Modelling evaporation from mine pit lakes: comparing three models under current and future climate.

Mine pit lakes, formed in abandoned open-pit mines, undergo complex hydrological processes that affect their water balance and environmental behaviour. Accurate evaporation estimates can be crucial for predicting a pit lake’s long-term behaviour. However, the lack of in situ measurements means that evaporation estimates often rely on models that have not been validated for pit lakes. In this study, three evaporation models were evaluated: an aerodynamic model, incorporating an equilibrium temperature approach, and the General Lake Model, a one-dimensional hydrodynamic model and traditional pan coefficient approach. The aerodynamic and GLM models were calibrated and validated using in situ daily measurements of evaporation and surface water temperature taken in a pit lake in central Queensland, Australia over a 21-month period. Based on minimising the RMSE, the aerodynamic model had a calibration period RMSE of 1.2 mm/day while the GLM model had  1.3 mm/day, with corresponding surface water temperature RMSEs of 0.9 °C and 1.2 °C. Validation period performances were similar for both evaporation and temperature. A pan coefficient model using a commonly assumed coefficient of 0.7 produced a calibration period RMSE of 2.8 mm/day. The aerodynamic model estimate exceeded that of the GLM on average by 34 mm/month in summer and 37 mm/month in autumn, with lower differences of 5.4 mm/month in spring and winter.  This is because the aerodynamic model excludes rainfall-induced cooling and mixing of the lake in the summer wet season, leading to higher evaporation estimates. In spring and winter, with less rainfall and mixing, the models align more closely. These differences remained consistent under future climate scenarios due to low projected changes in summer rainfall. Sensitivity analysis of the GLM identified surface heat exchange parameters, wind speed, and radiation are key factors influencing evaporation and temperature simulations. It is concluded that the aerodynamic model is an accurate and easily applied model for estimating current and future evaporation in the case study region. There may be accuracy benefits of using the GLM model where long-term changes in lake inflows, such as increases in rainfall, can change the lake's energy balance.