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 watertable con trol in areas where groundwater flows through colluvial deposits overlying low permeability bedrock. It is also believed that BOS plantations can supp lement their water use requirements by exploiting shallow groundwater at th e breaks in topographic slope. We compared measurements of BOS plantation and pasture evapotranspiration d uring spring, when the weather was warm and soils moist, and late summer wh en drought conditions prevailed. Microlysimeters and ventilated chambers we re used to determine pasture and plantation floor evaporation, and heat pul se sensors were used to determine transpiration of the plantation. In sprin g, pasture evapotranspiration was 65% of that of the plantation, whereas, i n summer, pasture evapotranspiration was equivalent to only 35% of that of the plantation. Rainfall interception by the canopy of the plantation was t wice that of the pasture, reinforcing the notion that trees can help reduce groundwater recharge and alleviate dryland salinity and waterlogging. During the summer drought period, daily plantation transpiration was only 2 0% of that measured during spring, suggesting that the plantation was not u tilizing groundwater supplies but was instead drawing from soil water suppl ies. This hypothesis was supported by the comparison of relative abundances of the isotopes of water (H-2 and O-18) in soil and wood samples. We concl ude that the BOS plantation is not behaving in the manner predicted, and ou r findings raise doubts about the predicted advantages of establishing plan tations in break of slope positions.