Managing water – the science, reality and luck

As part of third five yearly review of the Regional Forest Agreement, the Tasmanian government is also reviewing its policy for maintaining a Permanent Native Forest Estate (2014).  In particular
“The maintenance of a permanent native forest estate to ensure that we maintain the resource base for all its various production, conservation and amenity values.”

According to the policy ” . . . The protection of water quality values including meeting salinity objectives is addressed through provisions in the Forest Practices Code. The Forest Practices Authority has advised that inclusion of salinity objectives consistent with the State Salinity Strategy will be considered in the next planned review of the Code. The Forest Practices Authority has also agreed that prior to the next Code review applications for clearance and conversion will be assessed having regard to available salinity risk mapping.”

I wasn’t aware that salinity is an issue in Tassie but it seems up to 5% of agricultural lands have lost productivity due to rising water tables and associated salinity. As indicated in the policy this dilemma is put down to permanent forest clearing, as if logging and/or die-back doesn’t lead to similar outcomes. However one of the issues that reduces confidence in any natural resource data is the methods employed to gather it. For example and although the data has never been made public, now abandoned water sampling in the Bega river catchment alluded to increasing water turbidity. Regrettably confidence in the data, produced by Forestry and the DPI, was low due to inappropriate sampling methods and other inconsistencies.

Interested in the water sampling methods Tasmanian authorities employ and data they collect, I randomly selected the Welcome River catchment, in the far north west of the state that has the following brief description.

“The entire 546 km2 catchment is relatively low-lying, and originally the Welcome River flowed through extensive, low-lying swamp forests. These have largely been cleared and drained to facilitate the expansion of beef cattle and dairy farming, which along with forestry is the main land-use activity in the area. Some swamp forest remains within the Dismal Swamp Nature Reserve and Welcome Swamp.”

The data below is the Electrical conductivity of water samples, an indicator of salinity and turbidity, for the catchment over a five year period.

Welcome River at Woolnorth – Electrical conductivity (μS/cm)
Year           Minimum         Median           Maximum
2004             179.3               944                 2120
2007               701              1363                 3430
2009               535              1272                 3700

Based on these figures one could easily get the impression that surface water quality has decreased, although a problem with the monthly sampling is that it may not coincide with minimum or maximum flows. In addition, if one were considering ESFM for example,  other limitations include a lack of pluviographs in the catchment, along with information on land clearing, logging, burning etc., so finding correlations between land management, rainfall and river flows is problematic. The benefit, at least for forestry, is business as usual, until it all falls over.

swamp fence

Back at the Murrah,  the fence across the unnatural swamp has finally been completed. Previously the now swampy area had a post and beam fence around it, but over several decades sediment deposited at the end of the catchment has impeded drainage, leading to the swamp developing and spreading.  As indicated in the shot above, the final design incorporates a collapsing section of fence, aimed at reducing potential mangling associated with a big flood. At these times a greatly swollen river begins to enter the swamp at the point in the back ground. Consequently the flow of water from the creek is diverted to the eastern side, where most of the sediment has been deposited, so woody debris and every else that floats, gathers behind the fence along this section.

In theory the force of water coupled with this material will make the section fall down, so debris passes over it without doing too much damage. With perhaps a lot a rain forecast in the next week a test may be quite soon. So a bit of luck may be helpful.


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