Browsing by Author "Dean, JF"

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    Biomass uptake and fire as controls on groundwater solute evolution on a southeast Australian granite: Aboriginal land management hypothesis
    (Copernicus Gesellschaft MBH, 2014-01-01) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
    The chemical composition of groundwater and surface water is often considered to be dominated by water-rock interactions, particularly weathering; however, it has been increasingly realised that plant uptake can deplete groundwater and surface water of nutrient elements. Here we show, using geochemical mass balance techniques, that water-rock interactions do not control the hydrochemistry at our study site within a granite terrain in southwest Victoria, Australia. Instead the chemical species provided by rainfall are depleted by plant biomass uptake and exported, predominantly through fire. Regular landscape burning by Aboriginal land users is hypothesized to have caused the depletion of chemical species in groundwater for at least the past 20 000 yr by accelerating the export of elements that would otherwise have been stored within the local biomass. These findings are likely to be applicable to silicate terrains throughout southeast Australia, as well as similar lithological and climatic regions elsewhere in the globe, and contrast with studies of groundwater and surface water chemistry in higher rainfall areas of the Northern Hemisphere, where water-rock interactions are the dominant hydrochemical control. © 2014, Copernicus Gesellschaft MBH.
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    A groundwater recharge perspective on locating tree plantations within low-rainfall catchments to limit water resource losses
    (Copernicus Publications, 2015-02-26) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
    Despite the many studies that consider the impacts of plantation forestry on groundwater recharge, and others that explore the spatial heterogeneity of recharge in low-rainfall regions, there is little marriage of the two subjects in forestry management guidelines and legislation. Here we carry out an in-depth analysis of the impact of reforestation on groundwater recharge in a low-rainfall (< 700 mm annually), high-evapotranspiration paired catchment characterized by ephemeral streams. Water table fluctuation (WTF) estimates of modern recharge indicate that little groundwater recharge occurs along the topographic highs of the catchments (average 18 mm yr−1); instead the steeper slopes in these areas direct runoff downslope to the lowland areas, where most recharge occurs (average 78 mm yr−1). Recharge estimates using the chloride mass balance (CMB) method were corrected by replacing the rainfall input Cl− value with that for streamflow, because most recharge occurs from infiltration of runoff through the streambed and adjacent low gradient slopes. The calculated CMB recharge values (average 10 mm yr−1) are lower than the WTF recharge values (average 47 mm yr−1), because they are representative of groundwater that was mostly recharged prior to European land clearance (> BP 200 years). The tree plantation has caused a progressive drawdown in groundwater levels due to tree water use; the decline is less in the upland areas. The results of this study show that spatial variations in recharge are important considerations for locating tree plantations. To conserve water resources for downstream users in low-rainfall, high-evapotranspiration regions, tree planting should be avoided in the dominant zone of recharge, i.e. the topographically low areas and along the drainage lines, and should be concentrated on the upper slopes, although this may negatively impact the economic viability of the plantation. © 2015 Author(s)
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    The importance of topographic controls on groundwater recharge for plantation forestry in southeast Australia
    (International Association of Hydrogeologists, 2013-09-16) Dean, JF; Webb, JA; Jacobsen, GE; Chisari, R; Dresel, PE
    The trade off between water use and carbon sequestration by F plantation forestry is the subject of ongoing debate in Australia. Many tree plantations are planted without consideration for their impact on groundwater recharge. The spatial variation of recharge across a landscape can provide useful information on where to best situate a plantation while minimising its impact on groundwater recharge. Recharge is generally assumed to take place at topographic high points across a landscape, especially when rocky outcrops occur. This study analysed recharge rates at several points across a small paired catchment site to determine the most prevalent areas of recharge. Water-table fluctuations, Cl- mass balances, tritium and 14C ages were used to estimate recharge at 23 points across a 0.8 km2 catchment covered predominantly in a blue gum tree plantation, and an adjacent 0.5 km2 pasture catchment, in southwest Victoria, Australia. The CI- mass, balance method and water-table fluctuation methods were modified to better reflect the recharge processes taking place at the study site, including longer term hydrograph fluctuations and input from streams. Hydrographs from data loggers (since 2009) in bores situated close to the drainage line show considerably more recharge taking place compared to bores situated further upslope. Cl-mass balances show a similar trend, as do the hydrographs for the past 20-30 years, indicating that the majority of recharge is occurring along the drainage lines and towards the bottom of the slopes. Hydrograph fluctuations from these areas of the study site also often show significant response to runoff, measured at weirs on both catchments. Fracture heterogeneity contributes to some of the recharge variations, but the general trend is for recharge to occur more consistently in the topographically lower areas. ln order to minimise the interception of groundwater recharge by tree plantations in regions similar to the study area, the bulk of the plantation should be situated on the upper slopes, away from drainage lines and the immediately adjacent slopes. This would have less of an effect on the groundwater levels, as the plantation is not intercepting recharge in the zones where the majority of infiltration is occurring.