Hydrology report examines pine-to-pasture flood risk

Future pine to pasture conversions in the Upper Waikato River catchment between Taupo and Karapiro could generally increase the risk of localised flooding of streams feeding the upper river.

They could also contribute to higher water levels in Hamilton and other sites further downstream, particularly during the type of extreme flood that happens once every 500 years.

These findings, contained in an Environment Waikato draft report, are based on a projected total level of conversions of 542 square kilometres, much higher than has actually occurred so far (around 200 sq km). The draft report and supporting technical reports are now being released to stakeholders and are available on EW’s website.

EW’s 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, with the expectation that conversion of forest into farmland will continue in the Upper Waikato catchment.

“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, as a result of conversions.

“We will now be discussing the draft report’s findings with stakeholders before finalising the report. We want to hear their feedback on the report and its findings, and on their thinking on the issues raised.”

The report was commissioned in 2007 amid widespread pine to pasture conversions and predictions of much more to come. Advice to EW from forestry sources then was that some 542 sq km was earmarked for conversion over 15 years. Such conversions increase the amount of rain water that runs off the land into streams and rivers, although the actual impact varies depending on location.

Mr Fowlds said the study indicated that 542 sq km of Upper Waikato conversions would significantly increase the risk of localised flooding in the catchments of streams feeding into the Upper Waikato during events as frequent as once-in-five years and once-in-20 years scale rain storms. But the exact magnitude of change would depend on a range of factors.

However, the impact further downstream is generally much less. There would be 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 280-530 millimetres higher at Hamilton during an extreme event.

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

“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.