PEAT LAKE WATER LEVEL SETTING AND GROUDWATER INTERACTION
Brown, E. 1 Jenkins, B.R,1
1
Waikato Regional Council
Aims
Lake Maratoto is an 18 ha peat lake largely surrounded by farmland on drained peat swamp 10 km
south of Hamilton City (Figure 1). On the western edge of the lake is a small area of remaining
wetland vegetation under QEII covenant. Landcare (2014) considers that the water table under the
remaining wetland vegetation needs to be raised closer to the ground surface for improved
vegetation health.
Our aim is to understand how much higher the lake’s water levels need to be to have a beneficial
impact on the water table level under the QEII protected wetland. This informatiFon will support
the design of a new outlet control structure. Any decisions on lake level management need to
ensure there is no flooding of the surrounding farm land.
Method
To understand the interaction between lake level and groundwater levels a MODFLOW
groundwater model was developed.
Inputs to the model were:
 Lake level from the existing monitoring started in 2002.
 Recharge calculated from rainfall and evaporation measured at nearby Hamilton airport
and Ruakura climate stations.
 Geological information for the area was sourced from a study in 1985 (Green, & Lowe, 1985).
The depth of lake sediments and peat was ascertained at over 150 locations using probes,
auger and coring. These were taken at ~30 m intervals along 12 transects.
Calibration of the model was to:
 Discharge from the lake estimated based on the weir equation for the control structure on
the Lake’s one outlet.
 Groundwater levels monitored over a two year period from 29 water-table dip wells. Five
of these were monitored continuously with water level loggers. The remaining dip wells
were monitored manually as time permitted. All the dip wells’ elevations were surveyed
using RTK GPS on multiple occasions during the monitoring period and linked to the
existing lake level recorder.
A trial and error process was used to calibrate the transient, weekly time step, MODFLOW model.
The calibration period was between November 2013 and November 2014. Validation of the model
was against independent data from November 2014 to July 2015. During the Validation period
changes were made to the outlet weir to raise the lake level and provide different environmental
stresses to test the model against.
The model calculates changes in groundwater level to a range of lake level scenarios. The lake
level scenarios were based on the existing record (since 2002) but modified to the expected levels
for different outflow control structure designs. The lake level scenarios were calculated using a
simple water balance model (Golder 2013).
Results
An acceptable calibration and validation of the groundwater model was achieved to groundwater
levels with a RMS error of 0.16 m. The model produced a good match with the observed
seasonality for the majority of monitored locations (Figure 2). The model is now being used to
understand if a modified lake level regime can provide benefit to the adjacent QE11 vegetation.
Figure 1: Location of Lake Maratoto.
Figure 2: Time series of observed and modeled water level for one dip well (left) and observed against
calculated water level for all 29 dip wells and time periods (right).
References
Golders, (2013): Lake Maratoto hydrological study; lake water balance and weir assessment. Prepared for
Waikato Regional Council. Golder Assoc. Contract Report 1278610961-002R, April 2013.
Green, J.D., & Lowe, D.J. (1985) Stratigraphy and development of c. 17 000 year old Lake Maratoto, North
Island, New Zealand, with some inferences about postglacial climatic change. New Zealand Journal of
Geology and Geophysics, 28, 675 – 699.
Landcare Research, (2014) Lake Maratoto Peatland: 20 years of vegetation change (1993-2013). Prepared
for Waikato Regional Council. Landcare Research Contract Report LC1809, June 2013 Hamilton. WRC
Document 3054977.