Observed and modeled multi-year evaporation from three field-scale experiments using water balance and Penman-Monteith methods: Profound effect of material type and wind exposure

Three instrumented experimental waste-rock piles at the Cu-Zn-Mo Antamina Mine in Peru are composed of distinct types of waste rock but are otherwise almost identical in size and geometry and experience the same atmospheric conditions with the exception of wind exposure. Evaporation from the piles was calculated using the water balance method over three- and four-year periods to determine the effect of material type and meteorological variability on evaporation. Annual changes in water storage were low or negligible except as a result of unusually high annual precipitation. Observed evaporation was high (44% - 75% of precipitation) and was extremely variable annually in the coarsest-grained waste-rock pile 1, most likely as a result of greater wind exposure and air circulation in that pile. Observed evaporation was moderate (36% - 48% of precipitation) with moderate annual variability in the finer-grained, relatively homogeneous waste-rock pile 2. Observed evaporation was low (24% - 32% of precipitation) with low annual variability in the finer-grained, relatively heterogeneous waste-rock pile 3, most likely as a result of low air circulation coupled with complex flow regimes that include high-velocity preferential flow paths. Slightly higher evaporation was observed on the slopes than on the crowns of Pile 2, while much lower evaporation was observed on the slopes than on the crowns of Piles 1 and 3. Evidence suggests that Piles 1 and 3 slope water-balance evaporation estimates are skewed by non-vertical flow and that, in general, evaporation is higher on the slopes than on the crowns of the piles. Evaporation was also estimated using the Food and Agriculture Organization of the United Nations modified Penman-Monteith method (FAO-PM; Allen et al., 1998) using base-case laboratory- and software- derived parameters. The base-case method underestimated observed evaporation calculated by the water balance method for Pile 1, overestimated observed evaporation for Pile 2, and greatly overestimated observed evaporation for Pile 3. The depth of the soil layer which is susceptible to evaporation, Z_e, was calibrated from the base-case value of Z_e= 0.10 m to values of Z_e= 0.27 m (Pile 1), Z_e= 0.05 m (Pile 2), and Z_e= 0.02 m (Pile 3) to provide a good fit with observed evaporation. It is hypothesized that the Pile 3 calibration of Z_e = 0.02 m is less physically realistic than accounting for preferential flow directly. A possible approach would be to modify the FAO-PM formulation by including a coefficient of preferential flow, K_PF, on days that exceed a specified threshold daily precipitation, P_i,TH. The results highlight the profound effect of material type on evaporation from waste rock. Fine-grained, relatively homogeneous materials tend to follow traditional evaporation patterns both on the crowns and slopes of waste rock piles. Coarser-grained and more heterogeneous waste rock can have higher or lower evaporation than expected.