NATION-WIDE GRIDDED BASEFLOW FROM RECHARGE AND GROUNDWATER
MODELS
Rogier Westerhoff1,2,3, Paul White2
1
Deltares, 2 GNS Science, 3 Waikato University
Introduction
Research in groundwater-surface water interaction (GWSWI) has gained considerable interest in New
Zealand in recent years. One important topic of GWSWI is the assessment of baseflow, which is usually
defined as the sum of deep subsurface flow and delayed shallow subsurface flow, or alternatively as
groundwater discharging to the surface. Baseflow estimates can be derived through analyses of river
hydrographs. At catchment scale, long-term baseflow can be estimated using rainfall recharge data, or
vice versa. Models of rainfall recharge are often challenged, as they combine uncertain estimates of
rainfall, evapotranspiration, terrain slope, and unsaturated zone parameters. Rainfall recharge is
therefore easiest to estimate on a local scale, as it can be easily calibrated against observations. This
process fits in the New Zealand policy framework, where regional councils manage water allocation.
However, a consistent overview of rainfall recharge is difficult to obtain at catchment and national
scales. This is partly due to data uncertainties, but also because data formats, density of ground
observations and local models often differ per region.
Aims
The aim of this research is to estimate baseflow in New Zealand using a nation-wide rainfall recharge
dataset and a simplified groundwater model.
Method
First, a nation-wide model of rainfall recharge was developed, which used estimates of hydraulic
conductivity, topography, rainfall, satellite evapotranspiration, and soil data. Uncertainty of all input
parameters was propagated through the model equation. Then, a water table was estimated using a
loosely coupled system of rainfall recharge (incl. uncertainty) and the Equilibrium Water Table (EWT,
Fan et al. 2013) method. This method calculates a seasonally varying long-term average ‘natural’ water
table. The term ‘natural’ implies that not all human impacts, like water abstraction and drainage, are
embedded. The EWT method has undergone improvements for application in New Zealand after
recommendations of Westerhoff and White (2014). All EWT results of groundwater discharging to
surface were then compiled to estimates of long-term baseflow. This coupling process further corrected
the recharge data.
Results
The resulting nation-wide datasets are: (1) an estimate of the contribution of each model cell to baseflow
(seasonal, resolution 200m x 200m); (2) a 1km x 1km monthly rainfall recharge estimate, including
uncertainty; and (3) a seasonal 200m x 200m natural water table estimate. The rainfall recharge data
validates well with known lysimeters and local model data in the Canterbury and Waikato regions. Model
results in ungauged areas show that it is likely that recharge is higher than expected in some
mountainous regions. This higher rainfall recharge results in high shallow baseflow (or interflow) in
these areas. Nation-wide water tables point out zones where shallow water tables exist. Figure 1 shows
the water table and baseflow estimates for the Hauraki Plains. Recommended further work is the
application of the baseflow estimates in identification of nutrient hotspots in areas with increased
nutrient loads from e.g. dairy industry. Further recommended work is catchment-by-catchment
validation and regional calibration of the model results.
All work has been undertaken as part of the Smart Aquifer Characterisation, funded by MBIE. It has
received co-funding from the European collaborative research programme eartH2Observe.
Figure 1 – Model outpus for the Hauraki Plains. Left: Equilibrium Water Table in metres below ground
level (m BGL). Right: contribution to baseflow per model pixel.
References
Fan, Y., H. Li, & G. Miguez-Macho (2013). Global Patterns of Groundwater Table Depth. Science, 339, 940–943.
Westerhoff, R.S. & White, P.A. (2014) Application of equilibrium water table estimates using satellite measurements
to the Canterbury Region, New Zealand. GNS Science report 2013/43.