The importance of plant water use on evapotranspiration covers in semi-arid Australia

Abstract. We estimated the evapotranspiration (ET) for an area vegetated with characteristic semi-arid native Australian plant species on ET mine waste cover systems. These systems aim to minimise drainage into underlying hazardous wastes by maximising evaporation (E) from the soil surface and transpiration from vegetation. An open top chamber was used to measure diurnal and daily ET of two plant species – Senna artemisioides (silver cassia) and Sclerolaena birchii (galvanised burr) – after a simulated rainfall event, as well as E from bare soil. Both ET and E decreased with increasing time after initial watering. Different temporal patterns were observed for daily ET from the two plant species and E from bare soil, revealing Senna artemisioides as intensive and Sclerolaena birchii as extensive water exploiters. A strong positive linear relationship was identified between ET (and E), and the atmospheric water demand represented by the vapour pressure deficit. The relationship always was more pronounced in the morning than in the afternoon, indicating a diminishing water supply from the soil associated with a declining unsaturated hydraulic conductivity of the soil in the afternoon. The slopes of the regression lines were steepest for Senna artemisioides, reflecting its intensive water-exploiting characteristics. We used the derived estimates of ET and E to predict the effect of species composition on plot ET in relation to total vegetation coverage. Although both species proved suitable for an operational ET cover system, vegetation coverage should exceed at least 50% in order to markedly influence plot ET, a value which is likely to be unsustainable in semi-arid climates.