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

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.

Importance of deep water uptake in tropical eucalypt forest

“…Other studies have shown similar dynamics of water‐table depth in Eucalyptus plantations as compared to our measurements and simulations (Manzione, Tanikawa & Soldera ; Dean et al . ; Smethurst, Almeida & Loos ). During the second stage of growth following canopy closure, we show that tree water uptake also comes from the capillary fringe, and therefore, the saturated zone acts as another buffering pool of water, being filled the first 2 years after clear‐cutting the previous stand and partly used after canopy closure.…”

Coastal Forests and Groundwater: Using Case Studies to Understand the Effects of Drivers and Stressors for Resource Management

“…We emphasize the need for field data to properly quantify the potential effects of long-term pumping on forest conditions. Furthermore, a change in the species composition alters the vulnerability to drought and the relative magnitude of water balance components through changes in ET, both in terms of interception and transpiration [32,33,64]. For example, transpiration rates for a given diameter yellow poplar (Liriodendron tulipifera; diffuse porous xylem) were nearly twofold greater than for hickory (Carya spp.…”

“…Some cases in point are studies of the water table response in three locations in the coastal plain region [26,39], and data have indicated that the specific yield of the surficial aquifer sediments was 0.1 or less, such that 5 mm of water loss by evapotranspiration (ET) can cause the water table depth to drop by at least 5 cm [40][41][42]. A comparable study in a drier climate, that used water table hydrograph data to understand groundwater recharge, found that the management of eucalypt stands in Australia can affect the water balance, but that the landscape position in that region was more important [32,33]. Many tectonically-passive coastal regions like the southern and southeastern U.S. face widespread impacts of water imbalances because of the generally flat topography.…”

“…Such guiding documents can initially be used to assess and identify project-level design and mitigation measures for a specific set of GDE types, as well as the major physical and biological characteristics (geological setting, vegetation, soil, hydrology, and terrestrial and aquatic . P refers to precipitation and ET is evapotranspiration [33]. While initial considerations of groundwater-supported ecosystems have been discussed [18,19], it may not be appropriate to develop final local or regional classifications and distinctions from a survey guide.…”

“…An ephemeral stream channel or prairie-pothole wetland in arid to semi-arid climates may flood, even if the principal water table lies far below it, and those systems may be sensitive to changes in weather, but less so to alterations in regional aquifer water elevations [32,33]. In shallow water table settings, however, seemingly small changes in the water budget of the groundwater system may cause large changes in the surface hydrology, such as surface runoff, wetland water content, and groundwater discharge as baseflow to streams and rivers [23,34].…”