Conference paper presented at the MODSIM07, International Congress on Modelling and Simulation, held December 2007, University of Canterbury, New Zealand.Regional-scale prediction of the effects of nitrate
discharge from land use on the quality of the
underlying groundwater requires two major model
components: (1) a climate-driven model of
agricultural land use that predicts nitrate discharge
at the bottom of the plant root zone, for locations
over horizontal space; and (2) a groundwater
transport model, which predicts nitrate
concentration at horizontal and vertical locations
within the aquifer as well as the nitrate discharge
to surface-water bodies. AquiferSim is a recently developed
groundwater transport model, for which
the inputs of recharge and nitrate from the land
surface are received from a GIS user interface that
accesses the root zone nitrate discharge model.
The AquiferSim groundwater transport model is
designed to address two particular requirements.
The first is that model run times should allow for
real-time examination of land use scenarios and
assessment of uncertainty. The second is that
dimensions of computational cells should allow for
realistic transport dispersion in both horizontal and
vertical dimensions, as well as allowing improved
accuracy of flowpaths to surface-water bodies.
These latter requirements imply very large
numbers of computational cells for regional-scale
studies, with associated costs in model run time.
These issues are addressed in the AquiferSim model by: assuming steady-state groundwater flow
and transport; solving 2D horizontal groundwater
flow on ~10⁶ computational cells with a fast, fullmulti-
grid solver; and then solving flow and
transport on vertical sections of ~10⁴ cells along
selected groundwater flowpaths, with a successive over-
relaxation solver. The software was
developed entirely in Microsoft Visual C# on the
.NET framework. This enables the AquiferSim
engine to run on modern Windows PCs and on
Linux and clustered environments using the
MONO platform.
Computational time performance of the
AquiferSim engine enables the horizontal 2D
steady-flow groundwater problem and pathline
tracking to be solved in about 5 s for a region
occupying one third of the 1025 x 1025
computational grid. Solution for groundwater
flow, nitrate transport, and groundwater age on the
~ 10⁴ cells of one vertical slice requires up to 20 s.
Implementation of AquiferSim within a regional
council for environmental planning purposes is the
next phase of development. The major issues are
likely to be: quantifying the predictive uncertainty
caused by inadequate description of aquifer
recharge and properties; and software design for
interrogation of AquiferSim output to meet yet
unspecified requirements for end-user information
