Comparing Groundwater Recharge Rates Estimated Using Water Table
Fluctuations and Chloride Mass Balance Across the Australian Continent
Abstract
Estimation of groundwater recharge rates (GRRs) in space and time is
notoriously challenging but underpins water resource management. GRRs
derived from different techniques often differ as they estimate
different components (i.e., potential, net, gross) and/or estimate
different GRR mechanisms (i.e., diffuse or focused). We evaluate GRRs
from daily groundwater level data from ~400 bores across
Australia using the water table fluctuation (WTF) method. Specific yield
values are estimated using lithological information linked to literature
values. Comparisons were made between mean inter-annual GRRs from 224
bores and long-term GRRs derived from the chloride mass balance (CMB)
method. Mean inter-annual WTF-based GRRs were 365.5 mm/y for humid, 248
mm/y for dry subhumid, 128.6 mm/y for semi-arid and 50.3 mm/y for arid
zones. Inter-annual recharge variability is higher in arid and semi-arid
climate zones relative to wetter climates. WTF and CMB-based GRR
estimates exhibited low agreement in arid and semi-arid zones, where
most WTF-derived GRRs exceeded CMB values by over an order of magnitude.
While this can be explained by differing dominance of focused vs
diffusive recharge, we show influence from other factors including the
inability of the WTF method to quantify low GRRs, impacts of land use
change, and non-ideal conditions like river-aquifer connections. Major
differences between the WTF and CMB methods are attributed to CMB
reflecting pre-land clearing GRRs in many instances. This study serves
as a comparative framework for evaluating the appropriateness and
differences between the WTF and CMB methods which can be applied to
groundwater recharge studies globally.