The limited impact of vegetation on the water balance of mine waste cover systems in semi‐arid Australia

In (semi‐)arid environments, mine waste cover systems aim to minimize drainage into underlying hazardous wastes by maximizing evaporation from soil and transpiration from vegetation. We estimated the evapotranspiration (ET) for an area occupied by characteristic semi‐arid native Australian plant species. Using an open top chamber, we measured diurnal and daily ET of two plant species – Senna artemisioides (silver cassia) and Sclerolaena birchii (galvanized burr) – after a simulated rainfall event, as well as evaporation (E) from bare soil. Both ET and E decreased with increasing time after initial watering. However, we observed different temporal patterns for daily ET and E, indicating that S . artemisioides and S . birchii are relatively intensive and extensive water exploiters, respectively. We found a strong positive linear relationship between ET or E and the atmospheric water demand represented by the vapour pressure deficit. This correlation 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. We used the estimated values of ET and E to project the effect of species composition on plot ET in relation to total vegetation coverage. Although both species survive and grow under the dry conditions at the study site, their influence on plot ET was rather small because of the low vegetation coverage. On this basis, we conclude that the relevance of plants on ET cover systems is small under such water‐limited conditions of semi‐arid and arid climates.