Transpiration measurements on vegetated soil layers over waste rock and tailings

Numerous large-scale hard rock mines operate in Canada and will need to be reclaimed. The water balance of the reclaimed sites needs to be measured regularly to assess the reclamation performance and prevent environmental contamination from mining wastes. Transpiration is an important component of the water balance. However, it is rarely directly measured and is typically deduced from the measurement of other water balance components. This study used non-destructive sap flow sensors to evaluate transpiration rates of willow cuttings (Salix miyabeana clone sx64) planted in 2017 on experimental cells (ECs) composed of tailings and waste rock covered with overburden and topsoil layers. Data were collected in late summer 2020 and 2021. Consistently low transpiration rates of 0.76 mm and 0.8 mm per day were measured for waste rock ECs in 2020 and 2021, and 0.63 mm per day for tailings ECs in 2021. Transpiration accounted for almost one-third of the cumulative precipitation on the tailings and waste rock ECs (varying from 16% to 32%). Such high values demonstrate the importance of measuring the transpiration component of the water balance and its evolution in relation to vegetation development in the mining reclamation context to improve post-closure water management.