Catchment Class

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Catchment Class with regard to iTRARC

Catchment types allow for simplified reporting and management guidelines. A subcatchment is classified into its catchment class (1A, 3C etc) using the earliest available satellite imagery and changes in the structure of the riparian zone are scored against that reference point. For example if a catchment had forested headwaters and wetlands in the 1970s and that sub-catchment has subsequently had its headwaters thinned to woodland and the wetlands drained, then the catchment will not be reclassified into a 3E catchment, rather the catchment will have dropped (at least) 6 points in condition on account of head waters being cleared of sc+2 (-2 points), cleared of sc (-2 points) and wetlands drained (-2 points).

The maximum score represents a proxy for the range of riparian ecosystems encountered in that subcatchment. A high maximum score indicates a wide range of terrestrial and aquatic environments in the riparian zone of that subcatchment, such as may be encountered in a catchment with rainforest headwaters, floodplains and associated wetlands. A low maximum score on the other hands indicates a smaller range of habitats in the riparian zone, for example a single incised channel without floodplains and wetlands. Consequently the scoring system will allow identification of high biodiversity subcatchments as well as erosion hotspots (based on the scores of the erosion indicators). Please note that the scoring system makes no assumptions about the presence of ‘positive’ scoring attributes. For example if closed forest (sc+2) is not present on the floodplain (and it may never have been in the current climatic setting) then the catchment scores a 0 (rather than a negative) for that parameter. In other words a catchment is not ‘penalised’ for the absence of any particular feature.

The only assumption that has been made is that there was continuous riparian vegetation (no gaps) in the natural state. This assumption affects the iTRARC score calculated for any given date, but is valid in terms of comparing two dates. For example if the 1972 Landsat MSS data shows a subcatchment with a wooded upper catchment (2D) that has a fragmented riparian zone along the low order streams (and everything else is pristine) then the catchment will score 16/18, and if the subcatchment hasn’t changed since then, then the catchment will still score 16/18 (no change in condition). If land use practices elsewhere within the catchments have subsequently cleared the parts of the upper catchment (-1) and lead to fragmentation of riparian zones on most of the main channel (-2) then the catchment will drop 3 condition points to score 13/18. This score was calculated for each one of the 52 subcatchments that make up the Dry Tropics Catchments.

ClassDescription Max iTRARC Score
1ARainforest upper catchment with multichannel floodplain and wetlands26
1BRainforest upper catchment with floodplain and wetlands22
1CRainforest upper catchment with wetlands16
1DRainforest upper catchment with floodplain20
1ERainforest upper catchment14
2AForested upper catchment with multichannel floodplain and wetlands24
2BForested upper catchment with floodplain and wetlands20
2CForested upper catchment with wetlands14
2DForested upper catchment with floodplain18
2EForested upper catchment12
3AWooded upper catchment with multichannel floodplain and wetlands22
3BWooded upper catchment with floodplain and wetlands18
3CWooded upper catchment with wetlands12
3DWooded upper catchment with floodplain16
3EWooded upper catchment10
4AFloodplain subcatchment20

Catchment Class

Catchment Class

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