A linear system model of dynamic throughfall rates beneath forest canopies

[1] This paper describes a black box, linear system model to predict time-varying throughfall rates using only data of time-varying rainfall and storm total throughfall. For two forest stands in the Pacific Northwest the model predicted throughfall with mean efficiency of 0.84 when calibrated individually to 48 rainstorms and mean efficiency of 0.82 when calibrated to all storms simultaneously. The median of mean hydraulic residence times of precipitation in the canopy averaged across all storms ranged from 8 to 30 min by location within the two stands (overall median 12 min). Model predictions and performance were approximately equivalent for transfer functions based on either published equations describing storage and drip or of the form of exponential or gamma distributions. The model was insensitive to characteristics of rainstorms, so that calibrated models of water transfer through canopies were similar for storms of all sizes and intensities.