Abstract
Abstract. 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.
Highlights
IntroductionTree plantations are known to have the potential to reduce groundwater recharge and surface water flows (e.g. Bell et al, 1990; Benyon, 2002; Bosch and Hewlett, 1982; Jobbagy and Jackson, 2004; Scanlon et al, 2007; van Dijk et al, 2007), in low-rainfall, high-evapotranspiration regions where the high transpiration demands of the trees make them a significant user in the water balance (e.g. Benyon et al, 2006; Fekeima et al, 2010; Jackson et al, 2005; Schofield, 1992)
We present the findings from a paired catchment study in southwestern Victoria, Australia, where one catchment is planted with a tree plantation, and the adjacent catchment is covered with pasture
Tree plantations in Victoria cannot be planted within 20 m of drainage lines, to avoid erosion of creek banks when the trees are removed (Dept. of Environment and Primary Industries, Victoria); we suggest that this currently restricted area along the drainage lines be expanded to include as much of the low topography parts of the site as practicable
Summary
Tree plantations are known to have the potential to reduce groundwater recharge and surface water flows (e.g. Bell et al, 1990; Benyon, 2002; Bosch and Hewlett, 1982; Jobbagy and Jackson, 2004; Scanlon et al, 2007; van Dijk et al, 2007), in low-rainfall, high-evapotranspiration regions where the high transpiration demands of the trees make them a significant user in the water balance (e.g. Benyon et al, 2006; Fekeima et al, 2010; Jackson et al, 2005; Schofield, 1992). The study area consists of a pair of small, adjacent catchments at Mirranatwa in southwestern Victoria – one (referred to as the eucalypt catchment) is covered predominantly in a recently planted (July 2008) E. globulus (Blue Gum) plantation (0.8 km2), the other (referred to as the pasture catchment) is mostly pasture for grazing sheep (0.4 km; Fig. 1). Both catchments are underlain by the same weathered/fractured aquifer, the Devonian Dwyer Granite (390– 395 Ma; Hergt et al, 2007; Van den Berg, 2009). There is 50 m of relief in the eucalypt catchment, and 30 m in the pasture catchment; both catchments comprise reasonably steep hills separated by a marked break in slope from the more or less flat topography along the drainage lines (Fig. 1)
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