Managing water runoff

Catchment management problems arise when the soil water cycle becomes unbalanced due to artificial changes in the shape and function of a landscape through clearing, overgrazing, cropping or construction.

Historically, the year-round growth of native vegetation in southern Australia had greater ground cover by perennial species that used the moisture deep in the soil profile over summer and autumn, leading to less water and nutrient runoff and less leakage to the ground water. The replacement of native perennial vegetation and trees by winter and spring growing annuals that used less soil moisture in summer and autumn reversed this water balance, resulting in greater runoff and recharge.

Management of water runoff is critical for steep hill country. Processes such as saturation-excess runoff (runoff created when rain continues to fall on saturated soil) and infiltration-excess runoff (runoff created when heavy rainfall on a surface exceeds the rate at which water infiltrates the ground) are responsible for runoff generation in steep hill areas. Saturation-excess runoff produces larger overall quantities of runoff and occurs in late winter and early spring. Catchments with the most strongly convergent topography produce more saturation-excess runoff than other catchments.

Infiltration-excess runoff can occur at any time of the year and requires high rainfall intensities. Groundcover and plant density greatly affect water runoff. It has been found that higher peak flows were produced from set stocked areas than deferred grazing areas. These high peak flows had high erosive potential, with sufficient kinetic energy to move rocks weighing up to 0.8 kg.

Sustainable management practices for steep hill country avoid applying more fertiliser than is required to increase plant cover to the areas that provide most of the runoff. The application of phosphorus fertiliser can increase the phosphorus concentration of surface runoff. This does not always lead to lower water quality, particularly where runoff drains into a farm dam, because many farm dams are highly turbid due to dispersible subsoils. Inorganic phosphorus would bind to clay particles in the water. Furthermore, clay particles in the water limit the penetration of light, reducing the chances of toxic blue-green algal growth. However, where runoff is not turbid or is not intercepted by a farm dam, there is potential that fertilising of hill areas will reduce the quality of water in dams, wetlands, creeks and rivers, affecting aquatic animals such as frogs.