101 Questions answered
Why Mallee Seeps Form: Too Much Water, Shallow Clay, and Rising Salinity
Modern farming systems often involve weed control every year, which provides important moisture conservation for improved crop and pasture growth. Sandy soils have a low water holding capacity. When it rains, water that was previously used by deep rooted summer weeds quickly filters through these layers and pools in low lying areas and on top of clay subsoils.
Mallee seeps are a problem that starts with too much water. Too much water in a low rainfall area is intriguing, but here we are. Good farming in the Mallee is about turning every drop of rainfall into dollars, so it can be disappointing to have valuable moisture leading to long-term production losses.
Too much water might not be a problem to start. If rainfall isn’t too high, the extra water means crop growth is probably higher in these areas for the first few years (which also helps use the water). But if enough rain falls that the soil becomes too waterlogged for the crop to grow, when the soil dries out, it is bare. As the water evaporates from the soil surface, it draws water up from deeper in the soil, bringing subsoil salts with it. Repeat this cycle for a few seasons and the rootzone soil salinity gradually increases until it is too saline for crop growth.
The Mallee Seep animation explains how they form.
Generally, the impervious clay layer causes a perched water table and Mallee seep sits within two metres of the surface, but the severity of seep impacts depends on many factors, including the size of the catchment, where it sits in the landscape and the water quality. However, if the impervious clay layer is closer to where water is either collecting or moving across the landscape, it will generally increase the risk of the seep impacts.
Note that not all clay is impervious. There might be layers of sand, loam and clay above the perched water table, then a drier clay layer beneath.
