While some good rain has fallen across much of the state, many dams are unlikely to fill unless they are in the path of a thunderstorm.
Agriculture Victoria Land Management Officer Clem Sturmfels said the hot, dry summer and lack of autumn rainfall has left soil moisture levels well below average for this time of the year.
“Without an intense rainfall event, good runoff is unlikely until our soils become saturated,” Strumfels said.
“Developing a water budget can help with making decisions about remaining water levels and how to manage shortages.
“The amount of rain that is required to achieve saturation varies enormously across the state, depending on the soil depth, structure, soil moisture and pasture cover.
“Deep and well-structured soils may need more than 200mm, while shallower soils may require more than 150mm to saturate.
“In normal years, actively growing pastures use much of the stored soil moisture; however, plant growth and water use this year will be low due to the very late break.
“Even in an average year, plenty of rain is required to fill farm dams.
“The rate of runoff varies from about five per cent to 10 per cent of annual rainfall, and the location, climate, soil type and ground cover all impact on this figure.
“Using the rates above, a catchment area of 5–10 hectares is required to fill a small one megalitre dam, for example.
“Contour banks can help to increase the catchment area of a dam by collecting water from the adjoining area.
“They only need to be 200 – 300mm high and can be installed quickly and efficiently using a modern articulated grader. However, they should not be built on soils prone to tunnel erosion. A fall of about one in 200 is usually recommended for such banks.” Mr Sturmfels said.
Permits or licences may be required before undertaking this type of work. Landholders are encouraged to contact their local council planning officer and water authority for guidance.
For more information on dam management, visit the Agriculture Victoria website and Extension Aus for soil moisture monitoring.
