Towards a framework for predicting impacts of land-use on recharge: 1. A review of recharge studies in Australia

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.