Making the most of moisture in the Eden Valley
Abstract
This project showed that Eden Valley vineyards were capable of thoroughly drying the soil profile to 1.5 metres during summer. This demonstrates the topsoil and subsoil combined could provide about 1ML/ha of moisture storage.
Summary
The Eden Valley Wine Grape Growers Group (EVWGGG) is the main representation organisation for vignerons in the district. They face common irrigation management problems in the district: constrained water supplies; imposition of regulation of those supplies and the threat into the future of less rainfall and harsher summers from climate change.
The “Making The Most of Moisture in Eden Valley” project was started in order to characterise and compare different irrigation management currently practised in the district and to introduce if necessary, modified proven best practice systems. All vineyards were characterised for readily available water (RAW) storage in the topsoil and individual vineyard irrigation management recommendations were calculated. These were derived from the median vineyard topsoil RAW; the moisture holding capacity of the wetted soil envelope around each dripper and the delivery rate for the dripper.
A scheduling system for local conditions was developed based on the premise that the best use of limited irrigation water was to confine as far as possible, application to within the topsoil where the feeder roots and most nutrients occur and to minimise seepage into the underlying subsoil. The subsoil can be considered as a reservoir that is recharged annually in winter by rainfall. It supplies the moisture for the vines to keep healthy, if somewhat stressed, during summer.
Conventional irrigation management systems usually ignore most of the subsoil as there are few easily observable roots and often the lower subsoil is an inhospitable environment for roots. In Eden Valley there is compelling evidence from project findings that the subsoil is not necessarily hostile to vine roots. In this district where irrigation is supplementary to rainfall, this part of the soil profile plays an important role in the annual water budget of vineyards.
The project initially was planned for a year but funding was found from various sources to continue the project to a fourth and final year. Analyses of all four years’ irrigation showed no formally significant trend of yield increase with total irrigation applied.
Overall, the project has shown that in water short Eden Valley, it is possible to raise commercially acceptable crops of wine grapes with what would be considered sub-optimal irrigation amounts (often about 20% of normally recommended). Furthermore some vineyards that suffered drastic reductions in water supply from empty dams or from pump failure were resilient enough to continue to yield satisfactorily for at least that season.