Long term sustainability of precision irrigation
There is now strong evidence of serious soil structural decline under precision irrigation, but practical tools to identify the conditions for this degradation are not yet available. We will identify a range of paired (precision-irrigated / non-irrigated) texture-contrast soils in vineyards across the Barossa Valley and assess the impact of this irrigation on soil properties important for root growth and water uptake.
Soil profiles will be examined and penetration resistance and hydraulic measurements taken to indicate the degree of soil structural change in the field. Soil cores will be exposed in the laboratory to controlled, accelerated irrigation and tested for strength and hydraulic properties. Concurrent field trials will evaluate ‘best-bet’ strategies to avoid/ameliorate structural decline and to produce a “hazard rating” based on water quality and soil properties. We will determine the principal mechanisms of soil degradation under irrigation and recommend management strategies that minimize and reverse such decline.
Taking subsoil infiltration measurements in a drip-irrigated vine row near Tanunda in the Barossa Valley, South Australia.
With the wide adoption of drip irrigation to conserve water it’s important that we get it right. Research preceding this project suggested that the concentration of water that occurs immediately beneath drippers may cause serious subsoil structural decline.
The project embarked on a close comparison of subsoil structure under, and far away from drippers at vineyards in the but this work revealed no dramatic differences. However, as application rates in the Barossa Valley are usually less than 1 ML/ha, we have recently gathered similar data in and around Mildura and propose to do the same near Griffith shortly; at both locations water quality is good and application rates are much higher (5-10 ML/ha). The work in the Barossa Valley did however disclose universally poor subsoil structure in the 10 vineyards examined. In practically all cases drainage was poor, aeration was very poor, and mechanical resistance to root growth was high. Such adverse subsoil conditions point to a pressing need for improved preparation for permanent plantings and ongoing management of subsoil structure.
The alternative will be shallow-rooted plants dependent on frequent irrigation for survival; this is not consistent with the current need to maximize water use efficiency. We are in the process of confirming this hostility to root growth via root length density measurements on intact soil cores taken from the same locations. We are also pursuing study sites where more “heroic” and careful soil preparation has preceded planting.
- To establish the roles of soil type, irrigation water quality, amount of applied precision irrigation and methods of application of precision irrigation (e.g. pulse versus continuous) in soil degradation.
- To quantify the vulnerability of soils, in combination with various water qualities, to degradation under precision irrigation using a ‘hazard rating’.
- To develop and test management strategies to minimize and reverse soil degradation under precision irrigation.
- To prepare, with LWA editorial and publishing support, a grower-friendly guideline on best management practices for the industries using precision irrigation to reduce irrigation induced soil structure decline, and identification of any trade-offs between best management practices for soil structure, nutrients, pesticides and root zone salinity.
- To provide technical support to a separate project team charged with interpreting the projects findings for other climatic zones and commodities.
- To communicate new findings and additional outcomes to policy makers (SA, Vic, NSW, MDBC, NWC).
Initial work in the project has been at 18 sites within 8 vineyards. Water quality is generally good; only 4 of these sites have been irrigated with moderately saline water (EC >2dS/m). Infiltration rates were generally poor. Two thirds of the sites were below 2 mm/hour and more than one quarter were less than 0.4 mm/hr. The rates midway between, and directly under drippers were similar in 60% of cases. Otherwise they were much larger midway between the drippers.
Penetration resistance measured in the laboratory at field capacity (–10 kPa) was generally high with an average value of 1.1 MPa. At the 4 salt-affected sites, penetration resistance was 26% greater between drippers. At the other 14 sites, penetration resistance was 29% greater under drippers. Field penetration resistance 24-48 hours after irrigation was assessed at 35 points in 2 vineyards. In Vineyard 1, only 7% of measurements were below 1 MPa while 38% were above 2 MPa. In Vineyard 2, 80% of measurements were above 2 MPa and 40% were above 4 MPa. Air-filled porosity at field capacity (–10 kPa) was also poor. Two thirds of the samples studied were below 5% and almost all were below 10%. Air-filled porosity was generally greater (by about 50%) midway between drippers.
Precision irrigation, particularly drip irrigation, is used extensively in Australian vineyards to conserve water but there are soil related issues that may influence its long-term sustainability. Although real application rates (ML/ha) may be relatively low, the concentrated nature of precision irrigation causes subsoil beneath emitters to be wet for extended periods. When seen in relation to local rainfall, these artificial increases in the concentration of water have the potential to degrade soil structure. The quality of irrigation water is a further issue.
Decreasing availability of water for irrigation inevitably leads to compromises in water quality. Irrigation with saline water increases soil sodicity which can cause serious soil structural decline. This project aims to assess the extent of these problems in the Barossa Valley, to establish the roles of soil type and irrigation water in causing them, to quantify the vulnerability of soils to them and to develop and test management strategies to minimize and reverse them.
ProgramNational Program for Sustainable Irrigation
- Irrigation (442)