UNCERTAINTY IN MODELLING GROUNDWATER FLOW PATHS
BASED ON HEAD DATA ALONE: THE WAIHORA HILLSLOPE,
TAUPO
Woodward, S.J.R.1, Wöhling, Th.1,2, Stenger, R.1
1
Lincoln Agritech Ltd, Private Bag 3062, Hamilton, NZ
2
WESS, University of Tübingen, Germany
Introduction
The Waihora research site is a small (0.6 ha) pastoral hillslope that drains into a wetland
in the headwaters of the Tutaeuaua Stream on the northern shores of Lake Taupo. Highresolution piezometric head measurements have been collected at the site since 2004,
supplemented by slug test estimates of saturated hydraulic conductivity at several of the
wells, with the intention of describing the spatial and temporal patterns of flow paths that
transfer nitrate leached from the land surface, through the vadose zone-groundwater
continuum, to the wetland and stream.
Methods
A MODFLOW-NWT (Hunt et al., 2012) model of transient, 3D shallow groundwater flow
was developed for the site, with three model layers representing the Taupo ignimbrite
(TI), Paleosol (P) and Oruanui ignimbrite (OI) material layers, and a 2 x 2 m horizontal
grid resolution. Saturated hydraulic conductivity (Ksat) patterns in each of the layers
were calibrated using SVD-Assist and regularised Pilot Points in PEST, and the
uncertainty of the calibrated Ksat patterns subsequently assessed using Null Space
Monte Carlo analysis (Doherty, et al., 2010; Yoon et al., 2013). Finally, flow paths were
calculated for each of the Monte Carlo generated Ksat realisations that fitted the data
(Beven and Freer, 2001), using MODPATH version 5.
Results
Null Space Monte Carlo uncertainty analysis indicated that the available head and slug
test data were not sufficiently informative to definitively determine the spatial pattern of
hydraulic conductivity at the site, although modelled water table dynamics matched the
measured heads with high accuracy in space and time (NSE = 0.95). Particle tracking
analysis showed that the flow direction in the saturated zone was similar throughout the
year as the water table rose and fell, but was not aligned with either the ground surface
or subsurface material contours. Since the subsurface materials have overlapping
ranges of saturated hydraulic conductivity (as assessed by lab measurements, slug
tests, and model calibration), the material layers per se appear to have little effect on
saturated water flow at the site. Flow path uncertainty analysis (Figure 1) showed that
while accurate flow path direction or velocity could not be determined on the basis of the
available head and slug test data alone, the origin of well water samples relative to the
material layers and site contour could still be broadly deduced. Furthermore, comparison
with tritium-based mean residence time (MRT) estimates at several of the wells indicated
that predicted flow velocities (indicated by the length of the flow paths) were generally
reasonable.
Conclusions
This study highlighted both the challenge in collection of suitably informative field data,
and the power of modern calibration and uncertainty modelling techniques, to accurately
define flow paths in hillslopes and other small scale systems. Despite the high
uncertainty in predicted Ksat, the approach nevertheless allowed reasonable predictions
to be made of potential flow paths.
WR20-1 (oxidised)
WR26-1 (oxidised)
WR24-1 (oxidised)
WR26-2 (reduced)
WR24-2 (reduced)
MRT ≈ 0-3 y
WR20-5 (reduced)
MRT ≈ 18-25 y
MRT ≈ 14 y
Figure 1: Uncertainty in flow paths starting on 6 July 2007 and terminating at six wells on 27 November
2008, calculated using behavioural hydraulic conductivity results. Contours show the modelled elevation of
the Paleosols. Time postings are in months. MRT is tritium-estimated mean residence time in years. Trace
colours correspond to material layers (blue = TI, red = P, green= OI).
Acknowledgement
This research was conducted under the ‘Groundwater Assimilative Capacity’ Programme
funded by MBIE. Thanks to Aaron Wall, Juliet Clague, Brian Moorhead and Moritz Gold
for their excellent technical support.
References
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