Observations of evapotranspiration in a break of slope plantation susceptible to periodic drought stress

Break of slope (BOS) plantations are advocated as a means of water table control in areas where groundwater flows through colluvial deposits overlying low permeability bedrock. It is also believed that BOS plantations can supplement their water use requirements by exploiting shallow groundwater at the breaks in topographic slope. Although BOS plantations are attracting much interest from landholders, relevant research on their hydrologic function is limited. We compared measurements of BOS plantation and pasture evapotranspiration during spring, when the weather was warm and soils moist, and late summer when drought conditions prevailed In spring we found that pasture evapotranspiration was 65% of that of the plantation, whereas in summer, pasture evapotranspiration was equivalent to only 35% of plantation evapotranspiration. Rainfall interception by the canopy of the plantation was found to be twice that of the pasture. The greater evapotranspiration of the plantation reinforces the notion that trees can help to reduce groundwater recharge and alleviate dryland salinity and water logging. During the summer drought period, daily plantation transpiration was only 20% of that measured during spring. This suggested that the plantation was not utilising groundwater supplies but was instead 'mining' soil moisture supplies. Isotopic analysis of soil and tree water supported this hypothesis. The BOS plantation we studied is not behaving in the manner expected, and our findings raise some doubt as to mooted advantages of establishing plantations in 'break of slope' positions