growing in blocks where the drains run for three or more days following an irrigation are probably experiencing temporary waterlogging. To avoid temporary waterlogging, irrigation should be scheduled so that the amount applied matches the soil water deficit and the distribution uniformity of the system should be good, that is the system should deliver the same amount of water to all vines in a shift thereby avoiding the patchy over-irrigation associated with systems where the uniformity is poor.
River salinity varies over the season and some of this variation reflects decisions made regarding the management of river flows. At Loxton Centre we investigated the effect that the timing of saline irrigations has on yield. Over six seasons, four saline irrigation treatments were applied to drip irrigated field-grown Colombard vines on Ramsey rootstocks. We divided the eight-month irrigation season into four two-month periods, based on growth stages of the vines, and irrigated each treatment with saline water (3500 EC units) for one of the two-month periods and with river water for the remainder of the season. Results were compared with those from vines irrigated for the entire season with river water. Over the six seasons, the cumulative yield of vines irrigated with saline water from early November to early January (between flowering and veraison) was depressed by 7%, whereas the yield of vines which received saline irrigations from early January to late March (between veraison and harvest) was only depressed by 3%. The yields of vines irrigated with saline water either mid-September to early November or from late March to early May (bud-burst to flowering or harvest to leaf-fall, respectively) were not affected.
This study suggest that the detrimental effect of high salinity is greater between the early November and early January. In years where river flow is low, grapegrowers would obtain the greatest benefit from dilution flows that occurred within this period. (At veraison the berries of red grape varieties begin to colour and those of white grape varieties begin to soften and their skin becomes translucent.)
The entry of sodium and chloride into the grapevine is much easier through the leaves than the roots. At Loxton Centre we compared the chloride concentration in leaves from Colombard vines on Ramsey rootstock receiving river water or saline irrigation (440 and 3500 EC units) delivered by dripper with those from vines irrigated with overhead sprinklers. After three consecutive season of irrigation with either river water or saline water, the leaf chloride concentrations in vines irrigated with overhead sprinklers were double those in vines irrigated with drippers. With saline irrigation the maximum cumulative yield loss after three year was 3%, whereas with saline overhead irrigation it was 17%.
In the vineyard, keeping the concentration of salt in the soil water to a minimum and minimising the entry of toxic ions into the grapevines will reduce the deleterious effects of salinity on grapevine growth. Specific strategies to achieve these aims include:
- minimising the build-up of salt concentration in the rootzone soils through managing irrigation to avoid soil drought and to successfully leach salt from the rootzone,
- preferentially locating new plantings on sites where the soils in the lower rootzone are lighter textured,
- using salt excluding rootstocks,
- managing irrigation to avoid temporary waterlogged by scheduling irrigation to match the irrigation amounts with the soil water deficit and by achieving good distribution uniformity,
- in years of low river flow, lobbying for the release of river dilution flows between early November and early January,
- avoiding irrigation systems which wet the leaves or if other factors favour such a system then irrigating at night with a sprinkler revolution rate greater then once per minute so as to avoid evaporation concentrating the irrigation water as it lies on the leaves.
River salinity
Analysis of the year-to-year changes in the salinity of the River Murray has shown that it rises as the flow rate falls and falls as the flow rate rises. Once this effect is removed from the year-to-year records the remaining changes suggests that the river salinity is rising at a rate between two and five EC units per year. The main potential sources of salt in the river are inflows from groundwater residing in sediments layed down when this region was covered by the sea (the sea origin of these sediments is indicated by referring to them as marine sediments) and inflows from seepages associated with dryland salinity.
After the sea receded from the region, the Murray River formed by cutting a valley through the marine sediments. Water trapped in these sediments drained into the valley. Water has been added to these sediments by seepage of the small amount of the annual rainfall in excess of that stored by topsoils for use by native vegetation. This excess leached salts, fallings with the rain, and topped-up the saline groundwater sustaining the seepage out of these sediments into the river.
The rise in river salinity as the river flow rates fall suggests that most of the salt is being added at a constant rate and river salinity for a large part depends on the amount of water diluting these additions. The salt source for constant addition is largely that added by groundwater inflows from marine sediments.
In areas away from the main valley where the continuous porous sediments capable of draining this seepage toward the river are absent, the downward flow of rainfall over past millennia has deposited salts deep in the landscape. Changes in the movement of water brought about by the clearing of deep rooted vegetation have increased the amount of water percolating downward. This water has mobilised the salts, which were deposited over previous millennia, forming lenses of saline water which seep out to the surface lower in the landscape. This phenomena is known as dryland salinity.
Within the Murray-Darling basin the spread of irrigation, and coincidentally of dryland agriculture, is increasing the amount of water percolating downward below the rootzone. Drainage from the irrigation of land alongside the river (specifically that draining-water which by-passes the drains or forms on land where drains are not installed) seeps to the marine sediments and increases the rates of saline groundwater inflows from these sediments to the river. Dryland development in the upper catchment has removed much of the native vegetation which dried out the surface and deeper soils in summer with the deeper soil soaking up winter rainfall in excess of the plant’s immediate requirements. The pasture which replaced native vegetation has a shallower root system. In summer, it does not dry the deeper soils and, in winter, the excess rainfall drains downward causing dryland salinity. For the most part the effects of dryland salinity have yet to register in the river. However, it is conservatively predicted that 25 years from now the addition of salt from dryland salinity will add 140 EC units to the river salinity measured at Morgan.
Irrigation developments upstream of the Riverland have diverted water which once would have flowed to the sea diluting inflows of salinity measured at Morgan.
These effect of these changes is not irreversible. Returns per kilolitre of irrigation water used in the Riverland are greater than those achieved by irrigators of annual crops located upstream. It is likely that developments supporting cross-border trade in water will lead to a greater amount of water moving to where returns per kilolitre are high, that is from upstream to the Riverland. This increase will dilute saline inflows and reduce river salinity.
The drainage flows to saline aquifers alongside the river can be reduced. The volume of drainage currently produced is much greater than that required to sustain production. It can be reduced by better on-and off-farm irrigation management. Rehabilitation of irrigation areas which replaces open channels with pipes reduces drainage flows produced by channel leakage. The provision of water on-demand supports adoption of improved scheduling leading to a better match between irrigation amounts and soil water deficit, and thus, reduces the production of unnecessary drainage. Improvement in irrigation systems leads to more uniform application which avoids the unnecessary drainage caused when the amount of irrigation is increased to overcome the effect of poor uniformity. With good distribution uniformity, that is each vine in an irrigation shift receiving the same amount of water, the extra irrigation amount to overcome the poor uniformity becomes unnecessary.
In dryland areas, strategic replanting of native combined with the replacement of shallower rooted annual pasture with deeper perennial pasture is reducing the downward percolation of rainfall to below the rootzone.
Local increases in river salinity can be stalled by supporting:
- cross-border trading of water,
- rehabilitation of irrigation areas,
- improvements in irrigation management,
- programs (for example Landcare) aimed at stemming the spread of dryland salinity, especially in the upper catchment.
