Heretaunga Plains looking east from the Ngaruroro River towards Flaxmere and Hastings. Image: HBRC
A new study has uncovered a trend of declining groundwater levels in some areas of the Heretaunga and Ruataniwha Basins in the decade between 1994 and 2014 – and says pumping for irrigation could be to blame.
Due to be tabled at Hawke’s Bay Regional Council’s regional planning committee meeting tomorrow, the study used two methods to show long-term changes in groundwater levels at various sites monitored by the council, using measurements taken in February and August over the 10-year-period along with historic drilling records and patterns in rainfall to analyse any trends.
According to a staff summary on the study due to be presented at tomorrow’s meeting, the study found that areas of both the Heretaunga and Ruataniwha Plains experienced groundwater level declines from 1994 to 2014 – though for many sites, no statistically significant trends were detected.
The study discovered that on the Heretaunga Plains, there had been a continuous trend of declining groundwater levels in areas located near the major recharge area of the aquifer between Roy’s Hill, Fernhill and Flaxmere.
On the Ruataniwha Plains, the areas of declining groundwater were mostly located along the western margins of the aquifer, near Tikokino and Ongaonga.
Groundwater level declines in the Ruataniwha Plains were generally greater than groundwater level declines on the Heretaunga Plains, the staff report said.
While the range of decline on the Heretaunga Plains varied between wells, the study estimated there had been a maximum decline of 2 metres in the level of groundwater over the 10 years at the recharge point - equating to a decline of approximately 2 to 10 cms per year.
Over the confined zone of the aquifer, no statistically significant trends in groundwater levels were detected.
“Nonetheless, groundwater levels are generally lower both in the unconfined zone, and to a lesser degree in the confined zone, from 1994 to 2014,” the staff report said.
“However, anecdotal information from drilling in 1867 suggests groundwater levels in the Meeanee area are similar to groundwater levels measured today.
“This suggests groundwater levels have probably not changed significantly in this area since the mid-19th century."
There were some inter-annual fluctuations in groundwater levels which correlated with abnormally dry or wet periods some years, the report said.
“However trends in rainfall are different to trends in groundwater levels, which indicates some other factor besides rainfall is causing the declines observed in the groundwater levels.”
It said the declines observed in groundwater levels probably “reflected increased pumping over time”.
“However information about actual pumping is poor, with most information available only for the last five years. This makes it difficult to directly examine the impact of pumping on groundwater levels. Another alternative, but less likely cause for groundwater level decline, is a reduction in aquifer recharge.”
The study said areas where groundwater levels were showing a trend of continuous decline were along the western margins of the Ruataniwha Plains, near Tikokino and Ongaonga range, at a rate of approximately 3 to 28 cms per year.
The maximum total decline in the groundwater level between 1994-2014 was estimated to be about 5.6m near Takapau. At other sites, the total decline in groundwater ranged between 0.6m to 4.8m.
Inter-annual fluctuations recorded were consistent across most wells monitored on the Ruataniwha Plains and again correlated with abnormally dry or wet periods, which “suggests that extreme variations in weather patterns affect both the Heretaunga and Ruataniwha aquifers in a similar way.”
But “in a similar way to the Heretaunga, trends in rainfall and in Ruataniwha groundwater levels are not related, suggesting something else is causing observed declines in groundwater levels,” said the rpoert, inferring pumping for irrigation was the likely cause.