© 2000 Heron Publishing—Victoria, Canada
Observations of evapotranspiration in a break of slope plantation susceptible to periodic drought stress
D. L. McJannet (1), R. A. Vertessy (2, 4) and C. A. Clifton (3, 4)
1. Department of Civil Engineering, Monash University, Wellington Road, Clayton, Victoria, 3168, Australia (David.Mcjannet@eng.monash.edu.au) / 2. CSIRO Land and Water, GPO Box 1666, Canberra, ACT, 2601, Australia / 3. Centre for Land Protection Research, Bendigo, Victoria, 3550, Australia / 4. Cooperative Research Centre for Catchment Hydrology / Received December 23, 1998
Summary
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.
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. Microlysimeters and ventilated chambers were used to determine pasture
and plantation floor evaporation, and heat pulse sensors were used to determine transpiration of the plantation. In spring,
pasture evapotranspiration was 65% of that of the plantation, whereas, in summer, pasture evapotranspiration was equivalent
to only 35% of that of the plantation. Rainfall interception by the canopy of the plantation was twice 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 20% of that measured during spring, suggesting that
the plantation was not utilizing groundwater supplies but was instead drawing from soil water supplies. This hypothesis was
supported by the comparison of relative abundances of the isotopes of water (2H and 18O) in soil and wood samples. We conclude that the BOS plantation is not behaving in the manner predicted, and our findings
raise doubts about the predicted advantages of establishing plantations in break of slope positions.
Keywords:
Eucalyptus globulus, isotopic analysis, leaf water potential, stomatal conductance.
