Critical source areas

Critical source areas (CSAs) are small, low-lying parts of farms such as gullies and swales where runoff accumulates in high concentration.

Runoff from CSAs carries sediment and nutrients to waterways. Managing these areas well is a great way to reduce sediment and nutrient loss from your farm.

Actions for managing critical source areas

Identify where CSAs are on your farm.
Fence off CSAs to create a grass buffer zone to filter contaminants and prevent stock access. The faster the water is flowing across a buffer zone, the wider the buffer zone should be to provide time for effective filtering.
When constructing new tile drains, direct them into areas where runoff can be filtered, such as wetlands or grass buffers, before entering waterways.

Benefits of managing critical source areas

Loss of valuable topsoil is reduce.
Nutrient and sediment loss to waterways is reduced.
Keeping animals out of CSAs can improve hoof health and reduce the incidence of mastitis.

Filtering overland flow

Buffer zones or grass strips in and around CSAs and next to waterways act as filters by slowing overland flow to trap suspended contaminants.

The buffer zone should be left uncultivated and ungrazed to operate effectively.

The faster the water is flowing into a buffer zone, the wider the buffer zone will need to be to provide time for effective filtering. This is particularly important in a CSA or on sloping land.

Buffer zone fencing example in a critical source area
Good use of grass buffers in crop paddocks

To provide effective filtering, the buffer zone should be left uncultivated and ungrazed. The faster the water flows into the buffer zone, the wider it will need to be.
Unmanaged critical source areas without buffer zones can lead to loss of soil and nutrients

A wide buffer zone provides time for effective filtering, this is particularly important on sloping land or in a critical source area.