Attenuation of rainfall intensity by forest canopies

Graduation date: 2004Presentation date: 2003-08-29Interception of precipitation by forest canopies has been found previously to reduce rainfall\ud intensity, but this effect is poorly understood. The goal of this research is to quantify the attenuation of\ud rainfall intensity by forest canopy interception, with the specific objective of estimating the contribution\ud of this attenuation to hillslope hydrology and slope stability during extreme rainfall.\ud Measurements of throughfall in two forest stands in the Pacific Northwest showed peak rainfall\ud intensities were lagged in time and damped in intensity compared to rainfall measured in nearby\ud openings. Spatial patterns of intensity smoothing and throughfall amounts persisted among storms.\ud Modeling the responses of the soil water pore pressure of a hypothetical hillslope to the most intense\ud periods of rainfall resulted in estimates of slope stability under the forest canopy that were greater than\ud for the same hillslope without the forest canopy.\ud A black-box model to quantify lagging and damping of throughfall efficiently described timevarying\ud rates of throughfall using only data of rainfall and storm-total throughfall. However, the\ud prediction capabilities of this model were limited because variation of best-fit model parameters among\ud storms was unpredictable. Modeled mean hydraulic residence times for rainfall in canopies were\ud approximately 15 min.\ud Coupling the black-box model of throughfall with stochastic models of rainfall and evaporation\ud resulted in a stochastic model of throughfall. Modeled extreme rainfall events (> 20 yr return interval) in\ud rainfall occurred 39 to 70% as often in throughfall, depending on return period.\ud Measurements of water detained on branches under simulated rainfall in the laboratory revealed\ud higher storage at higher rainfall intensities. Conifers stored more water per leaf area than did\ud broadleaved species at all intensities, but storage on broadleaved species showed greater increases at\ud higher intensities. These storage increases, although short-lived at the branch scale (≤ 10 sec), indicate a\ud mechanism for intensity attenuation at the canopy scale (~ 10 min), in which storage is temporarily\ud increased during heavy rainfall and drains over time. Descriptions of canopy interactions with rainfall\ud must include this "dynamic storage" to properly predict time-varying throughfall