Catchment Management Units (CMUs)

Catchment Management Units are referred to as CMUs.

The FCP uses “Catchment Management Units” (CMUs) to divide the landscape up into units for visualising and managing the effects of clearfell harvesting in steepland forests. A CMU is a subcatchment upstream of where Zone 2 transitions to Zone 1. A CMU will therefore contain order 1 and 2 streams that have flowed together to make a 3rd order stream.

The aim of the FCP is to show the area and location of erosion-susceptible plantation forest within a CMU, since harvesting of this forest will influence the amount of erosion and therefore sediment that flows out of the CMU and into Zone 2 and Zone 3. To manage this erosion and sediment flowing out of the CMU, we need to manage the area of erosion-susceptible land within the CMU that is in the window of vulnerability at any point in time.

Structure

An important feature of catchments is that they usually have a structure like a tree. This structure can be understood in terms of stream orders (Figure 2), where the smallest and highest subcatchments in Zone 1 are classified stream order 1. Downstream, where two order 1 streams meet is the beginning of a stream order 2; where two order 2 streams meet, is the beginning of an order 3 stream and so on. By the time Zone 3 (floodplains) is reached, the main stem of the catchment “tree” may be classified as order 5, 6 or even 7.

CMUs are the basis for dividing up larger scale catchments as they deliver water, sediment, and woody debris to the main floodplain areas of larger catchments (Stream order 5 or greater).