Predicting evaporation from mountain streams

Evaporation can be an important control on stream temperature, particularly in summer when it acts to limit daily maximum stream temperature. Evaporation from streams is usually modelled with the use of a wind function that includes empirically derived coefficients. A small number of studies derived wind functions for individual streams; the fitted parameters varied substantially among sites. In this study, stream evaporation and above-stream meteorological conditions (at 0.5 and 1.5 m above the water surface) were measured at nine mountain streams in southwestern British Columbia, Canada, covering a range of stream widths, temperatures, and riparian vegetation. Evaporation was measured on several days at each stream, at approximately hourly intervals, using nine floating evaporation pans distributed across the channels. The wind function was fit using mixed-effects models to account explicitly for among-stream variability in the parameters. The fixed-effects parameters were tested using leave-one-out cross validation. The model based on 0.5-m measurements provided improved model performance compared to that based on 1.5-m values, with RMSE of 0.0162 and 0.0187 mm/hr, respectively, relative to a mean evaporation rate of 0.06 mm/hr. Inclusion of atmospheric stability and canopy openness as predictors improved model performance when using the 1.5-m meteorological measurements, with minimal improvement when based on 0.5-m measurements.