This work investigated the potential for developing generic relationships from
measurements of recharge made in previous studies that would allow the
assessment of the impact of land-use change on recharge. Forty-one studies
that measured recharge from across Australia were reviewed to generate a
database. Studies were characterised on the basis of broad soil type (sand or
non-sand), land-use/vegetation (annual, perennial, or trees), and annual
rainfall. Attempts to develop quantitative recharge relationships met with
limited success because of the limited geographical coverage of the studies,
lack of details on the study sites, and high variability in the data.
Nevertheless, the following relationships for annual vegetation were
statistically valid:
Ln(recharge) = –19.03 + 3.63 ln(rainfall) [for sandy
soils]; F(1, 96) = 149.03;
R2 = 0.60
Ln(recharge) = –12.65 + 2.41 ln(rainfall) [for
non-sandy soils]; F(1,151) = 46.87;
R2 = 0.23
The low degree of explanation of rainfall for the annual non-sand data
suggests that it is likely that soil structure becomes more important for
higher clay content soils. Recharge under trees was negligible compared with
that under annuals. These relationships should not be used in areas such as
those where: preferential pathway flow is the dominant recharge mechanism,
rainfall is summer dominant, lateral hydraulic gradients are high, water
holding capacities are very low, or there are fresh, high-yielding aquifers.
Collectively, the results show that: (1) rainfall explains a significant
proportion of the observed recharge variation; (2) there is a significant
difference between mean recharge under trees and annual vegetation; (3) there
is a significant difference between mean recharge under annual vegetation on
sand soils and non-sand soils; (4) the land-use groups had a greater influence
on recharge than the broad soil groups used in this study; (5) there is a lack
of annual recharge measurements under perennial pastures/crops, under
trees in high rainfall zones (i.e. >600 mm/year) and in areas of summer
dominant rainfall; (6) across a broad range of locations, recharge is higher
under shallow-rooted annual vegetation than deep-rooted vegetation; and (7)
the estimator of Zhang et al. (1999) for 'excess water' may
provide a useful indication of the upper limit to the long-term average
recharge measurements. Large variation in the data resulted from disparity in
the recharge techniques used, the coarse soil categories used, failure to
account for land management factors, and complications due to macropores and
shallow water tables. It is proposed that the value of the information
presented here may be enhanced in future studies by incorporating information
from qualitative studies, particularly paired-site studies, and by drawing
information from unsaturated zone and groundwater modelling studies,
particularly comparisons of different land-uses at similar locations.
Furthermore, the results of this study can be used to identify gaps in
knowledge and, hence, target areas for future research such as annual recharge
measurements beneath perennial vegetation.
dryland salinity, groundwater management, deep drainage.
