A groundwater recharge perspective on locating tree plantations within low-rainfall catchments to limit water resource losses

摘要

Despite the many studies that consider the impacts of plantation forestry ongroundwater recharge, and others that explore the spatial heterogeneity ofrecharge in low-rainfall regions, there is little marriage of the twosubjects in forestry management guidelines and legislation. Here we carryout an in-depth analysis of the impact of reforestation on groundwaterrecharge in a low-rainfall (< 700 mm annually),high-evapotranspiration paired catchment characterized by ephemeral streams.Water table fluctuation (WTF) estimates of modern recharge indicate thatlittle groundwater recharge occurs along the topographic highs of thecatchments (average 18 mm yr?1); instead the steeper slopes in these areasdirect 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 withthat for streamflow, because most recharge occurs from infiltration ofrunoff through the streambed and adjacent low gradient slopes. Thecalculated CMB recharge values (average 10 mm yr?1) are lower than the WTFrecharge values (average 47 mm yr?1), because they are representative ofgroundwater that was mostly recharged prior to European land clearance(> BP 200 years). The tree plantation has caused a progressivedrawdown in groundwater levels due to tree water use; the decline is less inthe upland areas.The results of this study show that spatial variations in recharge areimportant considerations for locating tree plantations. To conserve waterresources for downstream users in low-rainfall, high-evapotranspirationregions, tree planting should be avoided in the dominant zone of recharge,i.e. the topographically low areas and along the drainage lines, and shouldbe concentrated on the upper slopes, although this may negatively impact theeconomic viability of the plantation.