The coastal township of Whangamata's reticulated water supply is provided by a
number of groundwater bores, extracting water from local fractured rhyolite and
andesite aquifers. A need has arisen to create a greater understanding of the aquifers,
because of an increased demand for water abstraction. Water demand in Whangamata
increases dramatically during the summer vacation period. Occupant numbers
increase from 4,000 up to 50,000 during peak times, resulting in increased water
demand. Over the past five years an increase in groundwater abstraction has produced
an evident downward trend in bore water levels. Electrical conductivity is also
increasing in several aquifers, posing a realistic threat of sea water intrusion and
questioning the sustainability of current abstraction volumes.
Multiple linear regression and an artificial neural network model were investigated as
simple empirical forecasting tools for well drawdowns to predict the effect of future
increases in groundwater demand. This approach was adopted as opposed to a
groundwater numerical model because of poor time resolution of available data and
the complex, fractured nature of the aquifer. By using pumping volumes as variables,
seasonal bore water level variations and long term trends were predicted. The models
were evaluated with independent validation data sets. The actual ability of a model to
predict bore water level seasonal variation and long term trends was assessed using a
comparison with a moving average of the validation data set.
Multiple linear regression proved superior to the neural network in almost every bore
modelled. Although neural networks proved capable of modelling seasonal bore
water level variations it was not to the same degree of accuracy as the regression
approach. The regression approach yielded a modified index of agreement of 0.6-0.74
when comparing a moving average of observed data with the validation data sets. The
developed models were used to forecast well water levels with varying abstraction
volumes aiming to prevent further long term decline in bore water levels.
