How to talk about the complex interaction of vegetation, soils and watertable? Agriculture Victoria researchers have coopted the shorthand analogy of 'filling bathtubs' and 'leaking buckets' to dissect the soil water hydrology.
Water 'leaking' below shallow-rooted crops and pastures and the resultant rising saline groundwater are leading to widespread dryland salinity and other problems.
Using the buckets concept, Mark O'Connell and Des Whitfield are looking at the water take-up of crops and the option of intervening with a lucerne phase under different soil, rainfall and vegetation regimes in the southern grains region.
"We're finding annual crops and pastures rarely 'leak' in the lower-rainfall areas of the region. Lucerne leaks in the high-rainfall zone, but is otherwise effective over about one-third of the area," said Dr Whitfield.
The researchers defined an area-specific bucket as reflecting the inflow of winter rainfall in the majority of years. This sets a benchmark for winter water consumption/storage and is compared with the capacity of a vegetation-specific bucket which includes annual crops and pastures or lucerne.
"The vegetation bucket must exceed or match the area bucket to achieve hydrological stability," said Dr Whitfield. In other words, vegetation must draw out equivalent or more water than is flowing in, to keep the ground water level in balance.
The area-specific buckets for the southern grain regions were mapped on grids of 50 x 50 km.
Crop and lucerne buckets (135 mm and 210 mm capacity respectively) were compared with the area-specific bucket requirement.
Program 3.4.2 Contact: Mr Mark O'Connell or Dr Des Whitfield 03 5833 5222
Areas in the southern grains region where the crop/pasture bucket (red) and lucerne bucket (yellow) match the area-specific bucket shown above.
Seen here is the rapid increase in excess winter rainfall under annual crops and pastures when the area-specific bucket exceeds the crop/pasture soil water bucket (135 mm) in the higher rainfall zone of the southern grains region.