Pasture flooding alarm

Deforestation can cause streams to rise

Future pine-to-pasture conversions in the upper Waikato River catchment between Taupo and Karapiro could increase the risk of localised flooding of streams and contribute to higher water levels in Hamilton during extreme flood events.

Those are among the findings in an Environment Waikato (EW) draft hydrology report on the impact of projected land conversions of 542km sq downstream from Lake Taupo.

Conversion of plantation forested areas to pasture within the Waikato region is currently unregulated by statutory planning instruments, but can occur as of right and is expected to continue.

EW catchment services group manager Scott Fowlds said the report raised issues that the regional council, local councils, iwi, farmers and other stakeholders would need to consider closely.

"This report raises a number of flags that we all need to pay attention to as we consider how best to deal with land use change and potential increases in flooding and flood levels in the upper Waikato catchment and further downstream," he said.

EW now wants feedback from stakeholders.

The report, commissioned in 2007, is available on EW's website with supporting technical data.

Such land conversions increase the amount of rainwater that runs off the land into streams and rivers, although the actual impact varies depending on location.

Mr Fowlds said the study indicated that 542km sq of upper Waikato conversions expected over 15 years would significantly increase the risk of localised flooding in the catchments of streams feeding into the upper Waikato during once-in-five years and once-in-20 years' scale rain storms.

The impact further downstream was generally much less, with little or no change to flooding in the Waikato River itself during rain storms up to a once-in-20 years event.

But during large (once-in-100 years) and extreme (once-in-500 years) floods there was potential for much more elevated river levels further downstream.

For example, the Waikato River could be 280-530mm higher at Hamilton during an extreme event.

These sorts of findings were not a cause for major alarm at present given the current scale of conversions, Mr Fowlds said.

"But, once we've considered these findings further with stakeholders, we will need to collectively look at what our policy and operational responses are."

A final version of the report is due to be considered by EW's catchment services committee in April. Fairfax

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